Simultaneous Aggregation and Height Bifurcation of Colloidal Particles near Electrodes in Oscillatory Electric Fields

Abstract

Micrometer-scale particles suspended in NaCl solutions aggregate laterally near the electrode upon application of a low-frequency (∼100 Hz) field, but the same particles suspended in NaOH solutions are instead observed to separate laterally. The underlying mechanism for the electrolyte dependence remains obscure. Recent work by Woehl et al. (PRX, 2015) revealed that, contrary to previous reports, particles suspended in NaOH solutions indeed aggregate under some conditions while simultaneously exhibiting a distinct bifurcation in average height above the electrode. Here we elaborate on this observation by demonstrating the existence of a critical frequency (∼25 Hz) below which particles in NaOH aggregate laterally and above which they separate. The results indicate that the current demarcation of electrolytes as either aggregating or separating is misleading and that the key role of the electrolyte instead is to set the magnitude of a critical frequency at which particles transition between the two behaviors.

Surface forces generated by the action of electric fields across liquid films

“…Whilst considering the origin of the measured forces, it is important to consider the possibility of either Joule heating or electrochemical (Faradic) irreversible processes playing a role; we have considered these possible artifacts in detail and explain in the Supplementary Information our reasons for having discounted them. We note the strong connection between our observation of this bifurcation to (i) measurements of colloidal forces under AC fields in electrolyte, in which both attraction and repulsion have been reported 35,36 , and (ii) the theoretical report of non-linear AREF (Asymmetric Rectified Electric Field) effects where it was pointed out that oscillating electric fields in liquids (electrolytes) create a long-range steady field with the absolute direction of the steady field can switch depending on the starting distance and AC frequency 39 . A direct AREF field could act to generate a force dependent on the asymmetry of the ion mobilities.…”

Oscillating Electric Fields in Liquids Create a Long-Range Steady Field

“…The mechanism for this levitation has been obscure, but the behavior is consistent with our AREF hypothesis: the long-range steady field causes the particles to move upward until the AREF magnitude diminishes sufficiently for the electrophoretic force to balance with gravity. The complicated spatial dependence of the AREF also explains why some particles were observed to move upward against gravity, while others moved downward [27,28]. Note in Fig.…”

“…Indeed, recent work [27,28] has established that oscillatory fields do cause microscale colloids in millimolar NaOH (δ = 3.96) to levitate many particle diameters upward against gravity, while the same particles in millimolar KCl (δ = 1.04) do not. The mechanism for this levitation has been obscure, but the behavior is consistent with our AREF hypothesis: the long-range steady field causes the particles to move upward until the AREF magnitude diminishes sufficiently for the electrophoretic force to balance with gravity.…”

Electrohydrodynamic aggregation with vertically inverted systems

“…[19] Frequency-dependent aggregation has been also been seen with NaOH electrolytes at one of these planes with a sufficiently low applied frequency (~25 Hz), an electrolyte which previously had only displayed separation behavior. [20] In addition to electrolyte and field effects, aggregate formation and packing behavior has been shown to vary with electrolyte strength. [21] In this paper, we further test the scaling model by varying the forces present in the model using a unique electrode orientation.…”

“…A voltage of 10 Vpp (Volts peak to peak) was applied, at a frequency of 100 Hz for NaCl and 25 Hz for NaOH to create conditions similar to those found to cause aggregation in each electrolyte. [9,20] This voltage was also chosen as particles were found to remain stable at top and bottom electrodes for periods greater than 1 hour, presumably due to a sufficiently large energy barrier from the depth of the secondary minimum as predicted in Table I and shown in Two types of experiments were performed to determine the aggregation rate of the system and aggregate packing behavior. Aggregate rate was measured with a 20x optical objective, achieving a 2 pixel per micrometer resolution at a framerate of 50 fps.…”