Superspreading: Mechanisms and Molecular Design

Abstract

The intriguing ability of certain surfactant molecules to drive the superspreading of liquids to complete wetting on hydrophobic substrates is central to numerous applications that range from coating flow technology to enhanced oil recovery. Despite significant experimental efforts, the precise mechanisms underlying superspreading remain unknown to date. Here, we isolate these mechanisms by analyzing coarse-grained molecular dynamics simulations of surfactant molecules of varying molecular architecture and substrate affinity. We observe that for superspreading to occur, two key conditions must be simultaneously satisfied: the adsorption of surfactants from the liquid-vapor surface onto the three-phase contact line augmented by local bilayer formation. Crucially, this must be coordinated with the rapid replenishment of liquid-vapor and solid-liquid interfaces with surfactants from the interior of the droplet. This article also highlights and explores the differences between superspreading and conventional surfactants, paving the way for the design of molecular architectures tailored specifically for applications that rely on the control of wetting.

The effect of adsorption kinetics on the rate of surfactant-enhanced spreading

“…Therefore, there must be additional mechanisms facilitating the fast spreading of surfactant solutions. The most probable candidates are: the 'caterpillar-like' motion at the moving three-phase contact line, providing lower energy dissipation [10]; bilayer formation at the leading edge of spreading [11,12]; and Marangoni flow [13,14]. The comprehensive discussion on all these mechanisms is given in Ref [15].…”

SAFT-γ force field for the simulation of molecular fluids: 8. Hetero-segmented coarse-grained models of perfluoroalkylalkanes assessed with new vapour–liquid interfacial tension data

“…The SAFT-γ Mie EoS is used here to inform CG force-field parameters representing the interactions between the chemical groups characteristic of PFAA molecules that can then be used as input in direct molecular simulation. The SAFT-γ Mie methodology has already been applied to develop an efficient parametrization of CG force fields over a broad range of conditions for a variety of molecular fluids, including carbon dioxide [45][46][47], other greenhouse gases and refrigerants [48], aromatic compounds [49], water and aqueous mixtures [50][51][52][53], n-alkanes [48,54], and amphiphilic systems comprising nonionic [55], light-switching [56], and super spreading surfactants [57]. The procedure for the determination of the molecular parameters can be further simplified with a corresponding states treatment [58].…”

Smoothing of contact lines in spreading droplets by trisiloxane surfactants and its relevance for superspreading

“…31 Finally, in a recent MD study using a coarse-grained potential, formation of precursor bilayers was observed for the spreading of aqueous solutions of trisiloxane surfactants. 32 The coarse-grained model's capability of representing surfactant-water mixtures are demonstrated in the ESI of ref. 32.…”