Electrohydrodynamic aggregation with vertically inverted systems

Abstract

Flow patterns surrounding particles suspended near electrodes within an electrolyte solution can be induced with an electric field due to an electrohydrodynamic (EHD) force. Depending on electrolyte, particle, and field properties, a variety of particle packing and stability states have been observed in EHD flow. In this work, we report evidence of EHD flow-induced aggregation of 2-μm sulfonated latex beads in NaCl and NaOH electrolyte solutions, in inverted particle-electrode orientations. Experimental conditions were chosen to match previous work where aggregation was observed at a bottom electrode. Particles remain stable at the top electrode for times greater than 1 h, and aggregation behavior is quantified in terms of growth rate and particle packing density and order. Similar aggregation behavior is seen at both top and bottom electrodes, with aggregate growth occurring more quickly within NaCl solutions than NaOH solutions at both the bottom and top electrode. In addition, an observed secondary location of stability for the vertical position of particles in NaOH electrolyte at the bottom electrode is not seen at the top electrode. Comparing these metrics to predictions made by a scaling model for EHD flow, particle aggregation behavior is successfully predicted at both top and bottom electrodes, but some of the observed differences in aggregate packing are not. Thus we suggest that modifications to existing models or their interpretation may be needed to improve predictions of the behavior of such systems.

Extreme Levitation of Colloidal Particles in Response to Oscillatory Electric Fields

“…Finally, consistent with the predicted voltage dependency, the magnitude of the experimentally applied potential was observed to have a weak effect above a threshold voltage. The previously proposed EHD flow mechanism for particle height bifurcation , fails to capture these observed dependencies and is unable to account for the extreme levitation heights measured experimentally.…”

“…These results suggested that the key role of the surrounding electrolyte is to set the magnitude of the critical frequency (∼25 Hz in 1 mM NaOH) at which particles transition between separating and aggregating behaviors. The existence of a critical aggregation frequency has since been corroborated by Ruud and Dutcher who also showed that particles “levitated” away from the upper electrode as well to a stable height some distance beneath it. It should be noted that below the critical frequency, particles levitated out of the visible focal plane to extreme distances above the electrode .…”

“…This extreme levitation is not readily explained by the height bifurcation mechanism originally proposed by Woehl et al , Here, it was assumed that the EHD flow generates a vertical lift force on the particles causing them to levitate, and the magnitude of this predicted EHD flow was shown to decay as ∼1/ h 3 . Thus, the vertical driving force for the height bifurcation would be negligible at such large reported distances (>10 particle diameters above the electrode , ), suggesting that the EHD fluid flow is not the sole explanation for the particle height bifurcation.…”

“…Figure ). They hypothesized that a radially inward electrohydrodynamic (EHD) flow generated on the electrode yields a vertical lift force that pushes particles away from the bottom , or top of the electrode surface (depending on the electrode orientation), ultimately resulting in the pronounced height bifurcation. The balance between this EHD lift force and gravity resulted in the formation of a new “tertiary” energy minimum.…”

On hybrid electroosmotic kinetics for field‐effect‐reconfigurable nanoparticle trapping in a four‐terminal spiral microelectrode array

“…To handle fluidic samples of low particle density, a concentration procedure is usually inevitable to confine the sample volume to an effective working range (nanoliter to microliter) that microfluidic devices can handle . For such, a number of microfluidic particle manipulation methods, such as hydrodynamics , ultrasonics , magnetism , concentration polarization , CE , electrohydrodynamics (EHD) , and dielectrophoresis (DEP) , have been studied in‐depth.…”