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

Abstract

The surfactant-driven spreading of droplets is an essential process in many applications ranging from coating flow technology to enhanced oil recovery. Despite the significant advancement in describing spreading processes in surfactantladen droplets, including the exciting phenomena of superspreading, many features of the underlying mechanisms require further understanding. Here, we have carried out molecular dynamics simulations of a coarse-grained model with force-field obtained from the Statistical Associating Fluid Theory to study droplets laden with common and superspreading surfactants. We have confirmed the important elements of the superspreading mechanism, i.e. the adsorption of surfactant at the contact line (CL) and the fast replenishment of surfactant from the bulk. Through a detailed comparison of a range of droplets with different surfactants, our analysis has indicated that the ability of surfactant to adsorb at the interfaces is the key feature of the superspreading mechanism. To this end, surfactants that tend to form aggregates and have a strong hydrophobic attraction in the aggregated cores prevent the fast replenishment of the interfaces, resulting in reduced spreading ability. We also show that various surfactant properties, such as diffusion and architecture, play a secondary role in the spreading process. Moreover, we discuss various drop properties such as the height, contact angle, and surfactant surface concentration highlighting dif-Preprint submitted to Colloids and surfaces A: Physicochemical and engineering aspectsSeptember 19, 2019 arXiv:1909.00775v1 [physics.flu-dyn] 2 Sep 2019ferences between superspreading and common surfactants. We anticipate that our study will provide further insight for applications requiring the control of wetting. IntroductionSuperspreading of surfactant-laden aqueous droplets is an exciting phenomenon, which has received a great deal of attention over the last six decades [1][2][3][4][5][6]. It refers to the unexpectedly rapid spreading of aqueous droplets on hydrophobic substrates, due to the presence of surfactant molecules known as superspreaders [7,8]. This phenomenon is of fundamental importance for diverse applications, such as coating technology, drug and herbicides delivery, and enhanced oil recovery [2,[9][10][11][12]. Although the first reports of superspreading date back to over 50 years ago [3], this phenomenon still attracts considerable attention from both theory and experiment [4, 5, 13-20]. While experimental [18-21] and theoretical [13-17, 22] studies have discussed possible mechanisms of the superspreading for surfactant-laden droplets, certain aspects of this phenomenon require further discussion. This includes the distribution of surfactant molecules within the droplet and the role of surfactant aggregation and diffusion in the spreading process. 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 tow...

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

“…It cannot be discounted that aggregation formation may be important as the simulations were not sufficient to analyse possible differences between aggregates [21]. The diffusion dynamics of the three different types of surfactant are similar in agreement with current results from CG simulation [26], which may indicate a lesser role of diffusion dynamics for superspreading. Still, the adsorption ability of the surfactant may play a role, as has been discussed in later studies [19,20].…”

“…Most importantly, it was also found that the T-shaped surfactants undergo weaker micellisation, which may result in faster spreading [71]. The latter result has been recently corroborated by MD simulations based on the SAFT CG force-field [26].…”

“…The SAFT CG force-field has been used for the study of water-surfactant systems [19,66,67,[101][102][103] and particularly in the context of superspreading (Figure 4) [5,19,20,26,72]. This CG model is derived from the SAFT-γ molecular-based equation of state (EoS), which can describe analytically experimental data [104,105].…”

“…ether) chemical groups, and 'CM' to −CH 2 − CH 2 − CH 2 − (alkane). By using these beads, one may model a wide range of different surfactants, which include both superspreading and nonsupersprading (common) surfactants ( Figure 4) [26]. The model also takes also into account the different masses of the chemical beads.…”

“…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.…”