H.M. acknowledges funding from King Abdullah University of Science and Technology under BAS/1/1070-01-01 and KAUST access to nanofabrication core lab facilities.

1. Design
<ol>
	<li>Design 16 arrays, each comprising 625 circles (diameter, D = 100 &#956;m; pitch, L = 400 &#956;m), along with alignment marks to be translated onto 4&#34; SiO2(2 &#181;m)/Si(300 &#181;m)/SiO2(2 &#181;m) wafers using appropriate design software (see Table of Materials; Figure 3)38.</li>
</ol>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/60583/60583fig3.jpg" />
Figure 3: Designs of circular arrays. This design pattern was transferred onto SiO2(2 &#181;m)/Si(300 &#181;m)/SiO2(2 &#181;m) wafers through photolithography. Shown are (A) the entire wafer, (B,C) zoomed-in views, and (D,E) alignment marks used for the manual back alignment. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60583/60583fig3alarge.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<ol start="2">
	<li>Transfer features onto a 5&#34; soda lime glass (CaxHyNazOn) mask with a 50 nm coating of chromium and thin film of photoresist (a positive photoresist; see Table of Materials) through UV exposure in a direct-writing system (exposure time = 25 ms, defocus = +10).</li>
	<li>Mask development
	<ol>
		<li>Develop the photoresist by immersing the mask in 200 mL of developer (Table of Materials) for 60 s to expose the chrome underneath. Wash the mask with deionized (DI) water.</li>
		<li>Remove the exposed chromium by immersing the mask in a 200 mL bath of chrome etchant for 90 s. Wash the mask with DI water.</li>
		<li>Carry out a UV flood exposure (i.e., without mask) for 15 s.</li>
		<li>Completely remove photoresist from the mask by immersing in a 200 mL bath of developer until the photoresist completely disappears (60&#8211;120 s). Clean the mask surface with DI water and dry using a nitrogen (N2) gun.</li>
	</ol>
	</li>
</ol>
2. Wafer cleaning
<ol>
	<li>Immerse the silicon wafer in a freshly prepared piranha solution (H2SO4:H2O2 = 3:1 by volume) maintained at a temperature of 388 K for 10 min. 
	NOTE: Wear appropriate personalized protection equipment (PPE) while working with piranha solution on the wet bench.</li>
	<li>Rinse the wafer with DI water, two cycles in a wet bench, and dry it under a N2 environment in spin drier.</li>
</ol>
3. HMDS deposition
<ol>
	<li>Expose the wafer to the vapor of hexamethyldisilane (HMDS) to improve adhesion of the photoresist with the silica surface (details in Table 1).</li>
</ol>
<table border="1" fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td colspan="3">Stage 1: Dehydration and purging oxygen from chamber</td>
		</tr>
		<tr>
			<td>Step</td>
			<td>Function</td>
			<td>Time (min)</td>
		</tr>
		<tr>
			<td>1</td>
			<td>Vacuum (10 Torr)</td>
			<td>1</td>
		</tr>
		<tr>
			<td>2</td>
			<td>Nitrogen (760 Torr)</td>
			<td>3</td>
		</tr>
		<tr>
			<td>3</td>
			<td>Vacuum (10 Torr)</td>
			<td>1</td>
		</tr>
		<tr>
			<td>4</td>
			<td>Nitrogen (760 Torr)</td>
			<td>3</td>
		</tr>
		<tr>
			<td>5</td>
			<td>Vacuum (10 Torr)</td>
			<td>1</td>
		</tr>
		<tr>
			<td>6</td>
			<td>Nitrogen (760 Torr)</td>
			<td>3</td>
		</tr>
		<tr>
			<td colspan="3">Stage 2: Priming</td>
		</tr>
		<tr>
			<td>Step</td>
			<td>Function</td>
			<td>Time (min)</td>
		</tr>
		<tr>
			<td>1</td>
			<td>Vacuum (1 Torr)</td>
			<td>2</td>
		</tr>
		<tr>
			<td>2</td>
			<td>HMDS (6 Torr)</td>
			<td>5</td>
		</tr>
		<tr>
			<td colspan="3">Stage 3: Purging Prime Exhaust and Return to Atmosphere (Backfill)</td>
		</tr>
		<tr>
			<td>Step</td>
			<td>Function</td>
			<td>Time (min)</td>
		</tr>
		<tr>
			<td>1</td>
			<td>Vacuum</td>
			<td>1</td>
		</tr>
		<tr>
			<td>2</td>
			<td>Nitrogen</td>
			<td>2</td>
		</tr>
		<tr>
			<td>3</td>
			<td>Vacuum</td>
			<td>2</td>
		</tr>
		<tr>
			<td>4</td>
			<td>Nitrogen</td>
			<td>3</td>
		</tr>
	</tbody>
</table>
Table 1: HMDS priming process details.
4. Lithography
<ol>
	<li>Transfer the wafer onto a vacuum-chuck of a spin coater to spin coat the photoresist. Use AZ 5214 photoresist as a negative tone to achieve a 1.6 &#181;m thick uniform film of the photoresist (the spin coating parameters are listed in Table 2). 
	NOTE: AZ 5214 can be used as a positive or negative tone photoresist based on the heat treatment (i.e., pre-baking and post-baking). If prebaked at 110 &#176;C for 2 min, the photoresist behaves as a positive tone, such that exposed areas become dissolved during development. For the negative tone, the photoresist is prebaked at 105 &#176; C for 2 min followed by UV exposure and post-baking at 120 &#176;C for 2 min.
	<ol>
		<li>Bake the photoresist-coated wafer at 105 &#176;C on a hotplate for 2 min. This dries and hardens the photoresist film, which otherwise sticks to the glass mask and causes contamination issues during UV exposure, and it also improves adhesion of the photoresist to the silica surface. 
		NOTE: The pre-bake temperature should not be too high, as this may cause the partial destruction of light-sensitive components of the photoresist, reducing its sensitivity.</li>
	</ol>
	</li>
</ol>
<table border="1" fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td>Step</td>
			<td>Speed (rpm)</td>
			<td>Ramp (rpm/s)</td>
			<td>Time (s)</td>
		</tr>
		<tr>
			<td>1</td>
			<td>800</td>
			<td>1000</td>
			<td>3</td>
		</tr>
		<tr>
			<td>2</td>
			<td>1500</td>
			<td>1500</td>
			<td>3</td>
		</tr>
		<tr>
			<td>3</td>
			<td>3000</td>
			<td>3000</td>
			<td>30</td>
		</tr>
	</tbody>
</table>
Table 2: Parameters for spin coating recipe to obtain a 1.6 &#181;m layer of photoresist.
<ol start="2">
	<li>Expose the wafer under UV exposure (80 mJ/cm2) for 15 s through the chrome mask using a mask alignment system (EVG 6200) to achieve the desired design on the photoresist.</li>
	<li>Bake the realized wafer at 120 &#176;C on a hotplate for 2 min. During this step, the exposed negative photoresist film further cross-links. As a result, the UV-exposed parts of the photoresist are no longer soluble in the developer solution, while the unexposed areas are soluble.</li>
	<li>Further expose the wafer under UV light (200 mJ/ cm2) for 15 s in a UV cure system (PRX-2000-20). 
