Ph.D. – Physical Chemistry, 2012
M.Sc. (Honours School)-Chemistry, 2009
Research Focus: Directive Assembly of Mixed Colloidal Dispersions
Colloidal particles with strongly attractive interactions snap on contact and form permanent, but disordered aggregates. In contrast, AC electric fields allow directional assembly of chains or crystals from repulsive particles by dielectrophoresis (DEP), but these structures fall apart once the field is switched off. Our goal is to demonstrate how well-organized, permanent clusters and chains of micron sized particles can be assembled by applying DEP to mixtures of oppositely charged microspheres. We found that the length of the formed chains depends on size ratio as well as the number ratio of the two species, and formulated a statistical model for this assembly mechanism, which is in excellent agreement with the experimental results. Permanent chains of polymer particles with adjustable lengths suspended in liquid medium can find application in novel types of biparticle gels and chain fluids exhibiting unusual directional rheology response. Liquid-borne, or permanently immobilized materials containing oriented chain clusters from inorganic or metal particles will exhibit anisotropy in heat and electrical conductance that may lead to potential uses in microdevice heat management and wiring.
Fundamentals of DEP assembly of biparticle dispersions
The two different sequences of events leading to biparticle chaining of equal-sized and different-sized latex microspheres are sketched in Figure 1. The size ratio of the particles matters because the long-range drag experienced by them in the electric field is strongly dependent on their volume. If the size of positive and negative particles is similar, all particles experience nearly the same DEP force and are dragged with equal probability into a given position in the chains. In contrast, when the sizes of the positive and negative particles are significantly different, a much greater DEP force is exerted on the large particles than on the small ones during the field-induced chaining due to the large difference in particle volume. Accordingly, switching off the external AC-field will give different permanent chain length distributions in the two cases.
Figure 1: Proposed steps of the biparticle chaining process. (Left) Sketch of the two-step chaining process of equal sized particles; (Right) Sketch of the three-step process leading to permanent chaining of unequal-sized particles (here green and red spheres represent the positively and negatively charged particles).
Bharti, B.; Fameau, A.-L.; Rubinstein, M.; Velev, O.D. Nanocapillarity-mediated Magnetic Assembly of Nanoparticles into Ultraflexible Filaments and Reconfigurable Networks. Nat. Mater. 2015. DOI: 10.1038/nmat4364.