Passive pumping requires a big drop placed in the outlet and a small one placed at the inlet. This will cause the pressure difference needed to drive flow. Alternating drops of different fluids will cause packets to be flown in the channel and eventually be collected at the outlet.
High flow rates can be achieved by delivering low volume droplets at a frequency high enough to achieve a stable fluid shell at the inlet. Hi, I'm Pedro Reto from the Justin Williams lab in the Department of Biomedical Engineering at the University of Wisconsin Medicine. And I'm Brian Mogan, also from the Williams lab.
And I'm Aaron Burier from Professor S Lab, also in the biomedical engineering department at a W Medicine. Today we're gonna show you a procedure to do passive pumping in microfluidic devices. In our lab, we use this procedure to study flow within micro channels and to study fluidic exchange within the channel for biological applications.
So let's get started. First, using the soft lithography technique, create a microfluidic device using polymethyl suboxone. Next, cut out the device and then reversibly.
Attach the PDMS device to a glass slide by pressing it onto the slide and squeezing out any air bubbles. A reversible attachment allows the device to be reused multiple times. Now fill the device with liquid.
We use water with food coloring. If the liquid does not go into the channel by itself, put a drop of liquid at the inlet and use a pipette at the opposite end to suck the liquid through the channel. After filling the device with liquid, place a small drop on the inlet and a bigger drop on the outlet.
Make sure that fluid is moving by watching the small drop at the inlet collapse while the outlet drop gets bigger. Before we go any further, let's explain the basic relationships of passive pumping. Dynamics of pumped drops depend on several things, channel dimensions with height and length, and on the size of the drop that is placed at the inlet.
To give you a better idea of how these factors impact flow, we will give a few examples. Let's start out with two similar channels. They have the same width and height, but one is longer than the other.
Now we place drops of the same size at the inlet of each channel. Watch how the shorter channels inlet drop collapses faster than the longer channels drop. This is because a longer channel has greater fluidic resistance and flow inside of it is slower.
Next, let's explore two more channels with the same length and height, but different widths. Let's place identical drops at the inlet of each channel and watch them collapse. See how the thinner channel takes longer to collapse the inlet drop?
This is because the thinner channel possesses larger fluidic resistance than the wider channel. Lastly, let's take two identical channels and place different size drops at the inlet of each. Notice how the larger inlet drop takes longer to collapse than the smaller inlet drop.
This is because a smaller drop has less volume and greater inner pressure than the larger one. Knowing that smaller drops are better than larger ones. And having observed how fast these small drops can collapse, we now ask ourselves how to deliver volumetrically precise drops to the inlet of a channel in such a way that we can do biological experiments.
Using the Lee Company's micro dispensing starting kit, put together one or more valves each consisting of the vol valve, a nozzle with orifice size of 0.0100 inches, and a soft tube adapter. So here we have the valve connected to the control box via the tube valve connectors, which we did in the previous step. And then we have a spike voltage source, which is this root over here.
And then we have our control voltage source, followed by the hold voltage source. And finally, in connection to ground To hold the valves in place while aiming at the inlet. Use bioscience tools, miniature holders.
These provide a way to precisely aim and hold the valve at a certain position. During experimentation, using three quarter ounce syringes, make a reservoir system to be placed a few feet above the PDMS device. The reservoir provides a pressure head to drive the nozzles.
Attach a syringe needle to the syringe. Now attach the syringe needle to 1.14 millimeter inner tubing, and then attach that to 1.58 millimeter inner diameter tubing. Using PDMS as a sealant.
Now connect the 1.58 millimeter inner diameter tubing to the soft tube adapter of the valve. Now that there is a line between the syringe needle and the valve, fill the syringe reservoirs with liquid to purge the valves and allow the tubing to be filled with water. Place a magnet to the side of the valve and watch liquid begin to flow from the reservoir through the valve and out the 0.01 inch nozzle.
We control this system with a computer using lab view and the Lee Company's spike and hold control box. To calibrate the system, aim one valve to a Petri dish or a whey boat for weighing. Now open the valve for whatever amount of time you choose and weigh the total volume of water coming out of the nozzle.
Divide this volume by the total open time of the system. This calibration allows the user to find the volume of water that shoots out of the nozzle for any given period of time. Having calibrated the system, aim the nozzle to the inlet of a channel based on the previous calibration shoot from the nozzle, a volume such that a drop is shot at the inlet of the channel without being too big to cause a mess or too small to be unnoticed.
Remember to remove bubbles between experiments. Now that the basic principles of passive pumping are understood and system calibration has been explained, let's look at what a user should see if things are going right. The following video is a flow through a microfluidic channel going from higher volumes at lower frequencies to low volumes at high frequencies.
This should give the user an understanding of the capabilities of past pumping along with an automated fluidic delivery system. The device dimensions being used in the following videos are 2.22 millimeter width, 10 millimeter length and 260 micrometer. Height inlet and outlet diameters are 1.78 millimeter and 5.11 millimeter respectively.
Two or more valves are necessary to create alternating flows as shown up. Next is a video showing the maximum flow rate possible out of the current device being used. A single valve is used to deliver fluid in this case.
To maintain system balance, it is important to aim the valve directly from the top of the inlet. Notice the shell that the drop forms at the inlet. This represents the constant pressure maintained by delivering water to the inlet as fast as the device can passively pump it through.
Also, notice how the inlet drop overflows at some point. This is most likely caused by perturbing, the outlet drop. By removing the overflow at the inlet, the system regains equilibrium and flow continues.
Normally, We've just shown you how to set up, calibrate and operate a passive pumping system using commercially available micro nozzles. When doing these experiments is always important to remember to have a steady pressure source to avoid bubbles getting into your channel, to know the trade-offs between channel geometry, drop size in order to achieve the desired flows and exchanges. To not let the outlet drop get too big, for example, it'll make a mess or reach your inlet and to keep your PDMS devices clean and dry.
So that's it. Thank you for watching and good luck for your experiments.