	NOTE: During this step, the photoresist areas that were not previously exposed (step 4.3) are exposed and can later be dissolved in the developer, leaving behind the desired structures on the wafer. This step is tolerant of overexposure because the desired features (in the negative tone) are no longer photosensitive after the post-baking step.</li>
	<li>Immerse the wafer in a 50 mL bath of the AZ-726 photoresist-developer (in glassware) for 60 s to achieve the desired photoresist pattern on the silicon wafer.</li>
	<li>Subsequently clean the wafer using DI water and further blow-dry it with N2.</li>
</ol>
5. Sputter
<ol>
	<li>Sputter chromium on the wafer for 200 s to obtain a 50 nm thick chromium layer. The deposition is performed using a magnetron-type DC reactive sputter with a standard 2&#34; round target source in an argon environment with the following parameters: 400 V, current = 1 A, and pressure = 5 mTorr. 
	NOTE: The chrome layer protects silica from dry etching under octafluorocyclobutane (C4F8).</li>
</ol>
6. Photoresist lift-off
<ol>
	<li>Sonicate the sputtered wafer in an acetone bath for 5 min to lift off the remaining photoresist (and chromium deposited on the photoresist) from the wafer, leaving behind the desired features with a chromium hard mask.</li>
</ol>
7. Processing of the other side of the wafer
<ol>
	<li>After rinsing the backside of the wafer with a copious amount of acetone and ethanol, blow-dry with an N2 gun, then repeat steps 4.1 and 4.2.</li>
</ol>
8. Manual back alignment
<ol>
	<li>Align the desired features on the backside with the front side of the wafer using the alignment marks in the design and the &#34;Manual back alignment with crosshair&#34; module in the contact aligner (EVG 6200). 
	NOTE: Manual back alignment is a crucial step in the microfabrication protocol. Thus, the designed alignment features on the photomask must be used effectively to avoid offset in pore alignment.</li>
</ol>
9. Lithography on the backside of the wafer
<ol>
	<li>For the backside of the wafer, repeat steps 4.3&#8211;4.7, section 5, and section 6 to generate the required design with chromium on both sides of the wafer. Note that the part of surface covered with chromium does not undergo etching; thus, spots in which chromium is absent on the wafer define the inlets and outlets of the pore.</li>
</ol>
10. Etching
<ol>
	<li>Undergo etching of the exposed SiO2 layer on both sides of the wafer by an inductively coupled plasma (ICP) reactive ion etcher (RIE) that employs fluorine (C4F8) and oxygen (O2) chemistries. The duration is 16 min (ICP-RIE parameters listed in Table 3) for each side.</li>
	<li>Process the wafer with five cycles of anisotropic etching using the Bosch process to create a notch in the silicon layer. This process is characterized by a flat sidewall profile using alternating depositions of C4F8 and sulfur hexafluoride (SF6) gases. By alternating anisotropic etching and polymer deposition, the silicon etches straight down (etching parameters listed in Table 3).</li>
	<li>Immerse the wafer in a bath of piranha solution (H2SO4:H2O2 = 3:1 by volume) maintained at a temperature of 388 K for 10 min. This removes the polymers deposited in the anisotropic step.</li>
	<li>To create the undercut, which yields the reentrant profile, undergo isotropic etch using an SF6-based recipe for a duration of 165 s (etching parameters listed in Table 3). 
	NOTE: This step is performed on each side of the wafer.</li>
	<li>Anisotropic silicon etching
	<ol>
		<li>Transfer the wafer to deep-ICP-RIE (Oxford instruments) to etch 150 &#181;m of silicon using 200 cycles of deep etching using the Bosch process (etching parameters are listed in Table 3).</li>
		<li>Repeat step 10.4.1 with the backside of the wafer.</li>
		<li>Undergo piranha cleaning of the wafer in the wet bench for 10 min to remove polymeric contaminants deposited from the etching process, which ensures uniform etching rates.</li>
		<li>Repeat steps 10.4.1&#8211;10.4.3 to realize through pores (which can be visualized by naked eyes under a light source) in the wafer having reentrant inlets and outlets.</li>
	</ol>
	</li>
	<li>Immerse the wafer in a 100 ml bath of chrome etchant for 60 s to remove chromium from both sides of the wafer.</li>
</ol>
<table border="1" fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td rowspan="2">Parameter</td>
			<td rowspan="2">Silica Etching</td>
			<td colspan="2">Anisotropic silicon etching /cycle</td>
			<td rowspan="2">Isotropic silicon etching</td>
		</tr>
		<tr>
			<td>Deposition</td>
			<td>Etching</td>
		</tr>
		<tr>
			<td>RF power (W)</td>
			<td>100</td>
			<td>5</td>
			<td>30</td>
			<td>20</td>
		</tr>
		<tr>
			<td>ICP power (W)</td>
			<td>1500</td>
			<td>1300</td>
			<td>1300</td>
			<td>1800</td>
		</tr>
		<tr>
			<td>Etching pressure (mTorr)</td>
			<td>10</td>
			<td>30</td>
			<td>30</td>
			<td>35</td>
		</tr>
		<tr>
			<td>Temperature (&#176;C)</td>
			<td>10</td>
			<td>15</td>
			<td>15</td>
			<td>15</td>
		</tr>
		<tr>
			<td>C4F8 flow (sccm)</td>
			<td>40</td>
			<td>100</td>
			<td>5</td>
			<td>-</td>
		</tr>
		<tr>
			<td>O2 flow (sccm)</td>
			<td>5</td>
			<td>-</td>
			<td>-</td>
			<td>-</td>
		</tr>
		<tr>
			<td>SF6 flow (sccm)</td>
			<td>-</td>
			<td>5</td>
			<td>100</td>
			<td>110</td>
		</tr>
		<tr>
			<td>Etching time (s)</td>
			<td>960</td>
			<td>5</td>
			<td>7</td>
			<td>165</td>
		</tr>
	</tbody>
</table>
Table 3: Parameters for SiO2/Si dry etching.
11. Final cleaning
<ol>
	<li>After the microfabrication process, clean the wafer with 100 mL of freshly prepared piranha solution (H2SO4:H2O2 = 3:1 by volume; T = 388 K) in a glass container for 10 min, then further blow-dry with a 99% pure N2 pressure gun.</li>
	<li>Place the samples in a glass Petri dish inside a clean vacuum oven at T = 323 K until the intrinsic contact angle of water on smooth SiO2 is stabilized at &#952;o &#8776; 40&#176; (after 48 h).</li>
	<li>Store the obtained dry samples (silica GEMs) in a N2 cabinet. 
	NOTE: The entire fabrication workflow is depicted in Figure 4.</li>
</ol>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/60583/60583fig4.jpg" />
Figure 4: Schematic illustration of the GEM microfabrication process. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60583/60583fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

R.D., S.A., and H.M. have filed an international patent, Application No. PCT/IB2019/054548.

