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

Abstract

ab Superspreading, the greatly enhanced spreading of aqueous solutions of trisiloxane surfactants on hydrophobic substrates, is of great interest in fundamental physics and technical applications. Despite numerous studies in the last 20 years, the superspreading mechanism is still not well understood, largely because the molecular scale cannot be resolved appropriately either experimentally or using continuum simulations. The absence of molecular-scale knowledge has led to a series of conflicting hypotheses based on different assumptions of surfactant behavior. We report a series of large-scale molecular dynamics simulations of aqueous solutions of superspreading and non-superspreading surfactants on different substrates. We find that the transition from the liquid-vapor to the solid-liquid interface is smooth for superspreading conditions, allowing direct adsorption through the contact line. This finding complements a study [Karapetsas et al., J. Fluid Mech., 2011, 670, 5-37], which predicts that superspreading can occur if this adsorption path is possible. Based on the observed mechanism, we provide plausible explanations for the influence of the substrate hydrophobicity, the surfactant chain length, and the surfactant concentration on the superspreading phenomenon. We also briefly address that the observed droplet shape is a mechanism to overcome the Huh-Scriven paradox of infinite viscous dissipation at the contact line.

Spreading of aqueous droplets with common and superspreading surfactants. A molecular dynamics study

“…Moreover, simulation studies have thus far only considered a limited selection of superspreading and common surfactants and a broader selection of surfactants would provide more information towards identifying similarities and differences between surfactant behaviour.The study of spreading phenomena by computer simulation is well justified, given the availability of reliable all-atom [23-26] and coarse-grained (CG) models [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] that enable the faithful simulation of these systems. Indeed, simulations of aqueous solutions with surfactants [13,22,[45][46][47][48][49][50][51] have established the connection between the behaviour of surfactants in the bulk and…”

Molecular Dynamics Simulation of the Superspreading of Surfactant-Laden Droplets. A Review

“…The above models are based on the direct use of the force-field parameters without any refinement of the parameters to simulate water-surfactant systems. In a more recent study by all-atom models, Isele-Holder et al [21,22] have obtained atomistic potentials for trisiloxane, alkyl ethoxylate and perfluoroalkane-based surfactants that are compatible with the TIP4P/2005 model for water [82]. The surface tension of the systems was in agreement with experiment [83].…”

“…Nowadays, the existence of reliable force-fields for water-surfactant systems based on all-atom [39][40][41][42] and coarse-grained (CG) models [43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60] enable the genuine simulation of such systems. For example, recent simulations of aqueous solutions with surfactants [21,22,[61][62][63][64][65][66][67] have established the connection between the behaviour of surfactants in the bulk and spreading [68][69][70][71], while the superspreading mechanism and the main characteristics of superspreading surfactants have been the focus of recent studies [5,[19][20][21][22][23][24]68,[70][71][72]. In view of these important advances in the superspreading arena, this review will highlight what we believe a...…”

“…The model of Isele-Holder et al [21,22] has focused on identifying key characteristics of the trisiloxane surfactantants. The authors have conducted a very careful refinement of the model parameters that led to an accurate force-field for water-surfactant systems capturing specific features pertaining to the studied molecules [93].…”

“…Superspreading is important for many applications in a wide range of different areas, such as coating technology, drug and herbicides delivery, and enhanced oil recovery [3,[7][8][9][10], manifested as enhanced fluid flow and efficient spreading. In view of the large spectrum of applications, superspreading has motivated a wide range of experimental [11][12][13][14][15][16][17][18], as well as numerical and simulation research in the recent years [19][20][21][22][23][24][25][26]. The main focus of these studies has been the underlying mechanisms of this phenomenon and identifying key features that distinguish superspreaders from other common (or conventional) surfactants.…”