Microfluidics Design & Chip Application Reporter: AGNES Purwidyantri Student ID no: D Biomedical Engineering Dept.

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. Micro-channels Nano-tubes

Sample Loading And Injection Subatmospheric Pressure Chamber Electro-Osmotic Pump Electro-Pneumatic Distributor Small Volume Transport Microfluidic Device Micro-scale Handling System Microfluidic Technologies

The objectives of micro-fluidic systems Micro-Total-Analysis-Systems (  TAS) ▫One system to provide all of the possible required analyses for a given type problem ▫All processing steps are performed on the chip ▫No user interaction required except for initialization ▫Portable bedside systems possible Lab-on-a-chip Microarray Micro-fluidics in nature ▫Aveoli (Lung bubbles)

Micro-fluidics is Interdisciplinary Micro-Fabrication Chemistry Biology Mechanics Control Systems Micro-Scale Physics and Thermal/Fluidic Transport Numerical Modeling ▫Simulation of micro-flows Material Science Electronics …

The fluids in micro-fluidic system Simple fluids ▫liquids and gases Complex fluids ▫immersed structures, surfactants, polymers, DNA … Injection of a droplet into a micro-channel. Polymer flow in a micro-channel Cells in a micro- channel.

Typical fluidic components Micro-channels and channel- circuit Functional structures ▫Micro-pump and switches ▫Mixing and separating devices Channel-circuit Typical functional structre Electroosmotic Pumping

Length scales in micro-fluidic systems Micro-channel 1mm 100  m 10  m 1m1m 100nm 10nm Cellular scale Radius of Gyration of DNA Microstructure and micro-drops Extended lenght of DNA Typical size of a chip Colloid and polymer molecular size

Other flow features for micro-fluidics Low Reynolds number flow ▫Large viscous force Low Capillary number flow ▫Large surface force High Peclet number flow ▫Disperse and diffusion ▫Slow diffusion effects Special transport mechanism ▫Mixing: chaotic mixing ▫Separation: particle, polymer and DNA

Low Reynolds number flow (Stokes flow) Reynolds number (Re) is the ratio between inertial force to viscous force Scaling between intertial force and viscous force in NS equation ▫Length scale L ▫Velocity scale U Flow classification based on Re

Low Reynolds number flow (Stokes flow) In micro-fluidics, Re<1 ▫Laminar flow ▫the viscous force dominant the inertial force ▫Inertial irrelevance Purcell

Low Capillary number flow Capillary number (Ca) is the ratio between viscous force to surface force What is surface tension? ▫Stretch force along the material interface

Low Capillary number flow In micro-fluidics, Ca <<1 ▫Surface force dominant flow ▫Wetting effects Micro-fluidic pin-ball: routing

Separation in micro-fluidics External force used to move the solute Separating particle on different mobility ▫Large mass, small velocity ▫Dielectric properties

Driving Forces in Microfluidics Systems

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

Surface Tension Let U be the average total cohesive energy of a molecule, and δ be a characteristic dimension of a molecule such that δ 2 represents the effective surface area of a molecule, then surface tension is approximately Surface tension has units of J/m 2 = 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

Surface Tension Example Surface tension can be treated in two ways: as stored energy per unit area (J/m 2 ) or as a tangential force per unit length (N/m)

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.

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.

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.

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

Electrowetting

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

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.

Microfluidic Flow Cytometers Wlodkowic, D &Darzynkiewics, Z Methods Cell Biol. 102: 105–125.

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