Mechanical Engineering assistant professor Yangying Zhu receives a National Science Foundation Early CAREER Award to pursue phase-change research.

COVID-19 can be transmitted when an infected person talks, coughs, sneezes, or sings, expelling virus-containing respiratory droplets, which can reach the mouth, nose, or eyes of previously uninfected people. These aqueous droplets tend to fall out of the air rapidly and evaporate on the floor or the ground, but some smaller droplets can evaporate before reaching the ground, leaving virus nuclei floating through the air. Such infinitesimal aerosolized particles, or aerosols, can travel on air currents for hours and infect people, particularly when they spend prolonged periods of time in indoor settings that lack adequate ventilation. It is because transmission is possible via free-drifting aerosols that the coronavirus is referred to as an airborne disease.

During the pandemic, Yangying Zhu, an assistant professor of mechanical engineering at UC Santa Barbara, and her research lab have analyzed the evaporation and propagation of respiratory droplets and aerosols under different temperatures and humidity conditions. They found that, under some conditions, respiratory droplets traveled farther than the six feet recommended by the Centers for Disease Control (CDC) for safe social distancing, and that the effect increased in cooler and more-humid environments. Zhu says that it is important to expand her research into evaporation dynamics and respiratory droplets.

“We are so fortunate that vaccines were recently made available; however, our effort to understand the transmission of respiratory diseases should continue,” said Zhu, who co-wrote a paper on the project that was published in the journal Nano Letters.

Zhu plans to continue her respiratory-related work by focusing on phase change, or the process by which matter transitions from a solid, liquid, or gas into a different state. A phase change takes place because of heat transfer, or the exchange of thermal energy between physical systems. Liquids evaporate when sufficient heat energy is provided to break intermolecular bonds between molecules, allowing them to change into a gas. When a vapor comes in contact with the surface of matter with a lower temperature, it condenses into a liquid by releasing heat.

The evaporation of a respiratory droplet into an aerosol is an example of the phase change from liquid to vapor, as are boiling and condensation, which engineers have used to generate power, control building temperature, desalinate water, and cool electronics. Technologies having superior heat-transfer performance have made the above-mentioned applications possible. Zhu says that understanding a phase change and heat transfer at the microscale will unlock secrets that can lead to next-generation technologies and increased energy efficiency at the very large scale while providing further insight into the spread of respiratory droplets at the scale of viral transfer. She has proposed a project to develop a temperature-measurement technique that can directly probe the three-phase region, something that has been very difficult to realize before. The three-phase region refers to the location where liquid, vapor, and solid meet during the phase change, such as the base of a bubble on a solid surface during boiling.

“During a phase-change process, heat is mainly transported from one phase to another within the three-phase region, which usually has a length scale of several hundred nanometers or a few micrometers,” said Zhu. “While we can measure the macroscopic heat-transfer performance, it has been very challenging to understand what happens at the phase-change boundary at the microscale, which is necessary if we are to design next-generation engineered devices for improved thermal transport control.”

In recognition of her innovative and highly promising research in this area, Zhu has received a prestigious Early CAREER Award from the National Science Foundation. She will receive $500,000 over five years for her project titled “Understanding Thermal Transport Across Phase-Change Interfaces via in Situ Micro-Raman Thermography.” The award is part of the NSF Faculty Early Career Development program that encourages junior faculty to pursue cutting-edge research and advance excellence in education.

“I am grateful as a junior faculty member to receive support from the NSF to pursue my proposed research,” said Zhu, who joined UCSB’s Mechanical Engineering Department in July 2019, after completing her PhD in mechanical engineering at the Massachusetts Institute of Technology and a postdoctoral position at Stanford University. “I am motivated by the thermal-science community to pursue this particular research direction.”

“I offer sincere congratulations to Professor Zhu for receiving this highly esteemed award,” said Rod Alferness, dean of UCSB’s College of Engineering. “It reflects her tremendous potential to create new knowledge related to heat transfer and to design new phase-change technologies. It also recognizes her motivation to apply her findings to prevent future pandemics and to expand our understanding of respiratory-disease transmission.”

In her project, Zhu seeks to develop an innovative platform to measure, with unprecedented accuracy and spatial resolution, the temperature near the solid-liquid-vapor contact line for both evaporation and condensation processes. Conventional methods use resistance temperature detectors and infrared cameras, but they are limited, either by their spatial resolution or their remoteness from the region of interest. Zhu plans to use micro-Raman spectroscopy to measure temperatures and use a laser to probe the three-phase contact region non-invasively during phase change in an environmental chamber. Micro-Raman spectroscopy involves using scattered light to measure the vibrational energy modes of a microscopic sample, providing both chemical and structural information. Zhu, who says that the technique has never been used to study phase-change heat transfer, believes that the ability to take high-spatial-resolution measurements in situ, where the process is actually occurring, will provide an exciting opportunity to gain new insights.

Zhu also plans to investigate the transfer of heat during phase change in nanostructures of semiconducting materials. Taking the temperature of the nanostructures will enable her to determine what materials and conditions result in the smallest temperature rise during phase changes. That insight should, in turn, allow her to design devices having a high critical heat flux (CHF) and a high heat transfer coefficient (HFC). The CHF refers to the maximum temperature reached during a phase change, while the HFC indicates the ease with which heat is exchanged between two materials.  

“This experiment will produce data to better understand phase-change processes and identify factors that limit efficient heat transfer,” said Zhu. “The fundamental insights gained through this work will potentially lead to highly effective and improved phase-change devices that can enable energy savings and reduce freshwater withdrawals for power plant, as well as provide energy-efficient thermally driven desalination, effective heat dissipation for high-power-density electronic devices, and more-energy-efficient thermal control of buildings.”

Circling back to her work’s relevance to COVID-19, Zhu will apply her findings to the transmission of respiratory diseases from the perspective of heat and mass transfer. This work could lead to improved strategies for ensuring airflow and ventilation to prevent virus accumulation indoors, or to guidelines for facial masks that provide optimal filtering. Her findings will also be incorporated into her undergraduate and graduate courses through lab experiments, lectures, and projects to further demonstrate heat transfer and phase changes.

“The COVID-19 pandemic has prompted me to reflect on how I, as a thermofluid engineer, can contribute to solving real-world problems and demonstrate to the younger generation how science and engineering can help combat and control the disease,” she said. “This NSF-funded project will allow me to accomplish both.”