Introduction to Microfluidic System.

1
Introduction to Microfluidic System.

• Constructing extremely small laboratory
  elements on a chip by MEMS technology.

Figure. Microfluidic system on a chip. It
consists of small mixer, reactor, and analyzer,
etc.
ref. M. A. Burns et al., Science 1998, 282, 484.

• Typical cross-sectional dimension of
  microchannel used in the microfluidic system is
  approximately 100mm.

• The Reynolds number is usually lower than 100,
  which is representative of laminar flow and
  excludes the possibility of turbulent mixing.

• Therefore slow reagent mixing results from low
  Reynolds number regime is the most important
  performance limitation of microfluidic system.

2
Micromixer built in Microfluidic System.

• Static micromixer do not require any moving
  parts, and mixing is obtained by the natural
  motions of the fluids.

• There are various types of pressure driven
  micromixer shown below.

• Electroosmotical driven flow is an alternative
  to pressure driven flow in microchannel due to
  the controllability of fluids.

3
Electroosmotic Flow used in Microchannel.

• Recently, electroosmosis is used in
  microfluidic system due to the advantages of
  valveless control and transportation of fluids
  without dispersion.

• Electroosmotic flow is developed in a capillary
  when the capillary has electrical charges,
  the fluids are electrolytes and external electric
  fields are applied.

• Rapid reagent mixing is achieved by controlling
  surface charge distribution, external
  electric field or electrolyte concentration.

4
Electroosmotic Flow by Surface Charges.

• Non-uniform surface charges distribution
  generates vortical flow in microchannels.

ref. A. Ajdari, Phys. Rew. E 1996, 5, 4996.

• The two types of patterns act as a basis from
  which more general three dimensional flows.
  e.g., helical flow over bands of positive and
  negative charged bands running diagonally to the
  axis of channel ref. A. D. Stroock, Phys.
  Rew. Lett 2000, 84(15), 3314.

• However, the diagonal surface charge band
  induces not helical motion but small vortical
  flow.

Figure 2. Particle trajectories in a diagonal
surface charge band type microchannel. The
dimension of the microchannelis 200mm?70mm?2mm.
5
Electroosmotic Flow by External Electric Field.

• Electroosmotic flow runs parallel to the
  electric field.

• It is needed perpendicular electric field along
  with parallel external electric field to
  induce helical flow.

• We should obtain the entire electric field to
  get flow field in the microchannel.

6
Our Microchannel Description.

• Our microchannel has scale of length, 100mm and
  velocity 100mm/sec. Then the Reynolds number
  of our microchannel is less than 1.

where r is the density of fluid, m the viscosity
of fluid, Lc the length scale of
channel, Uc the velocity scale of channel.

• The electroosmotic helical flow field are
  governed by steady Stokes equations.

where v is the velocity of fluid, p the pressure.

• The boundary condition is as following.

where e is the permittivity , z the zeta
potential of channel, E the electric
field, x the position vector.
7
Numerical Description.

• Three-dimensional BEM is used to obtain the
  entire electric field.

• Three-dimensional FDM in Flow3D package is
  used to obtain the flow field.

• Scale of the microchannel is 100mm?100mm?1mm.

• Electric field strength is less than 10kV/m to
  avoid hydrolysis.

• Then the representative velocity is order of
  100mm/sec.

• We simulate the flow field and electric field
  within one set of additional electrode.

8
Electric Field at the Bottom Surface.

• Parallel electric field (E) strength is
  fixed, 10kV/m and perpendicular electric field
  (E?) strength is varied to generate various
  electric field shown below.

• As the ratio of E to E? decrease, the slop of
  electric field is getting more steeper.

• Electric field on the other surface is not much
  affected by additional electrode.

9
Electroosmotic Helical Flow Field in the
Microchannel.

• We obtain electroosmotic helical flow field
  with slip velocity approximation.

10
Reagent Mixing with Electroosmotic Helical Flow.

• We examine the reagent mixing by two groups of
  particles. They are mixed together within 1
  cycle (1mm).

11
Effect of Additional Electric Field Strength.

• We examine the mixing efficiency by different
  E to E? ratios.

(a)
(b)
t1.0
t2.0
t3.0
t4.0
t5.0
t6.0
t7.0
t8.0
t9.0
t10.0
Figure. Particle trajectories of two different
colored groups of particles at two different
electric field strength ratio. (a) EE?12,
(b) EE?10.5.
12
Conclusions.

• Nonuniform surface charge distribution can not
  produce the electroosmotic helical flow.

• Perpendicular electric field in addition to
  parallel external electric field is needed to
  generate electroosmotic helical flow.

• The electroosmotic helical flow gives rapid
  reagent mixing in microfluidic system as a
  passive micromixer.

• The suitable selection of additional electric
  field strength is 2 times greater than parallel
  external electric field, e.g., E1.4kV/m,
  E?2.8kV/m.

• In addition, the electroosmotic helical flow
  may use in micro protein isomer separator by
  amplifying small electrophoretic mobility
  differences of protein isomers.

Acknowledgement This work was supported by Grant
R01-2001-00410 from the Korea Science and
Engineering Foundation and also by the Ministry
of Education under the BK21 program.