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Microfluidics Effects of Surface Tension

Microfluidics Effects of Surface Tension. Schuyler Vowell Physics 486 March 12, 2009. Microfluidics. Microfluidics refers to the behavior and control of liquids constrained to volumes near the μ L range. Behavior of liquids in the micro domain differs greatly from macroscopic fluids.

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Microfluidics Effects of Surface Tension

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  1. MicrofluidicsEffects of Surface Tension Schuyler Vowell Physics 486 March 12, 2009

  2. Microfluidics • Microfluidics refers to the behavior and control of liquids constrained to volumes near the μL range. • Behavior of liquids in the micro domain differs greatly from macroscopic fluids. • Surface tension. • Laminar flow. • Fast thermal relaxation. • Diffusion. • Microfluidics was developed in the 1980s, mainly for use in inkjet printers. • Microfluidics is an multidisciplinary field with a wide variety of applications.

  3. Interface • An interface is a smooth surface separating two materials. • Real interfaces are not smooth, molecules from each material mingle at an interface.

  4. Surface Tension • Molecules in any medium experience an attractive force with other molecules. • Mainly hydrogen bonds for polar molecules • Van der Waals forces for other molecules • Imbalance of this attractive force at an interface leads to surface tension

  5. Surface Tension Let U be the average total cohesive energy of a molecule, and δ be a characteristic dimension of a molecule such that δ2 represents the effective surface area of a molecule, then surface tension is approximately Surface tension has units of J/m2 = N/m, and is usually given in mN/m. If S is the total surface are of an interface and γ is the surface tension, then the total energy stored in the interface is

  6. Surface Tension Example Surface tension can be treated in two ways: as stored energy per unit area (J/m2) or as a tangential force per unit length (N/m)

  7. Contact Angle: Young’s Law The contact angle at a triple point (intersection of three interfaces) is entirely determined by balancing the surface tensions of each interface. A more rigorous derivation from minimization of free energy yields the same result as a geometric argument.

  8. Capillary Action • Capillary action refers to the movement of liquid through thin tubes, not a specific force. • Several effects can contribute to capillary action, all of which relate to surface tension • Minimization of surface energy • Young-Laplace equation: pressure difference due to curvature of interface.

  9. Minimization of Surface Energy Like any type of energy stored in a system, surface energy wants to be minimized. Examples include • Soap films on wire frames form minimal surfaces. • Water in capillary tubes rises above or falls below the surrounding water level.

  10. Capillary Rise Capillary rise is a balance of surface energy and gravitational potential energy: For a contact angle less than 90o, the liquid will rise in the tube, but the liquid can also fall if the contact angle is greater than 90o. If the liquid is water, solids with a contact angle less than 90o are called hydrophilic, the opposite is hydrophobic.

  11. Young-Laplace Equation The Young-Laplace equation describes the relationship between a pressure difference across an interface and the curvature of the interface. The higher the curvature, the higher the pressure difference across the interface.

  12. Movement of a Liquid Plug R2 < R1 for a wetting surface (θ < 90o), hence P2 > P1 and the liquid plug moves to the right, towards the narrower part of the wedge.

  13. Marangoni Effect A gradient in the surface tension along an interface causes motion in surface molecules and thus motion in the bulk. This is called the Marangoni effect.

  14. Applications of Microfluidics:Biology (LOC) Lab on a Chip (LOC) for bacterial culturing and testing. Fast PCR using nanodroplets Kim, H. et al. “Nanodroplet real-time PCR system with laser assisted heating.” Optics Express Vol. 17 No. 1. 5 Jan 2009 Orenstein, D. “’Microfluidic’ chips may accelerate biomedical research.” Stanford Report, 18 Jan 2009. http://news-service.stanford.edu

  15. Lab-on-a-Robot Wireless mobile unit carrying an electrochemical detection unit and HVPS. After choosing a location, onboard GPS navigates the robot to the test site. At test site, a MEMS device diffuses a gas sample through 50 μL of buffer solution. A small sample of this solution is injected into a microfluidic device that electrophoretically separates the components of the gas. A detector sends real-time sampling data back to the base computer running a LabVIEW program, which can be used to relay new commands to the robot and analyze the data transmitted from the robot. Berg, C. et al. “Lab-on-a-robot: Integrated microchip CE, power supply, electrochemical detector, wireless unit, and mobile platform.” Electrophoresis Vol. 29, 2008.

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