Droplet Transport in a Nanochannel Coated by Hydrophobic Semiflexible Polymer Brushes: The Effect of Chain Stiffness

Abstract

We study the influence of chain stiffness on droplet flow in a nanochannel, coated with semiflexible hydrophobic polymers by means of nonequilibrium molecular dynamics simulations. The studied system is then a moving droplet in the slit channel, coexisting with its vapor and subjected to periodic boundary conditions in the flow direction. The polymer chains, grafted by the terminal bead to the confining walls, are described by a coarse-grained model that accounts for chain connectivity, excluded volume interactions and local chain stiffness. The rheological, frictional and dynamical properties of the brush are explored over a wide range of persistence lengths. We find a rich behavior of polymer conformations and concomitant changes in the friction properties over the wide range of studied polymer stiffnesses. A rapid decrease in the droplet velocity was observed as the rigidity of the chains is increased for polymers whose persistence length is smaller than their contour length. We find a strong relation between the internal dynamics of the brush and the droplet transport properties, which could be used to tailor flow properties by surface functionalization. The monomers of the brush layer, under the droplet, present a collective "treadmill belt" like dynamics which can only be present due the existence of grafted chains. We describe its changes in spatial extension upon variations of polymer stiffness, with bidimensional velocity and density profiles. The deformation of the polymer brushes due to the presence of the droplet is analyzed in detail. Lastly, the droplet-gas interaction is studied by varying the liquid to gas ratio, observing a 16% speed increase for droplets that flow close to each other, compared to a train of droplets that present a large gap between consecutive droplets.

Pressure responsive gating in nanochannels coated by semiflexible polymer brushes

“…The grafting density in this subsection is fixed at r g = 0.02s À2 , and solvent-monomer interaction parameter was set to e ms /e = 1/3, which is a well studied set of parameters. 24,25 Firstly, we note that for fully flexible polymer brushes (l p /l c = 0.1) the flow rate increases linearly with the driving force, as expected in a Poiseuille flow (black curve in Fig. 4).…”

“…A square lattice arrangement of the grafting sites was chosen over a random distribution, as used in other works. 14,25,36 This choice allows to avoid inhomogeneities that could cause spurious local effects on the system. The distance between nearest grafting sites (lattice constant, a l ) is related to the grafting density by r g = 1/a l 2 .…”

“…Previous studies show that, as the stiffness of the polymers increases, there is an associated change in the morphology of the brush, and the rheological properties are modified substantially. [24][25][26] Conformational changes in the polymer brush influence greatly the flow through nano-channels. 27,28 This suggests that the stiffness of the polymeric chains can have a considerable effect in the brush-solvent interplay in the gating process, which can lead to new and more efficient flow control mechanisms.…”

Supersolvophobic Soft Wetting: Nanoscale Elastocapillarity, Adhesion, and Retention of a Drop Behaving as a Nanoparticle

“…This is always the case for any MD simulation that considers a liquid-vapor interface. 51,57,58,60,61 Elastocapillary Statics As the liquid drop moves on the PBL, we track the instantaneous deformation of the PBL (as evidenced by Figure 1). For a PBL of a given stiffness, we average the deformation of the PBL at different time instants, i.e., we average the deformation of the PBL at different locations corresponding to the drop position at these locations at different time instants (a detailed description of this averaging procedure is provided in Supplemental Information).…”

“…We investigate through molecular dynamics (MD) simulations the unique drop behavior on a substrate that is simultaneously phobic and soft and represented as an SSP polymer brush layer (PBL) with controllable stiffness. Drop-PBL interactions have been studied previously; 45,46,51 although none of them have investigated the combined interplay of stiffness and repellency in dictating the drop behavior.…”