**Flow in Microfluidic Devices**

Microelectronics:

telecommunications

internet technology

other complex miniaturized systems

Microfluidics:

DNA analysis

cell manipulation

mixing

portable integrated miniature diagnostic kits

Medical

Anti-terrorism

Environmental

Animal care

LOC Devices

(Lab-On-Chip)

Components:

pumps

valves

mixers

reactors

dispenser

separator

sensors

Schematic cross-section of the LoC system for DNA research currently under development

...interconnected by microchannels...

integrated system composed of the pump,

the filter and channels

integrated components:

regulate

mix

manipulate

separate

detect the samples

**Syeda Shah**

**4th Year projects**

**Microfluidics**

Microfluidics: Study of fluid transport at micrometer scales

allow the potential capabilities of a conventional laboratory to be confined into a chip

Flow in Microchannels

Flow in Pipe

Flow in Microchannels

EDL

Electric Double Layer

Electro-Kinetic Effects

Microchannels

**Summary**

Electro-osmosis

Streaming Potential

Electrophoresis

Sedimentation Potential

Slit Microchannels

Cylinderical microchannels

Rectangular microchannels

Sector Microchannel

Electro-kinetic effects: the phenomena observed due

to the presence of the Electric Double Layer (EDL)

near the solid-liquid interface

Liquid motion is induced by

an external electric-field

liquid flows over a stationary charged surface

excess counterions in the diffuse layer of the EDL move under the applied electrical field

causes the ions in the EDL to move towards one electrode

gives rise to a body force on the liquid setting the liquid in motion

pressure-driven flow where the velocity is parabolic

electro-osmotic flow has constant velocity over the cross section

In compact layer, the velocity profile is not flat but it is negligibly small

Liquid set into motion by a pressure gradient in the absence of an electric-field

counterions in the diffuse

layer of the EDL

accumulate downstream,

carrying the surrounding liquid with them

result in a electric current in the pressure driven flow direction

The build up of electric field gives rise to a Conduction Current which opposes

the Streaming Current

Streaming potential

pressure drop measured across the capillary

particle motion induced by the electric field over a stationary liquid

the application of an external electric field to a solid, liquid or gas particle in a bulk liquid phase

potential difference generated as charged

particles settle or rise through a fluid by an

application of an applied force

Each of the particles in the liquid are surrounded by an atmosphere of opposite charges

The particles move in a stationary liquid between 2 electrodes, embracing a new atmosphere and leaving behind the old one.

Poisson Eqn

Poisson Boltzmann Eqn

Fluid Mechanics

dimensionless

Debye Huckel Parameter

linearize

The inverse of k is the characteristic thickness of the Electric Double Layer.

K separation distance or the

ratio of the channel's height

to the EDL thickness

Electrokinetics in Microfluidics

by Dongqing Li (2004)

A large K either means that there is a large separation distance between the two plates or the EDL thickness is very small since

K is the ratio of the channels height to the EDL thickness.

Solutions for slit Microchannel

Linearized

Exact

Poisson's Eqn in 1D

Linearized Poisson Boltzmann Eqn

Fourier Analysis

Boundary Conditions for rectangular microchannel

(non-homogeneous)

Numerical Solution

For large Psi, linear approximation not valid

Can use finite-difference method to approximate solutions to Differential Eqn

plots of the EDL potential field, against the non-dimensional height and width of the rectangular channel for a square microchannel.

increasing the value of K, confines the EDL potential field to near the surface of the rectangular microchannel.

for small K, there are significant effects at the corners

Electro-osmotic flow in sector microchannels

by Chien C Chang et al (2009)

Electrokinetics in Microfluidics

by Dongqing Li (2004)

Theta = 1/4pi

K=1

K=10

Theta = 3/4pi

At microscales, we have to take into consideration the effects of the EDL field

Project has explored the electrokinetic phenomena involved in microchannels with particular emphasis being placed on the electro-osmotic

The effect of the EDL potential field has been considered while varying the non-dimensional Debye-Huckel parameter.

Results have indicated that there are signicant effects on corners, for both rectangular and sector channels.

At low values of K, the EDL region is large

and has a signicant eect on the corners.

Transient Electro-osmotic flow

Channel Cross-sections

Further work: