Wetting transition of a nanodrop on switchable hydrophilic-hydrophobic surfaces
Introduction
The wetting properties of materials are largely dictated by the contact angle of the liquid with the solid surface and the solid-liquid surface tension (γSL) properties. These two factors are obviously dependent on the interfacial interactions between the solid framework and the liquid molecules. The main motivation towards wetting studies is to characterize the solid surfaces and making it relevant for practical applications. There are plenty of industrial, biological and manufacturing applications that are primarily based on phenomena of rapid wetting [1], [2]. On the other hand, there are applications that require poor wetting [3], [4], [5]. So the scientific community is mostly focused on the understanding of wetting behavior and designing materials or surfaces of desired engineering applications [6], [7]. In recent times a new domain has emerged based on nanotechnologies and nanosciences [8]. The wetting properties of nanodroplets in contact with various surfaces and the confinement or manipulation of liquids at the nanoscale [9], [10], [11] have several applications in nanotechnologies. Therefore, it is very obvious to have a clear understanding of the interfacial properties and the interactions between the liquid and the solid surface at the nanoscale.
Due to a lack of experimental methods for measuring the contact angle and various experimental challenges related to small length scale, molecular simulation is one of the viable options in predicting the preliminary insights of molecular level vision of liquids and their interfacial properties [12], [13], [14], [15]. The initial task for modeling a nanodroplet on a solid surface is to avoid the interactions between the replicated droplet images which are generated due to periodic boundary conditions. This is done by placing the water droplet at the center position of a huge solid surface.
The phenomena of wetting and dewetting process of a drop on solid surfaces is of practical relevance to various natural processes. Most of the systematic studies of wetting are typically based on the determination of properties at the equilibrium. But the practical relevance lies in the study of dynamics of wetting and dewetting behavior of the drop. The wettability of a drop on a solid surface is characterized by the calculation of the contact angle. For wetting and dewetting transitions, the contact angle varies with time with the movement of the three-phase (solid, liquid and vapor) contact line in a particular direction. The contact line motion of a drop is generally regulated by the frictional force operating between the liquid molecules and the solid surface. The origin of the frictional force due to the presence of viscous forces inside the moving liquid, which opposes the relaxation of the drop is known as hydrodynamic friction [16], [17]. In the hydrodynamic (HD) approach, the role of the solid surface is not taken into account. The other type is the molecular friction is defined as the resistance force operating between the moving liquid and the substrate [18], [19]. It is based on the molecular kinetic theory (MKT) and in this approach the microscopic detail of the solid surface is under consideration.
Thus, it is obvious that the nature of the surface solely responsible for changing the wettability of the drop. The nature of the surface can be modified by external stimuli. It would be a great advantage where the surface properties can be controlled, resulting in a controlled wetting and dewetting characteristic of the nanodroplet. In surface engineering, these types of surfaces can be termed as smart surfaces. In this paper, a wetting and dewetting characteristics of a nano water droplet is studied using atomistic molecular simulation. The drop is placed on an insulator surface covering two planar electrodes and perpendicularly in contact with a conductive tip. The system components resemble with typical electrowetting on dielectric (EWOD) setup generally considered for experiments [20]. The surface property of the insulator is modified by switching its properties from hydrophilic to the hydrophobic regime (implemented by modifying the Pt atom and oxygen atom water interaction energy). Experimentally, this switching is done by the application of the electric field. The variation of the interaction of the water molecules with the surface will lead to a change in contact angle as well as radius of curvature of the drop. The variable curvature may be used to focus objects at variable distances for imaging applications based on liquid lenses. The dynamical study would provide us valuable information about the accelerated response of the drop in the subnanosecond regime and its flexibility towards fast shape modification. The detailed investigation of the mechanism of the movement of the contact line of the drop will give a quantitative idea about the wetting and dewetting processes.
The organization of the remaining part of the paper as follows. Section 2 deals with the details of molecular dynamics simulation. The dynamics of the drop on varying the surface property from the hydrophobic and hydrophilic regime and vice-versa is discussed in section 3 and 4. Lastly, the conclusion of the study is briefly summarized in section 6. The present calculation shows that this idea can be implemented for designing tunable lenses [21] and optical devices [22].
Section snippets
Details of simulations
The present simulation system consists of a water nanodrop placed on an insulator surface and in contact with a conductive tip as shown in Fig. 1. At the bottom of the insulator layer, two planar platinum electrodes are placed. The Pt layers are 98 Å cubic simulation volume obtained by cutting a face-centered cubic platinum crystal with lattice parameter 3.9 Å angstorm along the (1 1 1) crystallographic plane. Platinum is chosen as electrode material because this element has been extensively
Surface Effect on the Drop
In the presence of a hydrophobic surface, the interaction of the water molecules is less and the drop gains a spherical shape. This spherical shape can be explained from the fact that when a liquid drop is brought into contact with a solid surface, the surface tension of the water molecules tends to minimize the surface area of the drop. This brings the drop to a minimum energy state with a spherical shape. In presence of a hydrophilic surface, the motions of the water molecules inside the drop
Dynamic Response of the drop
The dynamic response of a drop on a particular surface can be estimated by the variation of dynamic contact angle with time. The θ value will be meaningful if it is averaged over a sampling time of the order of nanoseconds. But the process of spreading of the drop in the hydrophilic case or retention in the hydrophobic case is very fast. So direct determination of θ(t) is the only suitable option. For the determination of the dynamic contact angle, it is considered that the drop doesn’t distort
Summary and conclusions
In this work, the dynamics of the wetting behavior of a nanodrop on an insulator surface is presented through MD simulation. The nature of the insulator surface is varied from hydrophilic to the hydrophobic regime. The study reveals that due to higher interaction of the water molecules with the surface the contact angle of the drop decreases in the hydrophilic case and the drop regain its original height as the surface is switched to hydrophobic one. An angle change of 58 ∘ is observed in
Supporting Information
Addition computational results are included for bigger dimension drop, regarding contact angle variation, variation of the height and time correlation function with time, the friction coefficient variation dependence with the drop’s contact line motion. The density profile of the near the insulator surface is also included for the smaller drop.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
Acknowledgment
Financial supports (File No.: TAR/2018/000103) from the Department of Science and Engineering Research Board (SERB) - Teacher Associateship for Research Excellence (TARE), Government of India, is gratefully acknowledged. The authors also thank the Management of BMS Institute of Technology and Management, Bangalore for financial assistance.
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