Particle focusing by AC electroosmosis with additional axial flow

ACEO-induced vortices have been used to manipulate fluids and particles for chemical and biochemical analysis in microscale systems. In such applications, a pressure-driven background flow is often applied to deliver the samples or particles into the target region. In the present study, the influence of an axial background flow on ACEO-induced vortices is experimentally investigated. An array of symmetric electrodes is used, perpendicular to the axial flow. Velocity fields of vortical flow are measured using astigmatism \(\mu\)-PTV. The strength of vortex is quantified as a function of the axial background velocity. Flow measurements with 2-\(\upmu \hbox {m}\) tracer particles reveal that the background flow compresses the ACEO-induced vortices and decreases the vortex size. This vortex region, separated from the background flow, can be seen as an “isolated area” in the microchannel. Next, 5-\(\upmu \hbox {m}\) particle dynamics in such flow with isolated vortical area is investigated. Experimental observations expose that 5-\(\upmu \hbox {m}\) particles do not follow the vortical flow and are focused toward the upper side of the channel. The velocity components of the 5-\(\upmu \hbox {m}\) particles are found to be different with the local flow visualized by the 2-\(\upmu \hbox {m}\) particles. This velocity difference of 5-\(\upmu \hbox {m}\) particles is probably due to the dielectrophoretic force acting on the particles. Furthermore, such particle focusing appears to be strongly related to the background velocity.