Selemon Bekele

Effect of Surface Energy on Freezing Temperature of Water

Previous studies have found that superhydrophobic surfaces are effective in delaying freezing of water droplets. However, the freezing process of water droplets on superhydrophobic surfaces depends on factors such as droplet size, surface area, roughness, and cooling rate. The role of surface energy, independent of any other parameters, in delaying freezing of water is not understood. Here, we have used infrared-visible sum frequency generation spectroscopy (SFG) to study the freezing of water next to solid substrates with water contact angles varying from 5° to 110°. We find that the freezing temperature of water decreases with increasing surface hydrophobicity only when the sample volume is small (∼10 μL). For a larger volume of water (∼300 μL), the freezing temperature is independent of surface energy. For water next to the surfaces with contact angle ≥54°, we observe a strong SFG peak associated with highly coordinated water. This research sheds new light on understanding the key factors in designing new anti-icing coatings.

Interfacial Properties of Oxidized Polystyrene and Its Interaction with Water

All-atom molecular dynamics simulations have been carried out to study the wetting of atactic polystyrene (aPS) thin films by water droplets. The effect of oxidation of the aPS surface on the contact angle has been studied as a function of oxygen concentration. Oxidation of aPS has been achieved by randomly replacing with oxygen the ortho and/or meta hydrogens on the aromatic rings within 1 nm of the aPS surface until the desired concentration of oxygen is reached. The simulated contact angle is found to decrease monotonically with increasing degree of oxidation, consistent with recent experimental results. The number of hydrogen bonds between water molecules and polystyrene at the interface is found to monotonically increase with oxygen concentration. By use of a modified Good-Girafalco-Fowkes-Young equation, the contribution of nondispersion interactions, γsl(P), to the interfacial energy at the aPS/water interface has been determined as a function of the degree of oxidation. The values of γsl(P) extracted appear to follow a quadratic dependence on oxygen concentration of the aPS surface. The roughness of the polystyrene surface appears to be independent of oxygen concentration when the polystyrene is exposed to vacuum, and it appears to increase slightly when it is in contact with water. The orientational ordering of the phenyl rings at the polystyrene surface exhibits no dependence on oxygen concentration for polystyrene in vacuum. However, the ordering appears to decrease slightly with increasing oxygen concentration when the polystyrene is in contact with water.

Effect of Polymer/Solid and Polymer/Vapor Instantaneous Interfaces on the Interfacial Structure and Dynamics of Polymer Melt Systems

Polymers are used in a wide range of applications that involve chemical and physical processes taking place at surfaces or interfaces which influence the interaction between the polymer material and the substance that comes into contact with it. Polymer surfaces are usually modified either chemically or physically for specific applications such as facilitating wetting, reducing friction, and enhancing adhesion. The variety and complexity of surface and interfacial processes requires a molecular-level understanding of the structural and dynamical properties of the surface/interface layer to help in the design of materials with desired functional properties. Using molecular dynamics (MD) simulations, we investigate the structure and dynamics at the surface of polymer films. We find that the density profiles of the films as a function of distance relative to an instantaneous surface have a structure indicative of a layering at the polymer/vapor interface similar to the typical layered structure observed at the polymer/substrate interface. However, the interfacial molecules at the polymer/vapor interface have a higher mobility compared to that in the bulk while the mobility of the molecules is lower at the polymer/substrate interface. Time correlation of the instantaneous polymer/vapor interface shows that surface fluctuations are strongly temperature dependent and are directly related to the mobility of polymer chains near the interface.