The objective of the project was to fundamentally understand thermal-fluid-interfacial interactions across multiple length and time scales with emphasis on condensation phenomena. The project developed theoretical and experimental framework for elucidating how nanoscale properties of surfaces define macroscale heat and mass transport properties during phase change. The intrinsic wettability and morphology of a surface, together, play an important role in phase change, ie., nucleation, growth and shedding of drops. One approach to enhance condensation is to promote drop mode of condensation. We studied the role of surface morphology, including hierarchical structures and wetting heterogeneities on wetting hysteresis and condensation performance. Droplets can reside on the surface in different states. In the Cassie state, the drop resides on top of roughness features, whereas a drop in the Wenzel state impales the roughness features. For low adhesion it is advantageous for droplets to remain in the Cassie state. In this state, the droplet assumes a nearly spherical shape and is able to easily shed from the surface. This rapid shedding leads to heat fluxes that are considerably higher than seen in conventional condensers. Therefore, it is important to understand the mechanism of Cassie to Wenzel wetting transition in order to take an advantage of designing energy efficient surfaces. In our study of wetting transitions, we investigate the effect of wetting dynamics on droplet transitions on textured hydrophobic surfaces. This study reveals the importance of compressibility and liquid-hammer effects in wetting transitions. Studies using environmental scanning electron microscopy showed that drop adhesion on hierarchical surfaces is governed by a self-similar depinning mechanism in which the macroscale contact line is composed of numerous micro-contact lines sitting at the bases of the micro-capillary bridges. The micro-contact lines will themselves be further subdivided into nano-capillary bridges with their own nano-capillary contact lines. We also demonstrate direct nano-to-microscale imaging of complex fluidic interfaces using cryostabilization in combination with cryogenic Focused Ion Beam milling and SEM imaging.  We show that application of this method yields quantitative information about the interfacial geometry of water condensate on nanostructured surfaces. This understanding has led to the design of surfaces with very low adhesion and enhancing the condensation rate by decreasing the maximum drop departure diameter significantly below the capillary length. We also studied drop coalescence during condensation. We show that isolated mobile and immobile coalescence between two drops, leads to tangential propulsion of mobile drops. These droplet-shedding modes comprise of multiple droplets merging during serial coalescence events, which culminate in formation of a drop that either departs or remains anchored to the surface. We find that favorable surface architecture consists of microscale features spaced close enough to enable transition of larger droplets into micro-Cassie state yet, at the same time, provides sufficient spacing in-between the features for occurrence of mobile coalescence. This project has lead to fundamental understanding of interfacial properties that can control nucleation, growth, and dynamic wetting phenomena and established the structure-property relationships between nanoscale surface morphology and macroscale heat and mass transport. As condensation processes are ubiquitous in many industries, the fundamental insights from the project can help design novel surfaces that can alter thermal-fluid-surface interactions for efficiency enhancements in various industries including energy and water. 

Last Modified: 01/25/2016
Modified by: Kripa K Varanasi