Design of Ice-free Nanostructured Surfaces Based on Repulsion of Impacting Water Droplets by Lidiya Mishchenko†, Benjamin Hatton‡, Vaibhav Bahadur†, J. Ashley Taylor§, Tom Krupenkin§, and Joanna Aizenberg†‡*

 

Wyss Institute for Biologically Inspired EngineeringHarvard University, Cambridge, Massachusetts 02138, United States
§Department of Mechanical Engineering, University of Wisconsin-Madison, 1513 University Avenue, Madison, Wisconsin 53706, United States
ACS Nano, Article ASAP
DOI: 10.1021/nn102557p
Publication Date (Web): November 9, 2010
Copyright © 2010 American Chemical Society
* Address correspondence to jaiz@seas.harvard.edu.

Abstract

Abstract Image

Materials that control ice accumulation are important to aircraft efficiency, highway and powerline maintenance, and building construction. Most current deicing systems include either physical or chemical removal of ice, both energy and resource-intensive. A more desirable approach would be to prevent ice formation rather than to fight its build-up. Much attention has been given recently to freezing of static water droplets resting on supercooled surfaces. Ice accretion, however, begins with the droplet/substrate collision followed by freezing. Here we focus on the behavior of dynamic droplets impacting supercooled nano- and microstructured surfaces. Detailed experimental analysis of the temperature-dependent droplet/surface interaction shows that highly ordered superhydrophobic materials can be designed to remain entirely ice-free down to ca. −25 to −30 °C, due to their ability to repel impacting water before ice nucleation occurs. Ice accumulated below these temperatures can be easily removed. Factors contributing to droplet retraction, pinning and freezing are addressed by combining classical nucleation theory with heat transfer and wetting dynamics, forming the foundation for the development of rationally designed ice-preventive materials. In particular, we emphasize the potential of hydrophobic polymeric coatings bearing closed-cell surface microstructures for their improved mechanical and pressure stability, amenability to facile replication and large-scale fabrication, and opportunities for greater tuning of their material and chemical properties.

Will I Stay or Will I Go? How Gender and Race Affect Turnover at ‘Up-or-Out’ Organizations by Harvard University Business School and Professor Kathleen McGinn

Gender and racial inequalities continue to persist at “up-or- out” knowledge organizations such as law firms, making it difficult for women and minorities to advance to senior levels. These inequalities reflect the fact that female and minority junior professionals don’t see senior professionals in their organization who look like them. To want to stay at a firm, women and minorities need to see other women and minorities in senior positions.

However, ”peer effects” have the opposite result. When women and minorities have a higher proportion of peers who are similar to them in terms of gender and race, it actually increases their likelihood of exiting an organization.

Bringing about the changes that are necessary to increase retention of women and minorities requires that firms and their leaders carefully examine how their practices and policies affect social cohesion and social comparison.

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