This work presents the design and fabrication of silica-GEMs, the first-ever DCMD membranes derived from hydrophilic materials. Microfabrication with the SiO2/Si system provides immense flexibility to create microtextures to test creative ideas. Of course, the scope of this work is limited to the proof-of-concept for GEMs, because SiO2/Si/SiO2 wafers and cleanroom microfabrication protocols are impractical for desalination membranes.
It should be noted that, even though GEMs architecture can prevent the intrusion of water upon immersion when the intrinsic contact angle is &#952;o &#8805; 40&#176;, this strategy fails if the surface is made superhydrophilic. For example, after exposure to oxygen plasma, silica surfaces exhibit &#952;o &#8776; 5&#176;, and these silica-GEMs lose air that is entrapped inside the pores spontaneously as bubbles, because the liquid meniscus is no longer pinned at the reentrant edges. However, common plastics, such as polyvinyl alcohol (&#952;o &#8776; 51&#176;) and poly(ethylene terephthalate) (&#952;o &#8776; 72&#176;), should be amenable to this approach. Thus, design principles learned from silica-GEMs can be scaled up using 3-D printing44, additive manufacturing45, laser micromachining46, and CNC milling37, etc.
Next, some crucial aspects of the microfabrication of silica-GEMs are discussed, which require special attention. The manual back alignment (section 8) of the features should be performed with as much care as possible to achieve vertically aligned pores. Offsets may result in pore-throats, and in the worst case, the misalignment may lead to only cavities on either side (no pores). Thus, it is suggested to use multi-scale alignment marks, with the smallest alignment mark being at least four times smaller than the pore diameter.
During the etching of the silica layer with C4F8 and O2 (step 10.1), prior usage (i.e., cleanliness) of the reaction chamber can influence etching rates. This is because of the presence of contaminants in the reaction chamber, a common occurrence in shared user facilities such as universities. Thus, it is recommended that this step is performed first on a dummy wafer to ensure that the system is clean and stable. Also, it is advised to use short periods for etching (e.g., no more than 5 min while monitoring the thickness of the silica layer using reflectometry). For example, if it takes 16 min to completely remove a 2 &#181;m SiO2 layer from a SiO2/Si/SiO2 wafer, then the etching process should be divided into four steps comprising three 5 min cycles followed by reflectometry, and one 1 min (optional) etching step, based on the reflectometry results.
To preserve the silica reentrant features during the Bosch process that is used to etch the silicon layer (step 10.4), it is crucial that a chromium hard mask is used. The Bosch process entails the deposition of C4F8 to ensure the anisotropic profile. However, over long etching cycles, this layer can become very thick and difficult to remove. Thus, it is recommended that the Bosch process should not be run for more than ~200 cycles, and it should be followed by piranha cleaning. It has also been observed that long cycles of deep etching also reduce the thickness of the silica layer, despite the presence of a chromium hard mask.
Most dry etching tools fail to achieve spatial uniformity in terms of etching rates. Thus, the features obtained in the center of a SiO2/Si/SiO2 wafer may not be the same as those at the boundary of the wafer. Here, high quality features were realized in the center of 4&#34; wafers, and samples were periodically observed under a microscope. In the case that some regions are etched more than others, the wafer should be broken into pieces that should be etched separately.
This fabrication protocol can be applied to SiO2/Si/SiO2 wafers of any thickness; however, a thicker layer means that a higher number of etching cycles is needed. It is suggested to use silicon wafers of &#60;300 &#181;m thickness, as long as this does not compromise the mechanical integrity of the wafer during handling and characterization.

