Left panel. The OTS molecules (shaded) are modeled as 17 carbon atoms alkyl chains each completed with the functional methyl group on top and with the hybrid LJ atom forming the head group. LJ hybrid atoms (dark gray) are arranged as a 111 plane of the fcc structure. One of the OTS chains is highlighted among a bunch of neighbors. In light gray, the 111 plane of the solid crystal, parallel to . In the initial configuration, the OTS molecules are normal to wall plane. Image created with VMD (Ref. 21). Right panel. Sketch of OTS orientation and nomenclature. The tilting angle is the angle between the -axis and the molecule axis, , oriented from the head group of the OTS (hybrid LJ atom) to the carbon atom of the functional group. The azimuth is the angle between the axis and the projection of the molecule on the -plane.
Snapshot of a simulation: a water drop lies on a OTS-SAM which coats the 111 surface of a LJ solid. The snapshot was taken after equilibrium is reached. A contact angle larger than is apparent. The system is formed by 625 OTS molecules, 2500 LJ atoms, and 4802 water molecules. The wall is parallel to -plane, the box dimensions are , and . Image created with VMD (Ref. 21).
Successive snapshots from a simulation of a water drop spreading over the LJ solid surface. The system is formed by 3125 LJ atoms and 4802 water molecules. As in Fig. 2, the wall is parallel to the -plane and the box dimensions are , , and . Image created with VMD (Ref. 21).
Snapshot of the Couette flow geometry with the OTS-SAM coating ( view). Water is confined between the two plane walls formed by five 111 planes of 324 LJ atoms each, the lower wall being coated with 324 OTS molecules. 85134 atoms are used, with box dimensions of . In the Couette flow simulations, after introducing a suitable void to isolate the basic computational cell from its neighbors in the wall normal direction, a constant force is applied tangentially to the upper plate to drive the wall. The lower wall is kept fixed.
Time evolution of the box dimensions normalized by their initial values. After a sharp change in the first few time steps (though hardly seen in the plot, the curves actually start at ), relatively smooth convergence to the equilibrium values is achieved. and substantially keep their initial values. instead decreases significantly.
Equilibrium density profile (no flow) in the nanochannel normalized with the bulk value . The nominal positions of the wall surfaces are shown by the two vertical lines. They are defined as average of the -coordinates of carbon atoms belonging to OTS functional groups as concerning the coated wall, and as average of the -coordinate of the LJ solid 111 plane atoms closest to the water for the opposite uncoated surface. The OTS nominal interface is at . Apparently, the water molecules distribution is different at the two walls, showing a larger peak at the LJ, hydrophilic wall.
Center of mass velocity ( component, in units of Å/ps) of the upper solid plate for case B of Table I.
Upper panel: Mean velocity profile (Å/ps) for case B (Table I). The dotted line provides the fit in the bulk of the nanochannel. The vertical dashed lines at and denote the positions of the two walls. The fitted velocity profile differs from zero at the nominal interface and vanishes 3.2 Å inside the coated wall (dot-dashed line). Bottom panel: Mean profile for simulation (Table I).
List of the simulations. The total purely tangential force applied to the upper plate (1620 LJ atoms) provided under the heading “forcing” is equally distributed on each atom of the plate. The 95% confidence interval on the slip length is of the order of 3 Å. Simulations A through E concern the SAM-coated interface, while F is a reference case with the uncoated (hydrophilic) surface.
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