Dipolophoresis of Janus nanoparticles in a microchannel

A nonlinear dipolophoretic analysis is applied to analytically explain the counterintuitive experimental results of Gangwal et al. that an uncharged micro/nanosize dielectric Janus particle is attracted to the wall of a microchannel when exposed to an AC-uniform electric field in the direction parallel to the no-slip boundaries. We employ the so-called "weak" field assumption and consider a metallodielectric Janus colloid comprising two semispheres of distinct dielectric properties subject to an oscillating-uniform electric field with moderate frequency (below the Maxwell-Wagner limit). The Debye scale (ratio of electric double layer thickness to particle size) is considered unrestricted. Under the low Reynolds number hypothesis, Faxén's theorem and the Green's function (Stokeslet) method of singularities, including appropriate images with respect to the no-slip boundary, are applied under the remote-field approximation to determine the dynamics and trajectory of a small colloid moving near a wall. When assuming maximum dielectric contrasts between hemispheres and relatively low Debye scale (compared to particle radius), a rather simple relation for the equilibrium position of the colloid (i.e. tilt angle and distance from the wall) is obtained and found to be in qualitative good agreement with the experimental observations of Gangwal et al. and the predictions of Kilic and Bazant.