As the stress on water/food/energy/environmental resources escalates, greener technologies and materials for desalination are needed1,2. In this context, direct contact membrane distillation (DCMD) process can utilize solar-thermal energy or waste industrial heat for water desalination3,4. DCMD exploits water-repellent membranes to separate counterflowing streams of hot seawater and cold deionized water, allowing only pure water vapor to transport across from the hot to cold side5,6,7,8,9. Commercial DCMD membranes almost exclusively exploit perfluorocarbons because of their water repellency, characterized by the intrinsic contact angle of water, &#952;o &#8776; 110&#176;10. However, perfluorocarbons are expensive, and they get damaged at elevated temperatures11 and upon harsh chemical cleaning12,13. Their non-biodegradability also raises environmental concerns14. Thus, new materials for DCMD have been explored, e.g., polypropylene15, carbon nanotubes16, and organosilica17, along with variations of the process, e.g., interfacial heating18 and photovoltaic-MD19. Nevertheless, all materials investigated for DCMD membranes thus far have been intrinsically water-repellent, characterized by &#952;o &#8805; 90&#176; for water).
Here, a protocol is described for exploiting water-loving (hydrophilic) materials towards achieving the function of water-repellent DCMD membranes i.e., separating water on either side by entrapping air robustly inside the membrane pores. Towards the proof-of-concept demonstration, double-sided polished silicon wafers with silica layers (2 &#181;m thick) on both sides (SiO2/Si/SiO2; 2 &#181;m/300 &#181;m/2 &#181;m, respectively) are used. Microfabrication processes are applied to achieve gas entrapping membranes (GEMs), which exploit a specific architecture to prevent liquids from entering the pores regardless of surface chemistry.
The inspiration for GEMs architecture originated from springtails (Collembola), soil-dwelling hexapods whose cuticles contain mushroom-shaped patterns20,21, and sea-skaters (Halobates germanus), insects living in the open ocean that have mushroom-shaped hair on their body22,23. The surface architecture, along with naturally secreted waxes, affords these insects with &#34;super&#34; water repellence, characterized by apparent contact angles for water (&#952;r &#8805; 150&#176;)24. As a result, in their resting state, sea-skaters are essentially floating in air at the sea-air interface22,25. If submerged in water, they instantaneously trap a layer of air around their body (also known as plastron), which facilitates respiration and buoyancy20,23. Inspired by springtails, Kim and co-workers showed that silica surfaces with arrays of mushroom-shaped pillars can repel droplets of liquids with low surface tensions26. This was a remarkable discovery; albeit, it was found that the liquid repellence of these surfaces could be lost catastrophically through localized defects or boundaries27,28. To remedy this problem, researchers microfabricated silica surfaces with cavities whose diameters at the inlets were abruptly smaller (i.e., with a 90&#176; turn) than the rest of the cavity27. These features are also known as &#34;reentrant&#34; edges, and the cavities are hereafter referred to as &#34;reentrant cavities&#34;.
Reentrant cavities robustly entrap air on contact with liquid drops or upon submersion27. The performance of cavities of different shapes (circular, square, and hexagonal), profiles (reentrant and doubly reentrant), and sharpness of corners in relation to the stability of entrapped air over time has been compared29. It has been found that circular reentrant cavities are the most optimal in terms of their robustness for air entrapment under wetting liquids and the complexity associated with manufacturing. Also, it has been demonstrated that intrinsically wetting materials with reentrant cavities can entrap air upon immersion in wetting liquids, and thus, achieve the function of omniphobic surfaces. Based on this body of work27,28,29,30 and previous experience with DCMD31, we decided to create membranes that have pores with reentrant inlets and outlets. It was envisioned that such a membrane could entrap air upon immersion in wetting liquids due to its microtexture, giving rise to the idea of GEMs.
Consider a membrane made from a hydrophilic material comprising simple cylindrical pores: when immersed in water, this membrane will imbibe water spontaneously (Figure 1A,B) reaching the fully-filled, or the Wenzel state32. On the other hand, if the inlets and outlets of the pores have reentrant profiles (e.g., &#34;T&#34;-shaped), they may prevent the wetting liquid from penetrating the pore and entrap air inside, leading to Cassie states33 (Figure 1C,D). Once the air is trapped inside the pore, it will further prevent liquid intrusion due to its compressibility and low solubility in water over time34,35.
Such a system will slowly transition from Cassie to Wenzel state, and the kinetics of this process can be tuned by the pore&#39;s shape, size, and profile, vapor pressure of the liquid, and solubility of the trapped air in the liquid29,34,36. Researchers have been able to realize GEMs using silicon wafers and polymethylmethacrylate sheets as the test substrates, and proof-of-concept applications for DCMD in a cross-flow configuration have been demonstrated37. Here, a detailed microfabrication protocol for the generation of silica-GEMs is presented, starting with double-side polished silicon wafers with silica layers (2 &#956;m thick) on both sides (SiO2/Si/SiO2; 2 &#956;m/300 &#956;m/2 &#956;m, respectively). Also, the ability of the silica-GEMs to entrap air underwater is assessed using a custom-built pressure cell and confocal microscopy.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/60583/60583fig1.jpg" />
Figure 1: Schematic representation of a membrane with simple cylindrical pores (A,B) and one with reentrant pores (C,D). In contrast to the simple cylindrical pores, the reentrant pores become sharply broader after inlets/outlets, and it is this discontinuity (or the reentrant edges) that prevents liquids from intruding into the pores. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60583/60583fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
In particular, this section describes the microfabrication protocol for carving arrays of pores with reentrant inlets and outlets using double-sided polished silicon wafers that are 300 &#181;m thick (p-doped, &#60;100&#62; orientation, 4&#34; diameter, 2 &#181;m thick thermally grown oxide layers on both sides). This is referred to hereafter as SiO2(2 &#181;m)/Si(300 &#181;m)/SiO2(2 &#181;m) (Figure 2).
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/60583/60583fig2.jpg" />
Figure 2: Flowchart listing key steps involved in the microfabrication of silica-GEMs. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60583/60583fig2alarge.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table><tbody><tr><td>3D Printer</td><td>BCN3D</td><td>020.180510.3103</td><td>BCN3D Sigma 3D printer for printing test module with PLA (polylactic acid) filament.</td></tr><tr><td>Acetone</td><td>BASF</td><td></td><td></td></tr><tr><td>AZ-5214 E photoresist</td><td>Merck</td><td></td><td></td></tr><tr><td>AZ-726 MIF developer</td><td>Merck</td><td></td><td></td></tr><tr><td>Chrome Etchant</td><td>MicroChemicals</td><td>TechniEtch Cr01</td><td>To remove chromium from silicon wafer and mask</td></tr><tr><td>Conductivity Meter</td><td>Hanna</td><td>HI98192</td><td>To measure conductivity of pure water during leak testing.</td></tr><tr><td>Confocal microscope</td><td>Zeiss</td><td>ZEISS LSM 710</td><td>For fluorescence imaging of water.</td></tr><tr><td>Contact Aligner</td><td>EVG</td><td>EVG6200</td><td>Mask aligner</td></tr><tr><td>Deep ICP-RIE</td><td>Oxford Instruments</td><td>Plasmalab system100</td><td></td></tr><tr><td>DI water</td><td></td><td></td><td></td></tr><tr><td>Direct writer</td><td>Heidelberg Instruments</td><td>&amp;micro;PG501 direct-writing system</td><td>UV exposure</td></tr><tr><td>Food Dye</td><td>Kroger</td><td></td><td>Green food dye to label salty water.</td></tr><tr><td>Glass Petri dish</td><td>VWR</td><td></td><td></td></tr><tr><td>HMDS vapor prime</td><td>Yield Engineering systems</td><td></td><td></td></tr><tr><td>Hot plate</td><td>Cost effective equipments</td><td>Model 1300</td><td></td></tr><tr><td>Hydrogen peroxide 30%</td><td>VWR chemicals</td><td></td><td>To prepare piranha solution.</td></tr><tr><td>Imaris software</td><td>Bitplane</td><td>Version 8</td><td>Postprocess confocal microscopy images</td></tr><tr><td>Nitrogen gas</td><td></td><td></td><td></td></tr><tr><td>Optical surface profiler</td><td>Zygo</td><td>Zygo newview 7300</td><td></td></tr><tr><td>Photomask</td><td>Nanofilm</td><td></td><td>5-inch soda lime glass mask</td></tr><tr><td>Profilometer</td><td>Veeco</td><td>Detak 8</td><td>Stylus profilometer</td></tr><tr><td>Reactive Sputter</td><td>Equipment Support Company Ltd</td><td></td><td>Chromium sputtering</td></tr><tr><td>Reactive-Ion Etching (RIE)</td><td>Oxford Instruments</td><td>Plasmalab system100</td><td></td></tr><tr><td>Reflectometer</td><td>Nanometrics</td><td>Nanospec 6100</td><td>To check remaining oxide layer thickness.</td></tr><tr><td>Rhodamine B</td><td>Merck</td><td>81-88-9</td><td>Dye for imaging water meniscus under confocal microscope.</td></tr><tr><td>SEM stub</td><td>Electron Microscopy Sciences</td><td></td><td></td></tr><tr><td>SEM-Quanta 3D</td><td>FEI</td><td>Quanta 3D FEG Dual Beam (SEM/FIB)</td><td></td></tr><tr><td>Silicon wafer</td><td>Silicon Valley Microelectronics</td><td></td><td>Double side polished, 4&quot; diamater, 300 &amp;micro;m thickness, 2 &amp;micro;m thick oxide layer, p-doped, &amp;lt;100&gt; orientation.</td></tr><tr><td>Sodium Chloride</td><td>Merck</td><td>7647-14-5</td><td>For preparing NaCl solution</td></tr><tr><td>Sonicator</td><td>Branson</td><td>1510</td><td></td></tr><tr><td>Spin coater</td><td>Headway Research,Inc.</td><td></td><td></td></tr><tr><td>Spin dryer</td><td>MicroProcess</td><td>Avenger Ultra Pure 6</td><td>Spin drying in Nitrogen environment.</td></tr><tr><td>Sputter</td><td>Quorum Technologies</td><td>Q150T S</td><td>Iridium sputter for SEM.</td></tr><tr><td>Sulfuric acid 96%</td><td>Technic</td><td>764-93-9</td><td>To prepare piranha solution.</td></tr><tr><td>Tanner EDA L-Edit software</td><td>Tanner EDA, Inc.</td><td></td><td>For designing photomask</td></tr><tr><td>Tweezers</td><td>Excelta</td><td></td><td></td></tr><tr><td>UV Cure</td><td>Tamarack Scientific Co. Inc.</td><td>PRX-2000-20</td><td>For flood exposure of wafer and photomask</td></tr><tr><td>Vaccum oven</td><td>Thermo Scientific</td><td>13-258-13</td><td>Lindberg/Blue M</td></tr><tr><td>Wet bench</td><td>JST Manufacturing Inc.</td><td>17391-015-00</td><td>Wet bench used for piranha cleaning</td></tr></tbody></table>

proof-of-concept for gas-entrapping membranes derived from water-loving sio2/si/sio2 wafers for green desalination

