1D Roughness Driven Depinning of Self-Assembly of Liquid Droplets.

The conventional hexagonal, uniform breath figure pattern formed over smooth substrates and substrates with constraints of the order of 50 µm is distorted when the underlying constraints are down to 1 µm. This paper explores this phenomenon further and concludes that, in addition to topology-based arguments presented by other authors previously, it is necessary to invoke the depinning effects of the three-phase contact line in order to explain the same. The influence of surface constraints on the self-assembly of liquid droplets is investigated. A semiquantitative explanation for large-scale pattern formation consisting of small-scale closely arranged droplets inside the large-scale distorted ring of droplets is presented in this paper. The scale at which the influence of constraints becomes dominant is also determined in this study. It is seen that the underlying roughness has a larger impact than the nature of polymer on pore size. Comparative studies of pore patterns formed on smooth and constrained substrates are reported. The simulated energy-minimized shapes of the droplets on smooth and constrained substrates are obtained using Surface Evolver.

Controlling breath figure patterns on PDMS by concentration variation of ethanol-methanol binary vapors.

In this paper, the self-assembly of condensed droplets on smooth and constrained surfaces under saturated vapor atmosphere of ethanol and methanol binary system is reported. Hexagonally ordered array of pores are obtained on smooth surfaces with saturated vapors of binary liquids without the assistance of any additives. The results show that the addition of a small amount of ethanol to methanol plays a role very similar to that of surface active agents in inducing the formation of a regular droplet array. The effect of constraints on a self-assembled droplet pattern such as the movement of the contact line and the depinning of the contact line is also investigated. It is observed that the pore size, pore shape, pore depth and ring diameter are influenced by the atmosphere of binary vapors in addition to the commonly held attribution to the surface tension of the solvent. Contact angle studies of the patterned substrates show hydrophobicity with high adhesiveness and transitions between the Wenzel and Cassie impregnating state over the entire concentration region.

Evidence of anomalous behavior of intermolecular interactions at low concentration of methanol in ethanol-methanol binary system.

At low concentrations of methanol in a binary system of ethanol and methanol, uniquely complex molecular interactions are reported here. Previous studies indicate that ethanol molecules form aggregates held together by hydrogen bonding (O-H-O) and also dispersive forces. Addition of small amount of methanol tends to break the hydrogen bond network of ethanol due to the larger polarity of methanol. This leads to the ethanol molecules becoming somewhat isolated from each other within a scaffolding network of methanol molecules, as seen from the present molecular dynamics simulations. This is an indication of a repulsive force that dominates among the two different alcohols. At higher molar concentration of methanol (Xm > 0.3817), the strength and extent (number) of formation of hydrogen bonds between ethanol and methanol increase. The geometry of molecular structure at high concentration favors the fitting of component molecules with each other. Intermolecular interactions in the ethanol-methanol binary system over the entire concentration range were investigated in detail using broadband dielectric spectroscopy, FTIR, surface tension and refractive index studies. Molecular dynamics simulations show that the hydrogen bond density is a direct function of the number of methanol molecules present, as the ethanol aggregates are not strictly hydrogen-bond constructed which is in agreement with the experimental results.