Dielectrophoresis - The Nanobiotechnology - page 412

particles. from. various. locations. travel. at. different. speeds. and. the. velocity. for. a. certain.
particle.( u x ).is.given.by

U x
h

x
h

u

x =

− . (17.15)

6 1

.

where
U .is.the.low.velocity
h .is.the.height.of.the.low.channel
x . is. the. height. of. the. particle. from. the. bottom. electrode. surface. (Gascoyne. and.
Vykoukal.2002)

Particles.can.be.fractionated.according.to.their.different.exist.moments.for.a.ixed.separation.
length.or.different.sediment.locations.for.a.given.residential.time..However,.the.use.of.a.
simple. electrode. arrangement. in. DEP-FFF. shown. in. Figure. 17.12a. could. lead. to. serious.
overlaps.of.various.particles:.(1).the.rapid.decay.of.the.magnitude.of.the.DEP.force.from.
the.electrode.surface;.this.results.in.weak.or.no.DEP.force.on.the.same.subgroup.particles.
when.they.are.far.away.from.the.electrode.surface;.(2).the.initial.random.distribution.of.
particles.in.the.same.group.experience.a.DEP.force.for.various.lengths.of.time..To.minimize.
these.issues,.the.practical.DEP-FFF.setup.often.comprises.two.sets.of.microelectrode.arrays:.
one.is.to.focus.all.particles.into.the.centerline.of.the.low.stream.by.n-DEP.or.converging.
tunnel.electrode.arrays.discussed.earlier.(Figure.17.11d.through.f)..This.helps.in.avoiding.
the. variations. of. the. DEP. force. on. particles. when. they. initially. stay. in. different. laminar.
luid.layers..When.the.focused.particles.pass.the.second.electrode.array,.the.difference.of.
the.DEP.force.on.these.particles.is.solely.determined.by.their.electric.properties,.leading.
to.much.clearly.deined.separation.bands.(as.shown.in.Figure.17.12b)..Such.designs.have.
been.successfully.used.in.the.separation.of.particles.and.three.main.cell.types.in.human.
blood.(Macrophages,.T.lymphocytes,.B.lymphocytes),.as.shown.in.Figure.17.12c..Till.date,.
DEP-FFF.has.been.successfully.used.to.separate.viable.from.dead.yeast.cells.(Markx.et.al..
1994b),.human.leukemia.cells.from.peripheral.blood.cells.(Huang.et.al..1997),.erythrocytes.
from.latex.beads.(Rousselet.et.al..1998),.and.human.breast.cancer.cells.from.CD34+.stem.
cells. (Huang. et. al.. 1999;. Wang. et. al.. 2000).. Although. DEP-FFF. separation. of. nanoscale.
objects,. such. as. DNA,. proteins,. and. virus,. seems. feasible. in. principle,. their. separation.
technologies.are.not.matured.yet.to.reach.desired.resolutions.

17.4.6 DeP for electrorotation and Transportation

DEP.trapping,.levitation,.and.separation.is.attributed.to.the.particle's.response.to.the.real.
part.of.the.Clausius-Mossotti.factor.and.the.magnitude.of.the.electric.ield..However,.the.
imaginary.part.of.the.DEP.forces.is.also.useful,.particularly.in.ROT.and.the.transportation.
of.particles.by.TWD..In.TWD.applications,.both.the.magnitude.and.phase.of.the.AC.signals.
play.important.roles:.Re[ K *].determines.the.levitation.of.the.particles.from.the.electrode.
surface.while.the.Im[ K *].controls.the.rotation.and.linear.motions.of.the.particle.along.the.
electrode. plane. (Fu. et. al.. 2004).. The. ROT. direction. is. determined. by. the. sign. of. Im[ K *]:.
when.Im[ K *].>.0,.particles.rotate.counterclockwise;.when.Im[ K *].<.0,.they.rotate.clockwise..
With.the.electrode.structures.shown.in.Figure.17.5d,.particles.are.caged.and.rotate.(usually.
too. hard. to. observe. directly. with. optical. microscopy). at. the. center. of. the. quadrupole.
microelectrode.under.the.polyphase.electrical.signals..However,.if.using.the.grid.electrode.