Desalination through direct contact membrane distillation (DCMD) exploits water-repellent membranes to robustly separate counterflowing streams of hot and salty seawater from cold and pure water, thus allowing only pure water vapor to pass through. To achieve this feat, commercial DCMD membranes are derived from or coated with water-repellent perfluorocarbons such as polytetrafluoroethylene (PTFE) and polyvinylidene difluoride (PVDF). However, the use of perfluorocarbons is limiting due to their high cost, non-biodegradability, and vulnerability to harsh operational conditions. Unveiled here is a new class of membranes referred to as gas-entrapping membranes (GEMs) that can robustly entrap air upon immersion in water. GEMs achieve this function by their microstructure rather than their chemical make-up. This work demonstrates a proof-of-concept for GEMs using intrinsically wetting SiO2/Si/SiO2 wafers as the model system; the contact angle of water on SiO2 is &#952;o &#8776; 40&#176;. Silica-GEMs had 300 &#956;m-long cylindrical pores whose diameters at the (2 &#956;m-long) inlet and outlet regions were significantly smaller; this geometrically discontinuous structure, with 90&#176; turns at the inlets and outlets, is known as the &#34;reentrant microtexture&#34;. The microfabrication protocol for silica-GEMs entails designing, photolithography, chrome sputtering, and isotropic and anisotropic etching. Despite the water loving nature of silica, water does not intrude silica-GEMs on submersion. In fact, they robustly entrap air underwater and keep it intact even after six weeks (&#62;106 seconds). On the other hand, silica membranes with simple cylindrical pores spontaneously imbibe water (&#60; 1 s). These findings highlight the potential of the GEMs architecture for separation processes. While the choice of SiO2/Si/SiO2 wafers for GEMs is limited to demonstrating the proof-of-concept, it is expected that the protocols and concepts presented here will advance the rational design of scalable GEMs using inexpensive common materials for desalination and beyond.

This section presents the underwater performance of silica-GEMs microfabricated using the abovementioned protocol. The pores of these GEMs were vertically aligned, the inlet/outlet diameters were D = 100 &#956;m, the center-to-center distance between the pores (pitch) was L = 400 &#956;m, the separation between the reentrant edges and the wall was w = 18 &#181;m, and length of the pores was h = 300 &#956;m (Figure 5). Due to the inhomogeneities incurred during etching steps and minor misalignment during microfabrication, the middle portion of the pores was a bit narrower compared to the portion below the inlets and outlets of the pores, however, it did not affect the mass flux significantly.
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/60583/60583fig5.jpg" />
Figure 5: Scanning electron micrographs of silica-GEMs. Shown are (A) a tilted cross-sectional view of silica-GEMs, (B) a magnified cross-sectional view of a single pore, and (C,D) magnified views of reentrant edges at the inlets and outlets of a pore. Panels (C) and (D) are reprinted from Das et al.37. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60583/60583fig5alarge.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Immersing silica-GEMs in water 
Silica (SiO2) is hydrophilic, as characterized by the intrinsic contact angle of water drops on it under saturated water vapor, &#952;o &#8776; 40&#176;. Thus, if cylindrical pores are created on a silica sheet, water would imbibe them, reaching the fully-filled, or the Wenzel state32. To test this, a custom-built module was employed that can secure a test-membrane between a reservoir of dyed salty water (~0.6 M NaCl with food coloring) and deionized water (T = 293 K and p = 1 atm). It also logs the electrical conductivity of the deionized water reservoir into a computer to monitor pore filling in situ (Figure 6A). Here, silica membranes with simple cylindrical holes were not able to prevent the mixing of the two reservoirs, since water infiltrated instantaneously as reflected by the release of the dye (Supplemental Movie). In sharp contrast, when silica-GEMs were tested under the same conditions, they robustly entrapped air and held it intact for over 6 weeks, confirmed by electrical conductivity measurements (detection limit = &#177; 0.01 &#956;S/cm), after which the experiment was discontinued (Figure 6B). These findings establish that the GEMs architecture can enable hydrophilic materials to robustly entrap air upon immersion in water. Also, a pore level scenario was presented in which the low solubility of the entrapped air in water and the curvature of the air-water interface prevented the liquid meniscus from intruding further into the pore (Figure 6C).
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/60583/60583fig6.jpg" />
Figure 6: Membrane robustness testing. (A) Schematic of the 3D-printed customized cell for testing the robustness of membranes at separating dyed salty water (~0.6 M NaCl with food coloring) from pure deionized water (T = 293 K, p = 1 atm), while simultaneously logging the electrical conductivity of DI water reservoir into a computer. (B) A semi-logarithmic plot of the electrical conductivity of the DI water reservoir overtime when silica-GEMs were used to separate the two reservoirs. Remarkably, silica-GEMs robustly entrapped air in every pore, such that water could not penetrate even a single pore for over 6 weeks, evidenced by the electrical conductivity data. (C) Pore level schematic, showing the air-water interface at either end. Panels (A) and (B) are reprinted from Das et al.37. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60583/60583fig6alarge.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
To gain deeper insight into the pinning and displacements of the air-water interface at the inlets and outlets of silica-GEMs underwater (~5 mm column), confocal microscopy was utilized. It is well-known that the laser used for illumination in confocal microscopy also heats up the system39, which can accelerate wetting transitions. Nevertheless, the high spatial resolution can yield useful insight. For comparison, the behavior of silica surfaces with reentrant cavities was also investigated29,40. In both scenarios, the additional heat supplied to the water reservoir above enhances the capillary condensation of water vapor inside the microtexture. In the case of reentrant cavities, the condensation of water vapor displaced the entrapped air, which caused bulging of the air-water interface upwards and destabilized the system (Figure 7A,C). Under those experimental conditions, water intruded into all the cavities in less than 2 h. In contrast, silica-GEMs remained free from bulging for a much longer period, even though the rate of heating was similar. These results were rationalized on the basis of preferential condensation of water vapor from the laser-heated reservoir onto the cooler air-water interface at the opposite side of the pore (Figure 7B,D). However, it was not possible to measure the rate of mass transfer in this experimental configuration.
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/60583/60583fig7.jpg" />
Figure 7: Air-water interfaces. (A) Computer-enhanced 3D reconstructions of the air-water interface at inlets of silica-GEMs underwater (column height, z &#8776; 5 mm; laser power = 0.6 mW) along with cross-sectional views along the white dotted lines (on left and right sides of the central image). Due to heating from the laser on the top side, water vapor condensed inside the cavities, displacing the entrapped air. This caused the air-water meniscus to bulge upward and become unstable. After 1.5 h, most of the cavities were intruded by water. (B) Confocal micrographs of silica-GEMs under similar conditions as in (A). (C) Schematic of the bulging of the air-water meniscus in the case of reentrant cavities underwater. (D) Schematic for a pore in silica-GEMs under similar conditions. Hot water vapor condenses everywhere, most notably on the cooler air-water interface on the side further from the laser. As a result of this mass transfer, there is a minimal pressure build-up in the pore. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60583/60583fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Direct contact membrane distillation with GEMs 
Having established that silica-GEMs can robustly separate two water reservoirs on either side, a static DCMD configuration was tested, in which the salty feed side (0.6 M NaCl at T = 333 K) and deionized permeate side (T = 288 K) were static reservoirs. Even though silica-GEMs prevented water intrusion, measurable fluxes were not observed. This was due to the fact that the thermal conductivity of silicon (k = 149 W-m-1 K-1)41 is orders of magnitude higher than that of typical DCMD membranes (i.e., k &#60; 1 W-m-1-K-1)2. Thus, the experimental set-up with silica-GEMs suffered from what is known as temperature polarization, wherein the hot side loses heat to the cold side, lowering the flux31.
It may be possible to reduce the thermal conductivity of silicon through nanostructuring42 (for instance, to enhance its thermoelectric properties43), but these avenues were not explored. Instead, the design principles from silica-GEMs were translated to polymethylmethacrylate (PMMA) sheets (&#952;o &#8776; 70&#176; for water, k = 0.19 W-m-1-K-1)40 to create PMMA-GEMs37. Indeed, the first (proof-of-concept) batch of PMMA-GEMs with a low porosity (of 0.08) exhibited robust separation of feed side and permeate and yielded a flux of 1 L-m2-h-1 over 90 h. Thus, it is possible to translate these silica-GEMs based studies to using more common materials for generation of greener, lower cost membranes for desalination.
Supplemental Movie. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60583/Movie_S1.mp4" target="_blank">Please click here to view this file (Right click to download).</a>

