Liquid–Solid Interfaces under Dynamic Shear Flow: Recent Insights into the Interfacial Slip

Abstract

The development of micro/nanofluidic techniques has recently revived interest in dynamic shear flow at liquid−solid interfaces. When the nature of the liquid−solid boundaries was revisited, the slip of the fluids relative to the solid wall was predicted theoretically and confirmed experimentally. This indicates that the molecular-level structures of the liquid−solid interfaces will be influenced by the liquid flow over certain temporal and spatial criteria. However, the fluid flow at the boundary layer still cannot be precisely predicted and effectively controlled, somehow limiting its practical applications. Here, we summarize the recent advances for the microscopic structures at the liquid−solid interfaces upon shear flow. Special attention was given to a second-order nonlinear optical technique, sum frequency generation vibrational spectroscopy, which is a powerful tool for exploring the molecular-level structures and structural dynamics at the liquid−solid interfaces and offering new insights into the molecular mechanisms of the fluid slip at the interfaces. Moreover, we discuss the possible approaches for controlling the interfacial slip at the molecular level and highlight the current challenges and opportunities. Although the theoretical framework of the slip at the liquid−solid interfaces is still incomplete, we hope that this Perspective will complement and enhance our understanding of various interfacial properties and phenomena with respect to practical non-equilibrium dynamic processes happening at the interfaces.

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“…In a recent study, we have examined the behavior of such an amphiphilic copolymer at the relevant solid/liquid interface using sum frequency generation (SFG) vibrational spectroscopy. SFG is a second-order nonlinear spectroscopic technique that can provide vibrational spectra of surfaces and interfaces with submonolayer specificity, which enables detailed characterization of the molecular structures of buried interfaces in situ and in real time. − SFG experiments showed that poly­(MMA-co-MPEG350MA) adsorbs effectively on poly­(ethylene terephthalate) (PET) surfaces with deuterated aliphatic chains (d4-PET; see Figure S1 in the SI for the molecular structure of the substrate) from aqueous solutions at very low copolymer concentrations. PET was used as a model for synthetic cloth textiles, while deuteration was necessary to avoid spectral confusion in the SFG data.…”