Edison Amah

Molecular-like hierarchical self-assembly of monolayers of mixtures of particles

We present a technique that uses an externally applied electric field to self-assemble monolayers of mixtures of particles into molecular-like hierarchical arrangements on fluid-liquid interfaces. The arrangements consist of composite particles (analogous to molecules) which are arranged in a pattern. The structure of a composite particle depends on factors such as the relative sizes of the particles and their polarizabilities, and the electric field intensity. If the particles sizes differ by a factor of two or more, the composite particle has a larger particle at its core and several smaller particles form a ring around it. The number of particles in the ring and the spacing between the composite particles depend on their polarizabilities and the electric field intensity. Approximately same sized particles form chains (analogous to polymeric molecules) in which positively and negatively polarized particles alternate.

Self-Assembly and Manipulation of Particles on Drop Surfaces

It was recently shown by us that particles distributed on the surface of a drop can be concentrated at the poles or equator of the drop by subjecting it to a uniform ac electric field. The dielectrophoretic and hydrodynamic forces which cause the motion of particles depend on the parameters such as the particles’ and drop’s radii, the dielectric properties of the fluids and particles, and the frequency of the electric field. The hydrodynamic force, which arises because of the induced motion in the liquids, is the main focus of this paper. We show that it can be used to control the distribution of particle monolayers on the surface of a drop.© 2014 ASME

Direct Numerical Simulations (DNS) of Particles in Spatially Varying Electric Fields

A numerical scheme is developed to simulate the motion of dielectric particles in uniform and nonuniform electric fields of a micro fluidic device. The particles are moved using a direct simulation scheme in which the fundamental equations of motion of fluid and solid particles are solved without the use of models. The motion of particles is tracked 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. One of the key features of the DLM method is that the fluid-particle system is treated implicitly by using a combined weak formulation where the forces and moments between the particles and fluid cancel, as they are internal to the combined system. The MST is obtained from the electric potential, which, in turn, is obtained by solving the electrostatic problem. In our numerical scheme 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. A comparison of the DNS results with those from the point-dipole approximation shows that the accuracy of the latter diminishes when the distance between the particles becomes comparable to the particle diameter; the domain size is comparable to the diameter; and also when the dielectric mismatch between the fluid and particles is relatively large.Copyright