Watch this Scientific Journal Video about Proof-of-Concept for Gas-Entrapping Membranes Derived from Water-Loving SiO2/Si/SiO2 Wafers for Green Desalination at JoVE.com

Proof-of-Concept for Gas-Entrapping Membranes Derived from Water-Loving SiO2/Si/SiO2 Wafers for Green Desalination

Presented here is a stepwise protocol for realizing gas-entrapping membranes (GEMs) from SiO2/Si wafers using integrated circuit microfabrication technology. When silica-GEMs are immersed in water, the intrusion of water is prevented, despite the water-loving composition of silica.

Proof-of-Concept for Gas-Entrapping Membranes Derived from Water-Loving SiO2/Si/SiO2 Wafers for Green Desalination

Desalination through direct contact membrane distillation (DCMD) exploits water-repellent membranes to robustly separate counterflowing streams of hot and ...

proof-concept-for-gas-entrapping-membranes-derived-from-water-loving

Nanyang Technological University

Research

JoVE Journal

Engineering

7.4K Views.  King Abdullah University of Science and Technology (KAUST). Gas indringende membranen, of GEMs, kan robuust vangen lucht op onderdompeling in bevochtiging vloeistoffen. Als gevolg hiervan bereiken ze deze functie door hun structuur, die van nut kan zijn, bijvoorbeeld bij ontzilting door membraandistillatie. Fotolithografie stelde ons in staat om complexe overhangende architecturen te creëren aan beide zijden van een siliciumwafer resulterend in GEMS.Het bood een pad om EDELSTENEN te fabriceren met behulp van conventionele microfabricagetechnie...

Video: Proof-of-Concept for Gas-Entrapping Membranes Derived from Water-Loving SiO2/Si/SiO2 Wafers for Green Desalination

Layer-by-layer Synthesis and Transfer of Freestanding Conjugated Microporous Polymer Nanomembranes

CMP as large surface area materials have attracted growing interest recently, due to their high variability in the incorporation of functional groups in combination with their outstanding thermal and chemical stability, and low densities. However, their insoluble nature causes problems in their processing since usually applied techniques such as spin coating are not available. Especially for membrane applications, where the processing of CMP as thin films is desirable, the processing problems have hindered their commercial application.
Here we describe the interfacial synthesis of CMP thin films on functionalized substrates via molecular layer-by-layer (l-b-l) synthesis. This process allows the preparation of films with desired thickness and composition and even desired composition gradients.
The use of sacrificial supports allows the preparation of freestanding membranes by dissolution of the support after the synthesis. To handle such ultra-thin freestanding membranes the protection with sacrificial coatings showed great promise, to avoid rupture of the nanomembranes. To transfer the nanomembranes to the desired substrate, the coated membranes are upfloated at the air-liquid interface and then transferred via dip coating.

In this paper we describe the interfacial synthesis of conjugated microporous polymers (CMP) on sacrificial substrates, and the dissolution of the substrate for the preparation of freestanding CMP nanomembranes. In addition, we will describe how the fragile nanomembranes can be transferred to other substrates.

CMP as large surface area materials have attracted growing interest recently, due to their high variability in the incorporation of functional groups in ...

Chemistry

Formation of Biomembrane Microarrays with a Squeegee-based Assembly Method

Lipid bilayer membranes form the plasma membranes of cells and define the boundaries of subcellular organelles. In nature, these membranes are heterogeneous mixtures of many types of lipids, contain membrane-bound proteins and are decorated with carbohydrates. In some experiments, it is desirable to decouple the biophysical or biochemical properties of the lipid bilayer from those of the natural membrane. Such cases call for the use of model systems such as giant vesicles, liposomes or supported lipid bilayers (SLBs). Arrays of SLBs are particularly attractive for sensing applications and mimicking cell-cell interactions. Here we describe a new method for forming SLB arrays. Submicron-diameter SiO2 beads are first coated with lipid bilayers to form spherical SLBs (SSLBs). The beads are then deposited into an array of micro-fabricated submicron-diameter microwells. The preparation technique uses a "squeegee" to clean the substrate surface, while leaving behind SSLBs that have settled into microwells. This method requires no chemical modification of the microwell substrate, nor any particular targeting ligands on the SSLB. Microwells are occupied by single beads because the well diameter is tuned to be just larger than the bead diameter. Typically, more 75% of the wells are occupied, while the rest remain empty. In buffer SSLB arrays display long-term stability of greater than one week. Multiple types of SSLBs can be placed in a single array by serial deposition, and the arrays can be used for sensing, which we demonstrate by characterizing the interaction of cholera toxin with ganglioside GM1. We also show that phospholipid vesicles without the bead supports and biomembranes from cellular sources can be arrayed with the same method and cell-specific membrane lipids can be identified.

Supported lipid bilayers and natural membrane particles are convenient systems that can approximate the properties of cell membranes and be incorporated in a variety of analytical strategies. Here we demonstrate a method for preparing microarrays composed of supported lipid bilayer-coated SiO2 beads, phospholipid vesicles or natural membrane particles.

