Size-dependent control of colloid transport via solute gradients in dead-end channels

Abstract

Transport of colloids in dead-end channels is involved in widespread applications including drug delivery and underground oil and gas recovery. In such geometries, Brownian motion may be considered as the sole mechanism that enables transport of colloidal particles into or out of the channels, but it is, unfortunately, an extremely inefficient transport mechanism for microscale particles. Here, we explore the possibility of diffusiophoresis as a means to control the colloid transport in dead-end channels by introducing a solute gradient. We demonstrate that the transport of colloidal particles into the dead-end channels can be either enhanced or completely prevented via diffusiophoresis. In addition, we show that size-dependent diffusiophoretic transport of particles can be achieved by considering a finite Debye layer thickness effect, which is commonly ignored. A combination of diffusiophoresis and Brownian motion leads to a strong size-dependent focusing effect such that the larger particles tend to concentrate more and reside deeper in the channel. Our findings have implications for all manners of controlled release processes, especially for site-specific delivery systems where localized targeting of particles with minimal dispersion to the nontarget area is essential.T he ability of a particle to migrate along a local solute concentration gradient, which is referred to as diffusiophoresis, has been exploited to direct transport in a variety of systems, e.g., artificial swimmers (1, 2) and collective behaviors of active colloids (3, 4). One physical mechanism for diffusiophoresis originates from surface-solute interactions, where the solute gradient sets up an osmotic pressure gradient within a narrow interaction region. This gradient leads to fluid flow along the surface of a particle, in which case propulsion occurs in the opposite direction and is referred to as chemiphoresis (5, 6). In addition, differences in diffusivities between anions and cations lead to spontaneous electrophoresis of a particle, giving an additional propulsion mechanism. A particular feature of diffusiophoresis is that the diffusiophoretic mobility, or the phoretic velocity, of a particle is independent of its size, as long as the thickness of the interaction region, e.g., the Debye screening layer when the interaction is electrostatic, is much thinner than the size of the particle (6). This feature allows the utilization of diffusiophoresis for enhancing transport of microscale particles, leading to orders of magnitude higher transport rates compared with pure diffusion (7). However, this size independence could also be a source of frustration because it precludes useful effects such as sorting or controlling transport by particle size.We anticipate that size-independent particle mobility breaks down when the thickness of the surface-solute interaction region becomes comparable to the size of the particle. Already more than a century ago this feature has been well appreciated in the field of electrokinetics as the Hückel limit (8) w...

Accumulation of Colloidal Particles in Flow Junctions Induced by Fluid Flow and Diffusiophoresis

“…Numerical simulations.-Because of the presence of three-dimensional effects including diffusioosmotic wall slip velocities and non-negligible secondary fluid advection velocities, a full 3D numerical solver was developed that couples the solution of the Navier-Stokes equations with the diffusioosmotic wall slip velocity, as well as the solute and particle advection-diffusion equations [12,17]. The diffusioosmotic wall slip condition is a function of the wall surface charge and local solute concentrations, and the particle diffusiophoretic velocity contribution is also a function of the local solute concentration.…”

“…Diffusiophoresis, which has received recent interest because of experimental and theoretical advances [7][8][9][10][11][12][13][14][15][16][17], refers to the motion of colloidal particles induced by the solute gradients. The local solute gradient gives rise to the particle motion due to the osmotic pressure gradient developed along the particle surface (chemiphoresis) and the liquid junction potential generated by the diffusion of ions with different diffusivities (electrophoresis).…”

“…More detailed insights into the diffusiophoretic particle dynamics can be obtained by performing full 3D numerical simulations of the Navier-Stokes equations coupled with the solute and particle advection-diffusion equations and applying the diffusioosmotic wall slip flow conditions (see Materials and Methods for more details) [12,17]. Computed particle trajectories near the pore entrance are shown on a slice through the pore center plane in Fig.…”

“…The trajectories approach an approximately fixed depth in the pore where advection velocities balance the diffusiophoretic contribution. Because of the circulating flow that diverges from the center toward the wall, which is induced by the diffusioosmotic flow along the charged channel wall [12,17], the particles are attracted toward the channel wall, leading to two distinct focusing locations. However, the crossflow brings asymmetry to the particle motion, and the particles are more likely to focus on the downstream side of the wall (right-hand side).…”

Micro-plumes for nano-velocimetry

“…Gradients arise for many reasons (Velegol et al 2016), implying that diffusiophoresis occurs in a wide range of micro-and nano-fluidic processes (e.g. Florea et al 2014;Banerjee et al 2016;Shin et al 2016). Because diffusiophoresis often scales with the logarithmic derivative of concentration, the exponential angular dependence of high-Pe Landau Squire plumes suggests diffusiophoresis will remain strong but localized around the jet.…”