Apparent hydrodynamic thickness of densely grafted polymer layers in a theta solvent

Apparent Slip of Newtonian Fluids Past Adsorbed Polymer Layers

“…For example, at D ) 8 nm (bottom panel), the hydrodynamic force was 10 times less than predicted by eq 2. Curvature in empirical data of this sort has been reported previously by several research groups including this one [26][27][28][29]34,37 but without, to the best of our knowledge, this quantitative analysis of its implications. Independent of subsequent analysis, it is worth emphasizing the extreme fluidity that is summarized in these data.…”

“…There is a standard textbook interpretation: the polymer layer renormalizes the location of the solid surface to a new location, the actual solid surface plus the “hydrodynamic radius” of surface-tethered polymer, and fluid on the surface of the renormalized moving body moves at the same rate as that body. − (Another limiting case, the near-field situation, occurs if the polymer-laden surfaces are so close together that the chain conformations overlap. Then the flow of solvent through these chains is perhaps akin to the flow of solvent through a porous medium, , albeit one whose structure adjusts to flow. − ,, ) Here, in studying the far-field situation, we show that this picture appears to break down when the flow rate (and therefore the shear stress on the chains at the moving surface) exceeds a critical level. The surprising result emerges that viscous dissipation of these Newtonian liquid solvents actually becomes less than for their flow past rigid surfaces in the absence of adsorbed polymer.…”

“…To measure hydrodynamic forces over a wide range of velocity, but at given surface−surface separations, we employed methods of piezoelectric actuation in a modified surface forces apparatus described elsewhere. ,, Multiple beam interferometry between silver sheets on the backside of cleaved single crystals of mica was used to measure the surface separation. After placing surfaces at a given spacing, they were pumped in the normal direction with small-amplitude oscillatory modulations of spacing, so that the peak velocity varied while keeping the spacing nearly constant.…”

The role of copolymer composition on the specific ion and thermo-response of ethylene glycol-based brushes

“…However, these models break down at separations encompassing the maximum extent of the brush due to additional contributions from compression of the brush by both the probe and the solvent flow. 56 Note, in these studies interaction force increased linearly with approach velocity at small separations. Attempts to fit a slip model to the data in Figure 4b were unsuccessful due to the presence of bridging interactions in the data.…”

No-Slip Boundary Condition Switches to Partial Slip When Fluid Contains Surfactant

“…This suggests an explanation of the success of “friction modifiers” in engine oils and gasoline , and points to a versatile new strategy for diminishing energy losses when fluid flows past a solid surface, provided that it is sufficiently smooth.…”