A Langevin model for fluctuating contact angle behaviour parametrised using molecular dynamics

Smith, Edward R.; Müller, Erich A.; Matar, Omar

doi:10.1039/c6sm01980c

Cited by 21 publications

(40 citation statements)

References 48 publications

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“…Clearly, the formation of bilayer at the CL does not allow for a strict interpretation of our measurements and a more accurate way of measuring the contact angle at the CL, based on the droplet curvature [14], is clearly not applicable in this case. Moreover, we do not attempt here to describe the fluctuating contact angle behaviour, which has been considered in a previous study in detail [84].…”

Section: Resultsmentioning

confidence: 99%

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

Theodorakis

Smith

Müller

2019

Colloids and Surfaces A: Physicochemical and Engineering Aspect

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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...

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Section: Resultsmentioning

confidence: 99%

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

Theodorakis

Smith

Müller

2019

Colloids and Surfaces A: Physicochemical and Engineering Aspect

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“…Clearly, other simple periodic geometries [102] might be used instead of the one proposed here with similar success. In any case, the proposed model can then be exploited for systematic sensitivity analysis, employed for further upscaling [103] or to provide for an in silico platform for testing advanced theories of wetting [64,104]. More importantly, it can be employed as a test system to understand outstanding inconsistencies between theories and simulations (and even resolving contradictions amongst simulations results), as pointed out by Svoboda et al [105].…”

Section: Discussionmentioning

confidence: 99%

Surrogate Models for Studying the Wettability of Nanoscale Natural Rough Surfaces Using Molecular Dynamics

et al. 2020

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A molecular modeling methodology is presented to analyze the wetting behavior of natural surfaces exhibiting roughness at the nanoscale. Using atomic force microscopy, the surface topology of a Ketton carbonate is measured with a nanometer resolution, and a mapped model is constructed with the aid of coarse-grained beads. A surrogate model is presented in which surfaces are represented by two-dimensional sinusoidal functions defined by both an amplitude and a wavelength. The wetting of the reconstructed surface by a fluid, obtained through equilibrium molecular dynamics simulations, is compared to that observed by the different realizations of the surrogate model. A least-squares fitting method is implemented to identify the apparent static contact angle, and the droplet curvature, relative to the effective plane of the solid surface. The apparent contact angle and curvature of the droplet are then used as wetting metrics. The nanoscale contact angle is seen to vary significantly with the surface roughness. In the particular case studied, a variation of over 65° is observed between the contact angle on a flat surface and on a highly spiked (Cassie–Baxter) limit. This work proposes a strategy for systematically studying the influence of nanoscale topography and, eventually, chemical heterogeneity on the wettability of surfaces.

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“…6 (b). As each electrowetting case has a different equilibrium angle θ e , we consider the relative change in angle θ − θ e and, for the four different electrowetting numbers Fig 6 (b), we can collapse these onto a single curve by multiplying velocity with an arbitrary function of wall interaction wall , here off mechanism seen in the molecular system also bears a striking qualitative resemblance to the one observed experimentally 16,132 . This limitation on the range of stability of a sheared liquid bridge places a constraint on the range of contact line dynamics that can be explored.…”

Section: Dynamic Contact Anglesmentioning

confidence: 99%

“…(7) with the Langevin Eq. (20), wetting is incorporated into a solver for the continuum thin-film equations 5 as shown in Fig 7 (b) (see Karapetsas et al6 for implementation details of the thin-film solver and Smith et al31 for the (a) Wall sliding speeds for four wetting numbers collapsed onto a single line by scaling using5/2 wall . (b) CFD solver using the thin-film form of the equations (5) for four values of electrowetting number wall = {0.38, 0.46, 0.52, 0.69} (red, blue, yellow, and green lines, respectively) showing the evolution of angle θ and the contact angle velocity dX c /dt (insert) as a function of time t. tuning of the contact line model).…”

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confidence: 99%

Moving Contact Lines: Linking Molecular Dynamics and Continuum-Scale Modeling

et al. 2018

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Despite decades of research, the modeling of moving contact lines has remained a formidable challenge in fluid dynamics whose resolution will impact numerous industrial, biological, and daily life applications. On the one hand, molecular dynamics (MD) simulation has the ability to provide unique insight into the microscopic details that determine the dynamic behavior of the contact line, which is not possible with either continuum-scale simulations or experiments. On the other hand, continuum-based models provide a link to the macroscopic description of the system. In this Feature Article, we explore the complex range of physical factors, including the presence of surfactants, which governs the contact line motion through MD simulations. We also discuss links between continuum- and molecular-scale modeling and highlight the opportunities for future developments in this area.

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A Langevin model for fluctuating contact angle behaviour parametrised using molecular dynamics

Cited by 21 publications

References 48 publications

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

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

Surrogate Models for Studying the Wettability of Nanoscale Natural Rough Surfaces Using Molecular Dynamics

Moving Contact Lines: Linking Molecular Dynamics and Continuum-Scale Modeling

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