Depletion at solid/liquid interfaces: Flowing hexadecane on functionalized surfaces

Abstract

We present a neutron reflectivity study on interfaces in contact with flowing hexadecane, which is known to exhibit surface slip on functionalized solid surfaces. The single crystalline silicon substrates were either chemically cleaned Si(100) or Si(100) coated by octadecyl-trichlorosilane (OTS), which resulted in different interfacial energies. The liquid was sheared in situ and changes in reflectivity profiles were compared to the static case. For the OTS surface, the temperature dependence was also recorded. For both types of interfaces, density depletion of the liquid at the interface was observed. In the case of the bare Si substrate, shear load altered the structure of the depletion layer, whereas for the OTS covered surface no effect of shear was observed. Possible links between the depletion layer and surface slip are discussed. The results show that, in contrast to water, for hexadecane the enhancement of the depletion layer with temperature and interfacial energy reduces the amount of slip. Thus a density depletion cannot be the origin of surface slip in this system.

Solid surface structure affects liquid order at the polystyrene–self-assembled-monolayer interface

“…Both facts appear to be clues to understanding (a) substantially different polymer slippage on silanized surfaces exhibiting identical surface energies and polystyrene contact angles and (b) large effective (comprising real and apparent) slip, both observed experimentally. Our findings corroborate ongoing research claiming conformational changes at the interface in the case of entangled polymeric liquids [20] and the interfacial liquid structure in the case of nonentangled oligomers as important parameters governing macroscopic slip [17,42]. Additionally, our results might also shed light on further interfacial phenomena such as depletion layers or glass-transition temperatures of thin polymer films.…”

“…Depletion effects of simple liquids have been observed in various cases [14][15][16][17] using x-ray reflectometry (XRR) and neutron reflectometry (NR). Their origin and the consequential link to macroscopic properties of liquids on solid surfaces such as hydrophobicity and also slippage are currently under debate [16][17][18].…”

Fluid Velocity Slip and Temperature Jump at a Solid Surface

“…The presence of a less viscous layer sandwiched between the surface and bulk flow has also been suggested as a possible cause of the anomalously high slip lengths observed in experiments [31]. Possible film types may constitute dissolved gases, coating of nanobubbles, or a densitydepletion layer, the last of which has recently been disputed [32]. Though the above forms of apparent slip do not arise from the true motion of liquid molecules relative to the surface, they may be exploited as the artificial approaches of inducing low interfacial friction.…”

The Search for Nanobubbles by Using Specular and Off-Specular Neutron Reflectometry

“…and demonstrated for other liquids at hydrophobic surfaces. [10][11][12][13] The situation becomes increasingly complicated if water supersaturated with gas is used as the exceeding gas may condense in the form of a nanoscale layer at a hydrophobic interface. 14 These so-called nanobubbles were initially assumed to be the cause of the hydrophobic gap 5 and motivated further reflectometry studies to probe the influence of different gas enrichments on the water depletion yielding controversial results.…”