Session G24: Microscale Flows: Particle Electrokinetics

Liquid dielectrophoresis controlled detachment and re-attachment of bubbles

Air bubbles attached to surfaces in liquids can block flow in channels, amplify shear wall stress, and form dry patches in heat exchangers. Here we present a new method of controlling the wetting, detachment, and re-attachment of an air bubble in a dielectric liquid through an interface-localized form of liquid dielectrophoresis. We show the contact of the bubble with the surface obeys a dielectrowetting equation, analogous to electrowetting. Moreover, the bubble can be completely detached on demand from the surface with the application of a voltage to a patterned electrode on the surface. Once detached, bubbles are free to float beneath the surface and can be prevented from re-attaching to the surface by a thin liquid layer maintained by an applied voltage. The thickness of this liquid has a logarithmic dependence on the magnitude of the voltage, which we accurately describe by a theory that considers the energy balance due to liquid-dielectrophoresis and buoyancy of the liquid-bubble system. The critical re-attachment voltage is shown to depend on the material parameters of the liquid, and on the geometrical parameters of the electrode pattern.
  

Direct Numerical Simulation of Particles in Spatially Varying Electric Fields

A numerical scheme is developed to simulate and study the motion of dielectric particles in the uniform and nonuniform electric fields of microfluidic devices. The motion of particles is simulated using a distributed Lagrange multiplier method (DLM) and the electric force acting on the particles is calculated by integrating the Maxwell stress tensor (MST) over the particle surfaces. The MST is obtained from the electric potential, which, in turn, is obtained by solving the electrostatic problem. In our numerical scheme, the domain is discretized using a finite element scheme and the Marchuk-Yanenko operator-splitting technique is used to decouple the difficulties associated with the incompressibility constraint, the nonlinear convection term, the rigid-body motion constraint and the electric force term. The MST results show that the ratio of the particle-particle interaction and dielectrophoretic forces decreases with increasing particle size. Consequently, when the spacing between the electrodes is comparable to the particle size, instead of collecting on the same electrode by forming chains, they collect at different electrodes.   

Separating two attractive elongated particles by hydrodynamic interactions under alternating external field

In applications where magnetic particles are used to detect and dose targeted molecules, it is important to prevent particle clustering and aggregation. Elongated ferromagnetic particles can be more interesting than spherical ones due to their large magnetic moment, which facilitates their separation by magnets or the detection of their orientation relaxation time. Under alternating magnetic field, the rotational dynamics of elongated ferromagnetic particles results from the balance between magnetic torque that tends to align the particle axis with the field direction and viscous torque. As for their translational motion, it results from a competition between direct magnetic particle-particle interactions and solvent-flow-mediated hydrodynamic interactions. Due to particle anisotropy, this may lead to intricate translation-rotation couplings. Using numerical simulations and theoretical modeling of the system, we show that two ellipsoidal magnetic particles, initially in a head-to-tail attractive configuration resulting from their remnant magnetization, can repel each other due to hydrodynamic interactions when alternating field is operated. The separation takes place in a finite range of low frequencies, depending on the field and magnetization intensities.