“When a droplet hits a hard surface, it spreads and bifurcates into multiple tiny droplets,” said Solomon Adera, assistant professor of mechanical engineering. “What we want to understand is the fundamental mechanism that causes the larger droplet to break up into smaller ones in the fashion that it does.”
When a raindrop strikes a hard surface, it breaks apart and spreads to cover a surrounding area. This phenomenon occurs by the same mechanism that is used when farmers go to seed a field, by police detectives when reconstructing a crime scene from blood splatter, and by manufacturers of computer chips and motherboards, the circuits of which can be laid by dripping liquid metals over a fiberglass plate and freezing them in place.

Adera has received an NSF CAREER Award for his project, “Drop impact dynamics and fingering on thin liquid films,” which will cover basic research into this ubiquitous phenomenon.
“The project is to study how droplets interact, specifically with a surface that is pre-lubricated with a water-immiscible oil, upon impact,” said Adera. “We’re using a pre-lubricated surface because such surfaces are common in nature.” Human skin, for example, is inlaid with many tiny pores that secrete oil in much the same way that plants and other natural surfaces do.
“The difficulty is that these things have not been studied in previous years,” said Adera. “The main challenge is, how do you create a surface that is pre-infused with a very thin layer of oil? How do you precisely control the lubricant thickness?”
In 2011, the Harvard laboratory in which Adera would later work as a post-doc took inspiration from the nepenthes pitcher plant – a carnivorous funnel-shaped plant with slippery skin – to create porous metals that can be treated with a layer of oil. In his years at the University of Michigan, Adera and his lab have repeatedly constructed such materials and adapted them for various applications by controlling the thickness of their overlying lubricant films.
“Our main work is to create these surfaces, to pre-infuse them with oil of controlled thickness, and characterize and model the interaction of a drop impacting such surfaces,” said Adera.
Adera and his lab will use various oils (such as silicone, Krytox, and mineral) to treat a series of porous structures, record water droplets impacting those surfaces at 10,000 frames-per-second or more, and then analyze the high-speed videos. The cause of the breakup mechanism and the post-impact dynamics will be studied by analyzing the time-lapse images (that is, frame-by-frame analysis of droplet-surface interaction).
“And then there is an air pocket that forms at the center,” said Adera. “When the droplet impacts the surface, air is trapped at its center, the role of which is not fully understood.”

Making use of optical microscopy, Adera’s team will measure the thickness of the air pocket and relate this information to the spread and breakup of the initial droplet. In this way, Adera’s team will provide fundamental insights into drop impact dynamics for future research and for the development of industry applications.
In addition to this research, Adera will partner with Xplore Engineering to host a series of experiential workshops with pre-college students to foster their interests in STEM fields. Partnering with the U-M Museum of Natural History, Adera and his lab will also present five public talks on drop impact dynamics and its future applications.
When asked how he felt about receiving funding for this project, Adera said, “I am honored to receive the prestigious NSF CAREER Award. The support helps me to continue my work on droplets and bubbles, a topic that is near and dear to me. I started working on droplets when I was a graduate student and the rich nature of the problem continues to amaze and engage me to this day. Despite its simplistic outlook and layman’s description, studying droplets is intellectually satisfying as there are always new things to learn every time you re-visit a well-established theory or past studies.”
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“The Faculty Early Career Development (CAREER) Program is a Foundation-wide activity that offers the National Science Foundation’s most prestigious awards in support of early-career faculty who have the potential to serve as academic role models in research and education and to lead advances in the mission of their department or organization.”