Non-equilibrium Phenomena in Colloidal Suspensions: Charged colloids in electric fields and thermal gradients
Prof. Jan Dhont (Forschungszentrum Jülich)

On applying external fields to colloidal suspensions, new phases, dynamical states, and diffusive mass transport can be induced. In this presentation I will address (i) the response of concentrated suspensions of charged colloids to external electric fields, and (ii) the diffusive mass transport that is induced by thermal gradients.

(i) Concentrated suspensions of long and thin, stiff rods (fd-virus particles) at low ionic strengths are found to exhibit various non-equilibrium phases and dynamical states under external electric fields, depending on the field amplitude and frequency. A non-chiral nematic, a chiral nematic and a homeotropically aligned homogeneous phase are observed, as well as a dynamical state where nematic domains melt and form. The dynamical state has been observed to exhibit “non-equilibrium critical behaviour”, in the sense that there is a diverging length and time scale on approach of a transition line. These phases and the dynamical state are the result of interactions between polarized double layers, polarized layers of condensed ions and/or electro-osmotic flow. The phase/state diagram in the field-amplitude versus frequency plane is determined by means of polarization microscopy, and the nature of the various transition lines is characterized with birefringence, dynamic light scattering and video-correlation spectroscopy.

(ii) A thermal gradient induces diffusive mass transport, which phenomenon is commonly known as the Ludwig-Soret effect. I will discuss the mechanism through which mass transport is induced as a result of the charges carried by the colloidal particles. An extension of Poisson-Boltmann theory to include a thermal gradient will be discussed, on the basis of which the thermal diffusion coefficient can be calculated. The theoretical predictions will be compared with experimental data.