1. Engr 1182.03 College of Engineering Engineering Education Innovation Center Engr 1182 Nano Pre-Lab Microfluidics Rev: 20XXMMDD, InitialsPresentation Short Title1

2. Engr 1182.03 Microfluidics: Objectives Understand capillary flow and how a capillary valve works. Explore how the flow of fluid in a microchannel depends on pressure and geometry. Practice delivering and cleansing mock samples. Rev: 20XXMMDD, InitialsPresentation Short Title2

3. Engr 1182.03 Review: hydrostatic pressure Rev: 20XXMMDD, InitialsPresentation Short Title3 The pressure at the bottom of an open container filled with liquid: h P PaPa

4. Engr 1182.03 Rev: 20XXMMDD, InitialsPresentation Short Title4 Example: for water (  = 1000 kg/m 3 ) at sea level ( g = 9.80 m/s 2 ) the hydrostatic pressure at a depth of h =10 m is: Pressure conversion factors: 1 atm = 1.01325 × 10 5 N/m 2 = 14.696 psi

5. Engr 1182.03 Surface tension Rev: 20XXMMDD, InitialsPresentation Short Title5 concave meniscus When a glass tube is immersed in water, liquid rises inside the tube due to surface tension and a concave meniscus forms. Surface tension can be thought of as a force, acting along the air/water/glass contact line, that “pulls” the liquid up the tube. Surface tension is caused by intermolecular forces.

6. Engr 1182.03 Capillary flow Surface tension can therefore cause fluid to flow in a capillary channel. Important factors are: tube orientation and the gravitational constant (g) diameter of tube density () and surface tension () of the liquid chemical nature of the tube walls Rev: 20XXMMDD, InitialsPresentation Short Title6

7. Engr 1182.03 A capillary “valve” If a tube initially filled with water is allowed to slowly drain, not all of the liquid drains out. In addition to surface tension at the top of the liquid, surface tension also acts to counter the expansion of surface area at the exit, and therefore prevents further flow. This is the basic principle behind a capillary check valve; undesired flow can be resisted by introducing a sudden expansion in a flow channel. Rev: 20XXMMDD, InitialsPresentation Short Title7

8. Engr 1182.03 The hydrostatic pressure at height h A is: P 1 - P 2 = gh A P 2 = P 1 – gh A Note that pressure P 2 (just beneath the surface) is not equal to P 1 ! This is a consequence of this interface being curved. Rev: 20XXMMDD, InitialsPresentation Short Title8 Let’s take another look at a vertical capillary tube immersed in liquid. Liquid spontaneously rises until it reaches an equilibrium height. hAhA P1P1 P1P1 P2P2

9. Engr 1182.03 Now let’s take another look at a capillary tube that is initially filled and allowed to slowly drain until it reaches the equilibrium state shown here. At equilibrium, P 3 = P 2 + gh B Substituting equation for P 2 : P 3 = P 1 – gh A + gh B P 3 – P 1 = g(h B – h A ) Rev: 20XXMMDD, InitialsPresentation Short Title9 P1P1 P2P2 P1P1 P3P3 hBhB (P 3 – P 1 ) is the pressure rating of this capillary valve. A pressure > (P 3 – P 1 ) is required to make liquid flow through this valve.

10. Engr 1182.03 Capillary check valves Capillary check valves can be used to prevent undesired flow into or out of a fluid reservoir in a device with micron-sized channels. Rev: 20XXMMDD, InitialsPresentation Short Title10