Nature Of Trapping Forces In Optically Induced Electrothermal Vortex Based Tweezers

Rapid electrokinetic patterning (REP) is an emerging microfluidic tweezer which can dynamically trap and manipulate micro- and nanoparticles via modulation of electrothermal vortices with optical patterns. By analyzing the trajectory of trapped particles with subpixel resolution, we show that the transverse trapping force in REP originates due to axisymmetric Stokes drag experienced by the particles in toroidal electrothermal vortices. The trapping force scales linearly with radial distance from the trap center and trap stiffness is on the order of femtonewtons/p.m in the transverse plane. This low trap stiffness is a direct feature of the fluidic nature of the REP trapping and would be useful for measuring femtonewton-scale forces.