Fog Collection on Vertical Wires
September 17, 2019
11:00 AM - 12:00 PM
945 W. Taylor St., Chicago, IL 60607
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Fog Collection on Vertical Wires
Presenter: K.-C. Kenneth Park, Assistant Professor of Mechanical Engineering, Northwestern University
Abstract: Fog collection is a promising solution to worldwide water scarcity while also being of vital importance in industrial processes. The upright, wire-like leaves of Stipagrostis sabulicala, a grass species in the Namib Desert, shows excellent fog collection performance. Inspired by such wire geometry, we aim to understand the mechanism of fog collection on individual vertical wires. To date, many studies have investigated the fog collection rate, a parameter that denotes the average performance over a period of time. However, it is important to study the fundamental correlation between the fog collection rate and the process of fog droplet capture, which has been largely underexplored, in order to understand the full span of the fog collection process and improve its collection efficiency. Moreover, the initial period (referred to as onset time) between the start of the fog flow and the collection of the captured liquid (a delay in time caused by droplet growth to a critical weight) has not been understood. In particular, a longer onset time may result in a more serious clogging issue that deteriorates the collection rate and, therefore, understanding this phenomenon is important. In the first part of this fog collection study, we aim to examine the correlation between the measured collection rate and the deposition step of fog collection on a wire, using spontaneous wetting of vertical, superhydrophilic wires that minimize the liquid loss during transport to precisely measure the volume of collected water. We also visualized how fog droplets are collected on a wire and measured the resulting fog collection rate while varying wire diameter, surface wettability, and wind speed. Surface wettability has a negligible effect on the fog collection rate on wires with large diameters and at low wind speeds (0.5 m/s). By contrast, there appears to be a strong effect of wettability on wires with small diameters and at high wind speeds (e.g., 3 m/s). The results show that the measured fog collection rate per unit area is linearly proportional to an empirically obtained deposition efficiency of aerosols, a function of the Stokes number. In the second part, we study how the onset time is determined by the capture and transport of fog droplets using single, vertical wires with various surface wettabilities and diameters, under different wind speeds. We then derive a scaling law that correlates the onset time with the fog capture process and droplet-surface retention force, governed by aerodynamics and interfacial phenomena, respectively. In particular, the onset time decreases with an increasing rate of fog capture or a decreasing droplet-surface retention. In addition to the controlled liquid transport by the modification of surface wettability, this study provides physical insights for the optimal design of fog collectors from an aerodynamics-centered perspective, benefitting the fight against the global water crisis.
Presenter bio: Dr. Kyoo-Chul (Kenneth) Park joined the Department of Mechanical Engineering at Northwestern University as an assistant professor in 2017. He received his PhD in mechanical engineering from the Massachusetts Institute of Technology in 2013 and worked as a postdoctoral fellow in the John A. Paulson School of Engineering and Applied Sciences and the Wyss Institute for Biologically Inspired Engineering at Harvard University. At both institutions, he was a recipient of four awards including the MIT Wunsch Foundation Silent Hoist and Crane Award for Outstanding Graduate Research and Harvard Postdoctoral Award for Professional Development.
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