Engr 1182 Nano Pre-Lab

1. Engr1182 Nano Pre-Lab Microfluidics Presentation Short Title

2. 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. Presentation Short Title

3. Review: hydrostatic pressure The pressure at the bottom of an open container filled with liquid: Pa h P Presentation Short Title

4. Example: for water (r = 1000 kg/m3) at sea level (g = 9.80 m/s2) the hydrostatic pressure at a depth of h =10 m is: Pressure conversion factors: 1 atm = 1.01325 × 105 N/m2 = 14.696 psi Presentation Short Title

5. Surface tension concave meniscus When a glass tube is immersed in water, liquid rises inside the tube due to surface tension and a concave meniscusforms. 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. Presentation Short Title

6. 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 (r) and surface tension (g) of the liquid • chemical nature of the tube walls Presentation Short Title

7. 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. Presentation Short Title

8. Let’s take another look at a vertical capillary tube immersed in liquid. Liquid spontaneously rises until it reaches an equilibrium height. • The hydrostatic pressure at height hA is: • P1 - P2 = rghA • P2 = P1 – rghA • Note that pressure P2 (just beneath the surface) is not equal to P1! This is a consequence of this interface being curved. P1 P2 hA P1 Presentation Short Title

9. P1 • 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, • P3 = P2 + rghB • Substituting equation for P2: • P3 = P1 – rghA + rghB • P3 – P1 = rg(hB – hA) P2 hB (P3– P1) is the pressure rating of this capillary valve. A pressure > (P3– P1) is required to make liquid flow through this valve. P3 P1 Presentation Short Title

10. 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. Presentation Short Title