Fluid Slip and Drag Reduction on Liquid-Infused Surfaces under High Static Pressure

Liquid-infused surfaces (LIS) have been shown to reduce the huge frictional drag affecting microfluidic flow and are expected to be more robust than superhydrophobic surfaces when exposed to external pressure as the lubricant in LIS is incompressible. Here, we investigate the effect of applying static pressure on the effective slip length measured on Teflon wrinkled surfaces infused with silicone oil through pressure measurements in microfluidic devices. The effect of static pressure on LIS was found to depend on air content in the flowing water: for degassed water, the average effective slip length was beff = 2.16 ± 0.90 μm, irrespective of applied pressure. In gassed water, the average effective slip length was beff = 4.32 ± 1.06 μm at zero applied pressure, decreased by 55% to 2.37 ± 0.90 μm when the pressure was increased to 50 kPa, and then remained constant up to 200 kPa. The result is due to nanobubbles present on LIS, which are compressed or partially dissolved under pressure, and the effect is more evident when the size and portion of surface nanobubbles are higher. In contrast, on superhydrophobic wrinkles, the decline in beff was more sensitive to applied pressure, with beff = 6.8 ± 1.4 μm at 0 kPa and, on average, beff = −1 ± 3 μm for pressures higher than 50 kPa for both gassed and degassed water. Large fluctuations in the experimental measurements were observed on superhydrophobic wrinkles, suggesting the nucleation of large bubbles on the surface. The same pressure increase did not affect the flow on smooth substrates, on which gas nanobubbles were not observed. Contrary to expectations, we observed that drag reduction in LIS is affected by applied pressure, which we conclude is because, in a similar manner to superhydrophobic surfaces, they lose the interfacial gas, which lubricates the flow

Receding Contact Line Motion on Nanopatterned and Micropatterned Polymer Surfaces

Surface properties such as topography and chemistry affect the motion of the three-phase contact line (solid/liquid/air), which in turn affects the contact angle of a liquid moving on a solid surface. In this work, the motion of the receding water contact line was studied on chemically and topographically patterned surfaces obtained from the dewetting of thin polymer films. The patterned surfaces consisted of hydrophilic poly­(4-vinyl­pyridine) (P4VP) bumps, which were either microsized and sparse or nanosized and dense, on top of a hydrophobic polystyrene (PS) background layer. These patterns are designed for atmospheric water capture, for which the easy roll off of water droplets is crucial to their efficient performance. The dynamic receding water contact angle and contact line height of the patterned surfaces were measured by vertically withdrawing the surfaces from a water bath and compared to those of a flat P4VP substrate. For both the micropatterned and nanopatterned surfaces, the height of the dynamic contact lines normalized by the capillary length was characterized by the equilibrium limit that was predicted from static states. The nanopatterned surface had a faster increase in the normalized height as the capillary number increased. The dynamic receding contact angles on all surfaces studied decreased with increasing withdrawing velocity. Surprisingly, even for these patterned surfaces with high hysteresis, the dynamic receding contact angle followed the Cox–Voinov relation at capillary numbers of between 1 × 10^–5 and 5 × 10^–5

Tunable Nanopatterns via the Constrained Dewetting of Polymer Brushes

Coarse-grained molecular dynamics simulations were used to investigate the morphology and dynamics of nanopatterns formed by grafted polymer brushes on a nonadsorbing substrate as a result of constrained dewetting. As a good solvent is made to gradually evaporate, polymer brushes with low to moderate grafting density collapse into discrete nanosized aggregates, with different types of nanopatterns possible, including pancake micelles and holey layers. The type of pattern, the size and number of features, and their dynamics depend on the grafting density of the polymer brush and amount of good solvent adsorbed. The final pattern morphology depends primarily on the total amount of material adsorbed to the surface, including both polymer and solvent. This result suggests the possibility for the use of polymer brushes as surfaces with reversibly tunable nanopatterns

Tunable Nanopatterns via the Constrained Dewetting of Polymer Brushes

Coarse-grained molecular dynamics simulations were used to investigate the morphology and dynamics of nanopatterns formed by grafted polymer brushes on a nonadsorbing substrate as a result of constrained dewetting. As a good solvent is made to gradually evaporate, polymer brushes with low to moderate grafting density collapse into discrete nanosized aggregates, with different types of nanopatterns possible, including pancake micelles and holey layers. The type of pattern, the size and number of features, and their dynamics depend on the grafting density of the polymer brush and amount of good solvent adsorbed. The final pattern morphology depends primarily on the total amount of material adsorbed to the surface, including both polymer and solvent. This result suggests the possibility for the use of polymer brushes as surfaces with reversibly tunable nanopatterns

Reliable Measurements of Interfacial Slip by Colloid Probe Atomic Force Microscopy. III. Shear-Rate-Dependent Slip