Lipid bilayer membranes form the plasma membranes of cells and define the boundaries of subcellular organelles. In nature, these membranes are ...

Bioengineering

Deposition of Porous Sorbents on Fabric Supports

A microwave deposition technique for silanes, previously described for production of oleophobic fabrics, is adapted to provide a fabric support material that can be subsequently treated by dip coating. Dip coating with a sol preparation provides a supported porous layer on the fabric. In this case, the porous layer is a porphyrin functionalized sorbent system based on a powdered material that has been demonstrated previously for the capture and conversion of phosgene. A representative coating is applied to cotton fabric at a loading level of 10 mg/g. This coating has minimal impact on water vapor transport through the fabric (93% of the support fabric rate) while significantly reducing transport of 2-chloroethyl ethyl sulfide (CEES) through the material (7% of support fabric rate). The described approaches are suitable for use with other fabrics providing amine and hydroxyl groups for modification and can be used in combination with other sol preparations to produce varying functionality.

This report details a microwave-initiated approach for deposition of porphyrin functionalized porous organosilicate sorbents on a cotton fabric and demonstrates reduction in 2-chloroethyl ethyl sulfide (CEES) transport through the fabric resulting from this treatment.

A microwave deposition technique for silanes, previously described for production of oleophobic fabrics, is adapted to provide a fabric support material ...

Electrophoretic Crystallization of Ultrathin High-performance Metal-organic Framework Membranes

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and non-porous supports (polymer, ceramic, metal, carbon, and graphene). We developed a novel crystallization technique, which is termed the ENACT approach: the electrophoretic nuclei assembly for the crystallization of highly intergrown thin films (ENACT). This approach allows for a high density of heterogeneous nucleation of MOFs on a chosen substrate via the electrophoretic deposition (EPD) directly from the precursor sol. The growth of well-packed MOF nuclei leads to a highly intergrown polycrystalline MOF film. We show that this simple approach can be used for the synthesis of thin, intergrown zeolite imidazole framework (ZIF)-7 and ZIF-8 films. The resulting 500 nm-thick ZIF-8 membranes show a considerably high H2 permeance (8.3 x 10-6 mol m-2 s-1 Pa-1) and ideal gas selectivities (7.3 for H2/CO2, 15.5 for H2/N2, 16.2 for H2/CH4, and 2655 for H2/C3H8). An attractive performance for C3H6/C3H8 separation is also achieved (a C3H6 permeance of 9.9 x 10-8 mol m-2 s-1 Pa-1 and a C3H6/C3H8 ideal selectivity of 31.6 at 25 &#176;C). Overall, the ENACT process, owing to its simplicity, can be extended to synthesize intergrown thin films of a wide range of nanoporous crystalline materials.

A simple, reproducible, and versatile approach for the synthesis of intergrown, polycrystalline metal-organic framework membranes on a wide range of unmodified porous and non-porous supports is presented.

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and ...

An Efficient Method for Selective Desalination of Radioactive Iodine Anions by Using Gold Nanoparticles-Embedded Membrane Filter

Here, we demonstrate a detail protocol for the preparation of nanomaterials-embedded composite membranes and its application to the efficient and ion-selective removal of radioactive iodines. By using citrate-stabilized gold nanoparticles (mean diameter: 13 nm) and cellulose acetate membranes, gold nanoparticle-embedded cellulose acetate membranes (Au-CAM) have easily been fabricated. The nano-adsorbents on Au-CAM were highly stable in the presence of high concentration of inorganic salts and organic molecules. The iodide ions in aqueous solutions could rapidly be captured by this engineered membrane. Through a filtration process using an Au-CAM containing filter unit, excellent removal efficiency (&gt;99%) as well as ion-selective desalination result was achieved in a short time. Moreover, Au-CAM provided good reusability without significant decrease of its performances. These results suggested that the present technology using the engineered hybrid membrane will be a promising process for the large-scale decontamination of radioactive iodine from liquid wastes.

An efficient method for the rapid and ion-selective desalination of radioactive iodine in several aqueous solutions is described by using gold nanoparticles-immobilized cellulose acetate membrane filters.

Here, we demonstrate a detail protocol for the preparation of nanomaterials-embedded composite membranes and its application to the efficient and ...

Rendering SiO2/Si Surfaces Omniphobic by Carving Gas-Entrapping Microtextures Comprising Reentrant and Doubly Reentrant Cavities or Pillars

We present microfabrication protocols for rendering intrinsically wetting materials repellent to liquids (omniphobic) by creating gas-entrapping microtextures (GEMs) on them comprising cavities and pillars with reentrant and doubly reentrant features. Specifically, we use SiO2/Si as the model system and share protocols for two-dimensional (2D) designing, photolithography, isotropic/anisotropic etching techniques, thermal oxide growth, piranha cleaning, and storage towards achieving those microtextures. Even though the conventional wisdom indicates that roughening intrinsically wetting surfaces (&#952;o &#60; 90&#176;) renders them even more wetting (&#952;r &#60; &#952;o &#60; 90&#176;), GEMs demonstrate liquid repellence despite the intrinsic wettability of the substrate. For instance, despite the intrinsic wettability of silica &#952;o &#8776; 40&#176; for the water/air system, and &#952;o &#8776; 20&#176; for the hexadecane/air system, GEMs comprising cavities entrap air robustly on immersion in those liquids, and the apparent contact angles for the droplets are &#952;r &#62; 90&#176;. The reentrant and doubly reentrant features in the GEMs stabilize the intruding liquid meniscus thereby trapping the liquid-solid-vapor system in metastable air-filled states (Cassie states) and delaying wetting transitions to the thermodynamically-stable fully-filled state (Wenzel state) by, for instance, hours to months. Similarly, SiO2/Si surfaces with arrays of reentrant and doubly reentrant micropillars demonstrate extremely high contact angles (&#952;r &#8776; 150&#176;&#8211;160&#176;) and low contact angle hysteresis for the probe liquids, thus characterized as superomniphobic. However, on immersion in the same liquids, those surfaces dramatically lose their superomniphobicity and get fully-filled within &#60;1 s. To address this challenge, we present protocols for hybrid designs that comprise arrays of doubly reentrant pillars surrounded by walls with doubly reentrant profiles. Indeed, hybrid microtextures entrap air on immersion in the probe liquids. To summarize, the protocols described here should enable the investigation of GEMs in the context of achieving omniphobicity without chemical coatings, such as perfluorocarbons, which might unlock the scope of inexpensive common materials for applications as omniphobic materials. Silica microtextures could also serve as templates for soft materials.

Rendering SiO2/Si Surfaces Omniphobic by Carving Gas-Entrapping Microtextures Comprising Reentrant and Doubly Reentrant Cavities or Pillars

This work presents microfabrication protocols for achieving cavities and pillars with reentrant and doubly reentrant profiles on SiO2/Si wafers using photolithography and dry etching. Resulting microtextured surfaces demonstrate remarkable liquid repellence, characterized by robust long-term entrapment of air under wetting liquids, despite the intrinsic wettability of silica.

