Dielectrophoresis force spectroscopy for colloidal clusters

Abstract

Optical trapping-based force spectroscopy was used to measure the frequency-dependent DEP forces and DEP crossover frequencies of colloidal polymethyl methacrylate spheres and clusters. A single sphere or cluster, held by an optical tweezer, was positioned near the center of a pair of gold-film electrodes where alternating current elecroosmosis flow was negligible. Use of amplitude modulation and phase-sensitive lock-in detection for accurate measurement of the DEP force yielded new insight into dielectric relaxation mechanisms near the crossover frequencies. On one hand, the size dependence of the DEP force near the crossover frequencies indicates that the dominant polarization mechanism is a volume effect. On the other hand, the power-law dependence of the crossover frequency on the particle radius with an exponent of -2 indicates the dielectric relaxation is more likely because of ionic diffusion across the particle surface, suggesting the dominant polarization mechanism may be a surface polarization effect. Better theories are needed to explain the experiment. Nevertheless, the strong size dependence of the crossover frequencies suggests the use of DEP for size sorting of micron-sized particles.

Low-frequency dielectrophoretic response of a single particle in aqueous suspensions

“…The particle was positioned near the center between the electrode tips to minimize the influence from AC electroosmosis, which, the same as DEP, is also E 2 dependent. 25 We use the same design principals described in details before 3,4 to determine the DEP crossover frequencies. Here, a 3 Vpp AC at radial frequencies was applied across the pair of electrodes, creating an electric field gradient.…”

“…The technique was introduced in our previous research on DEP-based lab-on-a-chip applications and was used for measurements of the DEP crossover frequency of individual micro colloidal particles 3 as well as that of clusters formed by geometrical stacking of spheres. 4 Those studies did not fully address the underlying mechanisms that would confirm the dielectric response function of charged colloidal particles as suggested by Schwarz. 15 In this single-particle study, we aim to examine the counterion relaxation of individual charged colloidal particles and to provide the microscopic foundation of the bulk dielectric response function as established by Schwarz. 15 Here, we measure the DEP crossover frequency of an individual particle as a function of particles radius, medium viscosity, and temperature.…”

“…The relative polarizability of the particle to the medium may change sign at a specific frequency, often referred to as the DEP crossover frequency, at which the direction of DEP force acting on the particle reverses. [3][4][5] In this case, the crossover frequency reflects the nature of the dielectric relaxation of the particle with its associated electrical double layer at the particle-liquid interface and the dielectric properties of the medium. Understanding the molecular mechanism of DEP crossover is important because it is of interest to particle manipulation in microfluidic environments, such as bio-separation and cell sorting.…”

“…As proposed by Schwarz, the low-frequency dielectric relaxation mechanism is mainly due to electric double layer diffusion. 4,[15][16][17] Although some success has been achieved to account for the effect of the particle sizes, a significant discrepancy lies between the measured DEP crossover frequency and the relaxation frequency predicted by Schwarz theory. 4,10,18,19 To investigate the diffusion theory by Schwarz and its applicability in explaining experimental data, we apply the technique of optical tweezers-based DEP force spectroscopy on a single colloidal particle.…”

“…4,[15][16][17] Although some success has been achieved to account for the effect of the particle sizes, a significant discrepancy lies between the measured DEP crossover frequency and the relaxation frequency predicted by Schwarz theory. 4,10,18,19 To investigate the diffusion theory by Schwarz and its applicability in explaining experimental data, we apply the technique of optical tweezers-based DEP force spectroscopy on a single colloidal particle. In contrast to particle image velocimetry, where the macroscopic behavior of a particle ensemble is studied, 18,20 single-particle force detection in conjunction with amplitude modulation and lock-in detection techniques allows us to avoid the complication effects due to electrokinetic mechanisms, such as electrophoresis and AC electroosmosis, 19,[21][22][23] so that only the effects due to DEP are measured.…”