We present experimental evidence and theoretical models that demonstrate that the slip length, which is the departure from the hydrodynamic no-slip boundary condition, cannot be constant as commonly assumed, but must decrease with increasing shear rate to avoid an unphysical divergence in the velocity of the fluid adjacent to the surface at small separations. The molecular origin of the shear rate dependence of the slip length is discussed. A new theoretical model for slip (the saturation model) is obtained, and it is shown to describe accurately colloid probe atomic force microscopy force measurements for all separations down to a few nanometers in two partially wetting situations (di-<i>n</i>-octyl phthalate on silanized silicon and bare silicon). Previous observations of slip length increasing with shear rate are explained as due to an imprecise calculation of the drag force on the cantilever. A new way of plotting experimental data is also presented, which provides a useful way to illustrate the slip length dependence on the shear rate

Tunable Nanopatterns via the Constrained Dewetting of Polymer Brushes

Coarse-grained molecular dynamics simulations were used to investigate the morphology and dynamics of nanopatterns formed by grafted polymer brushes on a nonadsorbing substrate as a result of constrained dewetting. As a good solvent is made to gradually evaporate, polymer brushes with low to moderate grafting density collapse into discrete nanosized aggregates, with different types of nanopatterns possible, including pancake micelles and holey layers. The type of pattern, the size and number of features, and their dynamics depend on the grafting density of the polymer brush and amount of good solvent adsorbed. The final pattern morphology depends primarily on the total amount of material adsorbed to the surface, including both polymer and solvent. This result suggests the possibility for the use of polymer brushes as surfaces with reversibly tunable nanopatterns

Tunable Nanopatterns via the Constrained Dewetting of Polymer Brushes

Coarse-grained molecular dynamics simulations were used to investigate the morphology and dynamics of nanopatterns formed by grafted polymer brushes on a nonadsorbing substrate as a result of constrained dewetting. As a good solvent is made to gradually evaporate, polymer brushes with low to moderate grafting density collapse into discrete nanosized aggregates, with different types of nanopatterns possible, including pancake micelles and holey layers. The type of pattern, the size and number of features, and their dynamics depend on the grafting density of the polymer brush and amount of good solvent adsorbed. The final pattern morphology depends primarily on the total amount of material adsorbed to the surface, including both polymer and solvent. This result suggests the possibility for the use of polymer brushes as surfaces with reversibly tunable nanopatterns

Tunable Nanopatterns via the Constrained Dewetting of Polymer Brushes

Coarse-grained molecular dynamics simulations were used to investigate the morphology and dynamics of nanopatterns formed by grafted polymer brushes on a nonadsorbing substrate as a result of constrained dewetting. As a good solvent is made to gradually evaporate, polymer brushes with low to moderate grafting density collapse into discrete nanosized aggregates, with different types of nanopatterns possible, including pancake micelles and holey layers. The type of pattern, the size and number of features, and their dynamics depend on the grafting density of the polymer brush and amount of good solvent adsorbed. The final pattern morphology depends primarily on the total amount of material adsorbed to the surface, including both polymer and solvent. This result suggests the possibility for the use of polymer brushes as surfaces with reversibly tunable nanopatterns

Tunable Nanopatterns via the Constrained Dewetting of Polymer Brushes

Coarse-grained molecular dynamics simulations were used to investigate the morphology and dynamics of nanopatterns formed by grafted polymer brushes on a nonadsorbing substrate as a result of constrained dewetting. As a good solvent is made to gradually evaporate, polymer brushes with low to moderate grafting density collapse into discrete nanosized aggregates, with different types of nanopatterns possible, including pancake micelles and holey layers. The type of pattern, the size and number of features, and their dynamics depend on the grafting density of the polymer brush and amount of good solvent adsorbed. The final pattern morphology depends primarily on the total amount of material adsorbed to the surface, including both polymer and solvent. This result suggests the possibility for the use of polymer brushes as surfaces with reversibly tunable nanopatterns

Tunable Nanopatterns via the Constrained Dewetting of Polymer Brushes

Coarse-grained molecular dynamics simulations were used to investigate the morphology and dynamics of nanopatterns formed by grafted polymer brushes on a nonadsorbing substrate as a result of constrained dewetting. As a good solvent is made to gradually evaporate, polymer brushes with low to moderate grafting density collapse into discrete nanosized aggregates, with different types of nanopatterns possible, including pancake micelles and holey layers. The type of pattern, the size and number of features, and their dynamics depend on the grafting density of the polymer brush and amount of good solvent adsorbed. The final pattern morphology depends primarily on the total amount of material adsorbed to the surface, including both polymer and solvent. This result suggests the possibility for the use of polymer brushes as surfaces with reversibly tunable nanopatterns