So-called lubricant-infused surfaces (LISs), which consist of a microporous surface infused with a thin oil layer, promote dropwise condensation, and can lead to smaller and more efficient energy and water harvesting systems. However, the widespread implementation of LISs in commercial applications is limited by an incomplete understanding of droplet-lubricant interactions, in particular droplet nucleation, growth, and mobility when interacting with the thin oil layer. The goal of this project is to elucidate these dynamics and to study their influence on condensation heat transfer rates. This research project aimed at providing a comprehensive understanding of the relationship of heat transfer, phase change, and fluid dynamics during condensation on lubricant-infused surfaces. It overcame previous limitations that hindered the direct observation and quantification of nucleation and heat transfer during dropwise condensation by combining high-speed imaging with high-resolution microscopy. Basic mathematical modeling complemented experiments to develop predictive tools and design criteria for condensation heat transfer enhancements. Two major findings of this project are discussed in more detail below. Furthermore, this project fostered interest in science and helped overcome the barriers for women and underrepresented minorities entering the field of engineering via outreach programs for black middle school children and research opportunities for undergraduate students.

Intellectual Merit:

LISs can be divided into lubricant-rich and lubricant-poor regions, which redistribute dynamically during condensation. Previously, we had shown that microdroplets can passively self-propel large distances due to overlapping oil menisci and resulting attractive capillary forces. Here, we hypothesized that the high droplet mobility and high sweeping frequencies enhance the nucleation rate density, which takes into account spatial and temporal information, and water collection rates on LISs compared to traditional solid hydrophobic surfaces. When comparing different LISs, the lowest lubricant viscosity always led to the highest nucleation rate density, as expected, due to lower friction. Interestingly and unexpectedly, however, we found that solid hydrophobic surfaces outperform high-viscosity LISs at high subcooling temperatures, but are generally inferior to any of the tested LISs at low temperature differences. To explain the observed non-linearity between LISs and the solid hydrophobic surface, we introduced two dominant regimes that influence the condensation efficiency: mobility-limited at lower vapor temperatures and coalescence-limited at higher vapor temperatures.

During experiments to characterize the nucleation rate density, we discovered that under certain conditions, microdroplets moved in opposite directions: some ascended the lubricant menisci, as expected based on capillary attraction, but some descended the meniscus and moved away from the larger central droplet. Using high-speed optical microscopy, confocal fluorescence imaging and numerical simulations, we found that temperature gradients at the oil-air interface lead to microscopic thermal-capillary Marangoni convection cells within the meniscus, whose magnitude and direction depend on the meniscus geometry and substrate temperature. On cooled substrates, as is the case during condensation, the oil surface flow is outward from larger droplets, i.e., downhill on the meniscus. The observed bi-directional droplet movement is then caused by the competition of unbalanced capillary forces at the droplet interface and drag forces from the convective flow. Larger microdroplets still ascend the meniscus, whereas smaller floating microdroplets are carried downward with the flow.

Broader Impact:

Graduate and undergraduate student mentoring and training was an integral part of this project. Students gained experience with various tools and techniques, allowing them to choose from a broad range of career paths in areas of heat transfer, fluid dynamics, and surface science after graduation. Overall, this project has supported one PhD thesis, one MS thesis, hosted two graduate visiting scholars, and allowed twelve undergraduate students to gain valuable research experience. Students acquired new skills through advanced research training in specific technical areas related to the project: experimental design, condensation phase change, nucleation, imaging, image and data analysis, research presentation, and writing of scientific manuscripts. Some of the trainees, including undergraduate students, even presented their research at national/international conferences, and received best poster and student keynote awards. Two of the undergraduate researchers also received NSF Graduate Fellowships to pursue PhDs upon graduation.

This project also supported various K-12 outreach efforts. Most notably, in 2021 (after the worst impairments of the pandemic were starting to subside), PI Weisensee got involved with the BrightPath STEAM Academy and co-organized their 6-week workshop series ?Young Black Scholars LEAP Program?, which invites 16 African-American middle school children to campus on six consecutive Saturdays for 2-hour long hands-on activities in different engineering disciplines. Starting in 2022, in addition to co-organizing the workshop series, the lab also hosted one of the activities, themed around droplet mobility on engineered surfaces. St. Louis is one of the most racially segregated cities in the US, and from student and parent feedback we learned just *how* important this program is for them. Possibly for the first time in their lives, the students and their families believe that a college education, maybe even at Washington University, is within their reach.

Last Modified: 07/07/2023
Modified by: Patricia B Weisensee