Contact Charge Electrophoresis: Fundamentals and Microfluidic Applications.

Contact charge electrophoresis (CCEP) uses steady electric fields to drive the oscillatory motion of conductive particles and droplets between two or more electrodes. In contrast to traditional forms of electrophoresis and dielectrophoresis, CCEP allows for rapid and sustained particle motions driven by low-power dc voltages. These attributes make CCEP a promising mechanism for powering active components for mobile microfluidic technologies. This Feature Article describes our current understanding of CCEP as well as recent strategies to harness it for applications in microfluidics and beyond.

Shape control and compartmentalization in active colloidal cells.

Small autonomous machines like biological cells or soft robots can convert energy input into control of function and form. It is desired that this behavior emerges spontaneously and can be easily switched over time. For this purpose we introduce an active matter system that is loosely inspired by biology and which we term an active colloidal cell. The active colloidal cell consists of a boundary and a fluid interior, both of which are built from identical rotating spinners whose activity creates convective flows. Similarly to biological cell motility, which is driven by cytoskeletal components spread throughout the entire volume of the cell, active colloidal cells are characterized by highly distributed energy conversion. We demonstrate that we can control the shape of the active colloidal cell and drive compartmentalization by varying the details of the boundary (hard vs. flexible) and the character of the spinners (passive vs. active). We report buckling of the boundary controlled by the pattern of boundary activity, as well as formation of core-shell and inverted Janus phase-separated configurations within the active cell interior. As the cell size is increased, the inverted Janus configuration spontaneously breaks its mirror symmetry. The result is a bubble-crescent configuration, which alternates between two degenerate states over time and exhibits collective migration of the fluid along the boundary. Our results are obtained using microscopic, non-momentum-conserving Langevin dynamics simulations and verified via a phase-field continuum model coupled to a Navier-Stokes equation.

Contact charge electrophoresis: experiment and theory.

Contact charge electrophoresis (CCEP) uses steady electric fields to drive the continuous, oscillatory motion of conductive particles and droplets between two or more electrodes. These rapid oscillations can be rectified to direct the motion of objects within microfluidic environments using low-power, dc voltage. Here, we compare high precision experimental measurements of CCEP within a microfluidic system to equally detailed theoretical predictions on the motion of a conductive particle between parallel electrodes. We use a simple, capillary microfluidic platform that combines high-speed imaging with precision electrical measurements to enable the synchronized acquisition of both the particle location and the electric current due to particle motion. The experimental results are compared to those of a theoretical model, which relies on a Stokesian dynamics approach to accurately describe both the electrostatic and hydrodynamic problems governing particle motion. We find remarkable agreement between theory and experiment, suggesting that particle motion can be accurately captured by a combination of classical electrostatics and low-Reynolds number hydrodynamics. Building on this agreement, we offer new insight into the charge transfer process that occurs when the particle nears contact with an electrode surface. In particular, we find that the particle does not make mechanical contact with the electrode but rather that charge transfer occurs at finite surface separations of >0.1 µm by means of an electric discharge through a thin lubricating film. We discuss the implications of these findings on the charging of the particle and its subsequent dynamics.

Microfluidic mixing of nonpolar liquids by contact charge electrophoresis.

We present a simple and effective ratcheted microfluidic mixer that uses contact charge electrophoresis (CCEP) of a micron-scale particle to rapidly mix nonpolar liquids. CCEP combines contact charging and electrostatic actuation to drive the continuous oscillatory motion of a conductive particle between two electrodes subject to a constant (DC) voltage. We show how this oscillatory motion can be harnessed to mix laminar flows by using dielectric "ramps" to direct the particle along non-reciprocal, orbital trajectories, which repeatedly stretch and fold the flowing streams. Complete mixing requires that the speed of the particle is much larger than the fluid velocity such that the particle completes many orbits as the fluid flows through the mixing region. The extent of mixing also depends strongly on the size of the particle and the shape of its trajectory; effective mixers relied on larger particles (comparable to the size of the channel) moving along non-reciprocal orbits. While the present study uses mineral oil as a convenient nonpolar liquid, we also screened fifteen common solvents to determine the applicability of CCEP for mixing other organic liquids. Owing to its simple design and low power requirements (~100 nW), the orbital mixer presented here demonstrates the utility and versatility of ratcheted electrostatic actuation in powering active microfluidic operations.

Self-assembly of nanoparticle amphiphiles with adaptive surface chemistry.

We investigate the self-assembly of amphiphilic nanoparticles (NPs) functionalized with mixed monolayers of hydrophobic and hydrophilic ligands in water. Unlike typical amphiphilic particles with "fixed" surface chemistries, the ligands used here are not bound irreversibly but can rearrange dynamically on the particles' surface during their assembly from solution. Depending on the assembly conditions, these adaptive amphiphiles form compact micellar clusters or extended chain-like assemblies in aqueous solution. By controlling the amount of hydrophobic ligands on the particles' surface, the average number of nearest neighbors--that is, the preferred coordination number--can be varied systematically from ∼ 1 (dimers) to ∼ 2 (linear chains) to ∼ 3 (extended clusters). To explain these experimental findings, we present an assembly mechanism in which hydrophobic ligands organize dynamically to form discrete patches between proximal NPs to minimize contact with their aqueous surroundings. Monte Carlo simulations incorporating these adaptive hydrophobic interactions reproduce the three-dimensional assemblies observed in experiment. These results suggest a general strategy based on reconfigurable "sticky" patches that may allow for tunable control over particle coordination number within self-assembled structures.

Ratcheted electrophoresis for rapid particle transport.

Ratcheted electrophoresis of contact-charged particles allows for high speed transport through microfluidic channels over large distances and even against fluid flows. Using a set of predictive design heuristics, we demonstrate an extension of this microfluidic ratchet to separate conductive particles from a particle suspension.

ac electric fields drive steady flows in flames.

We show that time-oscillating electric fields applied to plasmas present in flames create steady flows of gas. Ions generated within the flame move in the field and migrate a distance δ before recombining; the net flow of ions away from the flame creates a time-averaged force that drives the steady flows observed experimentally. A quantitative model describes the response of the flame and reveals how δ decreases as the frequency of the applied field increases. Interestingly, above a critical frequency, ac fields can be used to manipulate flames at a distance without the need for proximal electrodes.