Wetting transition of a nanodrop on switchable hydrophilic-hydrophobic surfaces

Abstract

The dynamic modulation of contact angle on switchable hydrophilic-hydrophobic surfaces is one of the important factors in designing miniaturized optical devices. A spreading and retention of a nano water droplet placed on an insulator surface covering two planar electrode layers and perpendicularly in contact with a conductive tip is studied using atomistic molecular simulation. The change in the droplet shape is initiated by modifying interaction energy between the insulator atoms and the water molecules. The contact angle analysis shows a unique dependence on the nature of the underlying substrate. Time correlation function analysis based on the calculation of variation of the height of the drop’s center of mass with time estimates the rate of relaxations for wetting-dewetting behavior subject to the condition of surface hydrophilic and hydrophobic nature. For the hydrophilic case, the relaxation rate of the drop is found to be twice slower than the hydrophilic one. The difference in the rate of relaxations is analyzed by considering the effects of the frictional force on the contact line motion of the drop. Friction coefficients are calculated on the basis of continuum hydrodynamic (HD) and molecular kinetic theory (MKT) theories. The dependence of the friction coefficients on the surface properties depicts the strong dominance of the liquid-substrate friction on the drop’s perimeter motion based on molecular kinetic theory.

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 (γ_SL) 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].