We present microfabrication protocols for rendering intrinsically wetting materials repellent to liquids (omniphobic) by creating gas-entrapping ...

Self-Assembly of Hybrid Lipid Membranes Doped with Hydrophobic Organic Molecules at the Water/Air Interface

Because of their unique properties, including an ultrathin thickness (3-4 nm), ultrahigh resistivity, fluidity and self-assembly ability, lipid bilayers can be readily functionalized and have been used in various applications such as bio-sensors and bio-devices. In this study, we introduced a planar organic molecule: copper (II) 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (CuPc) to dope lipid membranes. The CuPc/lipid hybrid membrane forms at the water/air interface by self-assembly. In this membrane, the hydrophobic CuPc molecules are located between the hydrophobic tails of lipid molecules, forming a lipid/CuPc/lipid sandwich structure. Interestingly, an air-stable hybrid lipid bilayer can be readily formed by transferring the hybrid membrane onto a Si substrate. We report a straightforward method for incorporating nanomaterials into a lipid bilayer system, which represents a new methodology for the fabrication of biosensors and biodevices.

We report a protocol for producing a hybrid lipid membrane at the water/air interface by doping the lipid bilayer with copper (II) 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (CuPc) molecules. The resulting hybrid lipid membrane has a lipid/CuPc/lipid sandwich structure. This protocol can also be applied to the formation of other functional nanomaterials.

Because of their unique properties, including an ultrathin thickness (3-4 nm), ultrahigh resistivity, fluidity and self-assembly ability, lipid bilayers ...

Epitaxial Nanostructured &alpha;-Quartz Films on Silicon: From the Material to New Devices

In this work, we show a detailed engineering route of the first piezoelectric nanostructured epitaxial quartz-based microcantilever. We will explain all the steps in the process starting from the material to the device fabrication. The epitaxial growth of &#945;-quartz film on SOI (100) substrate starts with the preparation of a strontium doped silica sol-gel and continues with the deposition of this gel into the SOI substrate in a thin film form using the dip-coating technique under atmospheric conditions at room temperature. Before crystallization of the gel film, nanostructuration is performed onto the film surface by nanoimprint lithography (NIL). Epitaxial film growth is reached at 1000 &#176;C, inducing a perfect crystallization of the patterned gel film. Fabrication of quartz crystal cantilever devices is a four-step process based on microfabrication techniques. The process starts with shaping the quartz surface, and then metal deposition for electrodes follows it. After removing the silicone, the cantilever is released from SOI substrate eliminating SiO2 between silicon and quartz. The device performance is analyzed by non-contact laser vibrometer (LDV) and atomic force microscopy (AFM). Among the different cantilever&#8217;s dimensions included in the fabricated chip, the nanostructured cantilever analyzed in this work exhibited a dimension of 40 &#181;m large and 100 &#181;m long and was fabricated with a 600 nm thick patterned quartz layer (nanopillar diameter and separation distance of 400 nm and 1 &#181;m, respectively) epitaxially grown on a 2 &#181;m thick Si device layer. The&#160;measured&#160;resonance frequency was&#160;267 kHz and the estimated quality factor, Q, of the whole mechanical structure was Q ~ 398 under low vacuum conditions. We observed the voltage-dependent linear displacement of cantilever with both techniques (i.e., AFM contact measurement and LDV). Therefore, proving that these devices can be activated through the indirect piezoelectric effect.

Epitaxial Nanostructured &#945;-Quartz Films on Silicon: From the Material to New Devices

This work presents a detailed protocol for the microfabrication of nanostructured &#945;-quartz cantilever on a Silicon-On-Insulator(SOI) technology substrate starting from the epitaxial growth of quartz film with the dip coating method and then nanostructuration of the thin film via nanoimprint lithography.

In this work, we show a detailed engineering route of the first piezoelectric nanostructured epitaxial quartz-based microcantilever. We will explain all ...

Ion-Exchange Membranes for the Fabrication of Reverse Electrodialysis Device

Reverse electrodialysis (RED) is an effective way to generate power by mixing two different salt concentrations in water using cation-exchange membranes (CEM) and anion-exchange membranes (AEM). The RED stack is composed of an alternating arrangement of the cation-exchange membrane and anion-exchange membrane. The RED device acts as a potential candidate for fulfilling the universal demand for future energy crises. Here, in this article, we demonstrate a procedure to fabricate a reverse electrodialysis device using laboratory-scale CEM and AEM for power production. The active area of the ion-exchange membrane is 49 cm2. In this article, we provide a step-by-step procedure for synthesizing the membrane, followed by the stack&#39;s assembly and power measurement. The measurement conditions and net power output calculation have also been explained. Furthermore, we describe the fundamental parameters that are taken into consideration for obtaining a reliable outcome. We also provide a theoretical parameter that affects the overall cell performance relating to the membrane and the feed solution. In short, this experiment describes how to assemble and measure RED cells on the same platform. It also contains the working principle and calculation used for estimating the net power output of the RED stack using CEM and AEM membranes.

We demonstrate the fabrication of a reverse electrodialysis device using a cation-exchange membrane (CEM) and anion-exchange membrane (AEM) for power generation.

Reverse electrodialysis (RED) is an effective way to generate power by mixing two different salt concentrations in water using cation-exchange membranes ...

Merging Ion Concentration Polarization between Juxtaposed Ion Exchange Membranes to Block the Propagation of the Polarization Zone

The ion concentration polarization (ICP) phenomenon is one of the most prevailing methods to preconcentrate low-abundance biological samples. The ICP induces a noninvasive region for charged biomolecules (i.e., the ion depletion zone), and targets can be preconcentrated on this region boundary. Despite the high preconcentration performances with ICP, it is difficult to find the operating conditions of non-propagating ion depletion zones. To overcome this narrow operating window, we recently developed a new platform for spatiotemporally fixed preconcentration. Unlike preceding methods that only use ion depletion, this platform also uses the opposite polarity of the ICP (i.e., ion enrichment) to stop the propagation of the ion depletion zone. By confronting the enrichment zone with the depletion zone, the two zones merge together and stop. In this paper, we describe a detailed experimental protocol to build this spatiotemporally defined ICP platform and characterize the preconcentration dynamics of the new platform by comparing them with those of the conventional device. Qualitative ion concentration profiles and current-time responses successfully capture the different dynamics between the merged ICP and the stand-alone ICP. In contrast to the conventional one that can fix the preconcentration location at only ~5 V, the new platform can produce a target-condensed plug at a specific location in the broad ranges of operating conditions: voltage (0.5-100 V), ionic strength (1-100 mM), and pH (3.7-10.3).

The protocol for a novel ion concentration polarization (ICP) platform that can stop the propagation of the ICP zone, regardless of the operating conditions is described. This unique ability of the platform lies in the use of merging ion depletion and enrichment, which are two polarities of the ICP phenomenon.

The ion concentration polarization (ICP) phenomenon is one of the most prevailing methods to preconcentrate low-abundance biological samples. The ICP ...

Proof-of-Concept for Gas-Entrapping Membranes Derived from Water-Loving SiO₂/Si/SiO₂ Wafers for Green Desalination

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