Boosting migration of large particles by solute contrasts.

Brownian diffusion is a keystone concept in a large variety of domains, from physics, chemistry to biology. Diffusive transport controls situations as diverse as reaction-diffusion processes in biology and chemistry, Brownian ratchet processes, dispersion in microfluidic devices or even double-diffusive instability and salt-fingering phenomena in the context of ocean mixing. Although these examples span a broad range of length scales, diffusive transport becomes increasingly inefficient for larger particles. Applications, for example, in microfluidics, usually have recourse to alternative driving methods involving external sources to induce and control migration. Here, we demonstrate experimentally a strongly enhanced migration of large particles, achieved by slaving their dynamics to that of a fast carrier species, a dilute salt. The underlying fast salt diffusion leads to an apparent diffusive-like dynamics of the large particles, which is up to two orders of magnitude faster than their natural 'bare' diffusion. Moreover both spreading and focusing of the particle assembly can be achieved on demand. A model description shows a remarkable quantitative agreement with all measured data. Applications of this process are illustrated in microfluidics for filtering and concentrating operations, as well as in conjunction with standard hydrodynamic focusing. In a wider perspective, this mechanism can affect a broad range of scales and phenomena, from biological transport to the dispersion of sediments and pollutants in oceanographic situations.