Basic Flows in a Microfluidic Device Vishal Tandon 1 Introduction Microfluidic devices are used to transport, mix, and separate analytes in small sample volumes for chemical or biological analysis. Understanding small-scale fluid mechanics is essential in device design and evaluating device performance. In this laboratory exercise, you will observe flow in two different microfluidic devices designed to highlight some important aspects of small-scale fluid mechanics. These devices are fabricated from poly(dimethylsiloxane), or PDMS for short. PDMS is a form of silicone. It initially exists as a viscous fluid, but when mixed with a crosslinker and baked, it hardens to a rubber-like consistency. The liquid PDMS can be poured over a mold, cured into a solid, peeled off the mold, and then bonded to a flat glass slide to make a device. PDMS is useful in the lab as a quick way of making microfluidic device, though it is not well-suited as a material for mass-production. 2 Materials Laminar Flow PDMS microfluidic device Step-Down PDMS microfluidic device Tygon Tubing Two-Way Valve with Luer-Lok fittings 5 ml Plastic Luer-Lok Syringes 23 Ga. Syringe Needles Deionized Water Food Coloring Fluorinert Microscope Clamp Stand Clamps with Test Tube Holders Weights Gloves Make sure to wear gloves for all of these experiments! Be careful with the syringe needles, they are sharp and can puncture your skin and/or the tygon tubing. 1
3 Laminar Flow Device 3.1 Setup 1. Draw 2 ml of food dye solution into one 5 ml syringe. Try to remove all the air bubbles from the syringe. 2. Draw 2 ml of deionized water into another 5 ml syringe. 3. Attach syringe needles to both syringes. Leave the caps on the needles while you are doing this. 4. Mount the syringes to the clamp stand using the clamps with test tube holders. Keep the syringes vertical, with the plunger ends facing upward. 5. Slide tygon tubes onto the syringe needles at the ends of the syringes. Use the end of the tube that does not have a pin. 6. Place the laminar flow PDMS device onto the microscope stand, and insert the pins from the tygon tubes connected to the syringes into the inlets of the device (the device is symmetrical, so either side is fine). There are holes punctured in the PDMS where you should put the pins. 7. Connect two tygon tubes to the outlets of the device, pushing the pin ends of the tubes into the holes in the PDMS. These tubes should go to a waste container 3.2 Hypotheses 1. If pressure is applied equally to both syringes, what will the flow look like? Draw a picture. 2. What will the fluid coming out of the outlets look like? 3.3 Experiment 1. Bring the center part of the device into focus in the microscope. 2. Lightly press on the syringe with food dye to fill the channel. The device may leak at the inlet if the pressure is too high. If this happens, clean up the leak with a q-tip or a napkin. 3. Try to press on both syringes equally. Once you get both fluids into the channel, balance the weights on top of the plungers so that you apply a roughly equal pressure to both syringes. 4. Observe the flow with the microscope, and record your observations below. 2
5. Move the chip so that you re looking at the branch point near the outlets. Record your observations below. 3.4 Questions 1. Draw what the flow in the center of the channel looks like when equal pressure is applied to both syringes. 2. Draw what the flow looks like at the junction at the end of the channel. 3. Do the fluids mix in the channel? 4. Is this flow more similar to that of glycerin (a very viscous fluid) in a large jar, or to water in a large jar? 5. Is mixing two fluids in a microfluidic device trivial? Possible? Why? 3
4 Step-Down Device 4.1 Setup 1. Draw 2 ml of food dye solution into one 5 ml syringe. Try to remove all the air bubbles from the syringe. 2. Draw 2 ml of fluorinert into another 5 ml syringe. Fluorinert is a liquid that is immiscible with water, but has similar viscosity. Again, remove the air bubbles. 3. Connect both syringes to the two-way valve using the Luer-Lok connection. Screw them in tightly so that they don t leak. 4. Screw a syringe needle onto the two-way valve. Leave the cap on the syringe needle while you do this. 5. Mount the food dye syringe to the clamp stand using a clamp such that the plunger is facing vertically upward. The fluorinert syringe will be at a right angle to the first. 6. Remove the cap from the syringe needle, and slide a tygon tube over it (the end that does not have a pin in it). 7. Place the step-down PDMS device on the microscope stage, and insert the pin from the tygon tube connected to the syringe needle into the inlet of the device (the side with the fattest channel). 8. Place a pin from another tygon tube into the outlet of the device. The other end of the tube should go into a waste container. 9. The valve stops flow in the direction it s pointing. Turn the valve toward the food dye syringe. Gently press on the fluorinert syringe to fill the device. The device may leak at the inlet if the pressure is too high. If this happens, clean up the leak with a q-tip or a napkin. 10. Turn the valve to the fluorinert syringe. Apply pressure to the food dye syringe either by hand or by carefully balancing a weight on top of the plunger for the syringe. Using the weight gives you a quantitative estimate of the applied pressure. 11. Switch between the two solutions until you see a clear interface between dyed solution and transparent solution moving through the channel when you apply pressure. You will observe this interface. 4.2 Hypotheses 1. For a given applied pressure, how will the fluid speed vary in the differently sized channels (as observable by looking at the motion of the interface between the two types of solution)? Why? 2. Which part of the channel has the highest fluidic resistance? The lowest? 4
4.3 Experiment 1. Center the widest channel on the microscope stage, and focus on it. 2. Balance a weight on top of the plunger of the food dye syringe, so as to apply a constant pressure and drive flow in the device. 3. Observe an interface between food dye and fluorinert as it moves through the channel. Get an idea of the speed with which it moves. Follow the interface by moving the chip if the interface moves out of the field of view of the microscope. 4. Pay special attention when the interface moves from the larger channel to the smaller one. Watch to see if it speeds up or slows down. 5. Observe the interface again as it passes to an even smaller channel. 4.4 Questions 1. In which section of the channel does the interface move the fastest for a given applied pressure? 2. What happens to the speed of the interface when it passes from a larger channel to a smaller one? 3. How are fluidic resistance and channel width related? 5 Extensions 1. Design an experiment to quantitatively measure the speed with which the interface moves through the step-down device. The dimensions of the device are labeled in the diagram above. Use this data to pose and answer questions about the system. 2. In the laminar flow device, come up with additional experiments to try. You have control over the pressures applied to the two syringes, the placement of inputs into the inlets/outlets, the fluids involved, etc. 5