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...

Silica nanoparticles (NPs) embedded in atactic polystyrene (PS) are simulated using coarse-grained (CG) potentials obtained via iterative Boltzmann inversion (IBI). The potentials are parametrized and validated on polystyrene of 2 kDa (i.e., chains containing 20 monomers). It is shown that the CG potentials are transferable between different systems. The structure of the polymer chains is strongly influenced by the NP. Layering, chain expansion, and preferential orientation of segments as well as of entire chains are found. The extent of the structural perturbation depends on the details of the system: bare NPs vs NPs grafted with PS chains, grafting density (0, 0.5, and 1 chains/nm 2 ), length of the grafted chains (2 and 8 kDa), and the matrix chains (2−20 kDa). For example, there is a change in the swelling state for the grafted corona (8 kDa, 1 chains/nm 2 ), when the matrix polymer is changed from 2 to > 8 kDa. This phenomenon, sometimes called "wet brush to dry brush transition", is in good agreement with neutron scattering investigations. Another example is the behavior of the radius of gyration of free polymer chains close to the NP. Short chains expand compared to the bulk, whereas chains whose unperturbed radius of gyration is larger than that of the NP contract.

Large scale atomistic molecular dynamics simulation for a nanoparticle in oligomeric poly(methyl methacrylate), composed of 20 repeating units, for a long time, up to 100 ns, are performed. Simulations are done for systems up to 87500 atoms, each containing a single bare or surface-grafted nanoparticle of various diameters and grafting densities. The effect of surface area, surface curvature, grafting density, and hydrogen bonding on the alteration of local structural and dynamical properties of the polymer is studied in details. Although atomistic simulations are only feasible for oligomeric chains in contact with surfaces, the results of the present simulation still discriminate the interphase thickness, defined in terms of local and global chain properties. In the case of structural properties, a minimum interphase thickness, ≈ 2 nm, is associated with local properties such as layering of individual polymer monomers. However, when probed in terms of global chain properties like the extension of chains from the interface to the polymer phase, a thicker interphase, three times the radius of gyration of the unperturbed chain (R g ≈ 1 nm), is observed. Our results on the chain structures are shown to be in good agreement with experiment where available. An examination of the dynamical properties shows that the surface influence on the polymer dynamics depends on the length and time scale of the corresponding bulk property. The change in time scales, in a 0.5 nm thick spherical shell, around the nanoparticle, is shown to cover a broad range from a few tens of a percent (for a short-time dynamical property, like the hydrogen bond formation) to 15–20 orders of magnitude (for a long-time dynamical property, such as the relaxation of end-to-end vector in grafted chains). Therefore, the influence and the range of surface effects on dynamical properties (interphase thickness) depend on the inherent time scale of those properties. In all cases, a thicker interphase is observed for global structural and long-time dynamical properties for chains in contact with a flatter and more densely grafted surface. Hydrogen-bond formation between polymer and surface decelerates the polymer dynamics. The effect is more pronounced at low temperatures.

. (2013) Reprinted with permission from the American Physical Society: Phys. Rev. Lett. 110, 028101 c (2013) by the American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modi ed, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society. Additional information:Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details.

Biological membranes are continuously brought out of equilibrium, as they shape organelles, package and transport cargo, or respond to external actions. Even the dynamics of plain lipid membranes in experimental model systems are very complex due to the tight interplay between the bilayer architecture, the shape dynamics, and the rearrangement of the lipid molecules. We formulate and numerically implement a continuum model of the shape dynamics and lipid hydrodynamics, which describes the bilayer by its midsurface and by a lipid density field for each monolayer. The viscoelastic response of bilayers is determined by the stretching and curvature elasticity, and by the inter-monolayer friction and the membrane interfacial shear viscosity. While the bilayer equilibria are well understood theoretically, dynamical calculations have relied on simplified continuum approaches of uncertain transferability, or on molecular simulations reaching very limited length and time scales. Our approach incorporates the main physics, is fully nonlinear, does not assume predefined shapes, and can access a wide range of time and length scales. We validate it with the well understood tether extension. We investigate the tubular lipid transport between cells, the dynamics of bud absorption by a planar membrane, and the fate of a localized lipid density asymmetry in vesicles. These axisymmetric examples bear biological relevance and highlight the diversity of dynamical regimes that bilayers can experience.