Abstract
Phoresis, the drift of particles induced by scalar gradients in a flow, can result in an effective compressibility, bringing together or repelling particles from each other. Here, we ask whether this effect can affect the transport of particles in a turbulent flow. To this end, we study how the dispersion of a cloud of phoretic particles is modified when injected in the flow, together with a blob of scalar, whose effect is to transiently bring particles together, or push them away from the center of the blob. The resulting phoretic effect can be quantified by a single dimensionless number. Phenomenological considerations lead to simple predictions for the mean separation between particles, which are consistent with results of direct numerical simulations. Using the numerical results presented here, as well as those from previous studies, we discuss quantitatively the experimental consequences of this work and the possible impact of such phoretic mechanisms in natural systems.
Highlights
The transport of particles and macro-molecules in a flow can be strongly affected by a scalar quantity present in the fluid
Recent studies show that a significant enhancement of the transport properties can be achieved by addition of a small salt concentration to the solution, which induces a phoretic mobility in response to the salt gradients [4, 5]
The enhanced mobility of the colloids observed in microfluidic channel experiments with salt gradients [4, 5] was initially interpreted in terms of an effective diffusion aided by diffusiophoresis
Summary
The transport of particles and macro-molecules in a flow can be strongly affected by a scalar quantity present in the fluid. Electrophoresis and magnetophoresis [2, 3], induced by a drift proportional to the electric or magnetic field, respectively, provide two extra examples of phoresis with potential practical utility It is well-known that the diffusive transport of particles and of macro-molecules in a laminar flow is generally very slow because of their very small intrinsic molecular diffusion coefficient. The inferred diffusion coefficient is larger than what is expected from a simple Brownian motion of colloidal particles and for certain salts it is almost two orders of magnitude larger This makes diffusiophoresis an important concept with a practical utility, which can be used to selectively control the mobility of colloids or macro-molecules in microfluidic devices for scientific and/or industrial purposes. Similar arguments based on effective diffusivity were later used to explain the experimental results on enhanced or delayed mixing of colloids in chaotic flows [6]
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