Yangying Zhu

COVID-19 can be transmitted when an infected person speaks, coughs, sneezes or sings, expelling respiratory droplets containing the virus that can reach the mouth, nose or eyes of previously uninfected people.

These watery droplets tend to fall quickly from the air and evaporate onto the ground or soil, but some smaller droplets may evaporate before reaching the ground, leaving the nuclei of the virus to float in the air. . These infinitesimal aerosolized particles, or aerosols, can travel in air currents for hours and infect people, especially when they spend long periods in indoor environments lacking 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, assistant professor of mechanical engineering at UC Santa Barbara, and his research lab analyzed the evaporation and spread of respiratory droplets and aerosols under different conditions of temperature and humidity.

They found that, under certain 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, wetter environments.

Zhu says it’s important to expand his research into the dynamics of evaporation and respiratory droplets.

“We are so lucky that vaccines have recently been made available; However, our efforts to understand the transmission of respiratory disease should continue, ”said Zhu, who co-authored an article on the project published in the journal Nano Letters.

In recognition of his innovative and highly promising research in this area, Zhu received a prestigious Early CAREER Award from the National Science Foundation. She will receive $ 500,000 over five years for her project entitled Understanding Thermal Transport Across Phase-Change Interfaces via in Situ Micro-Raman Thermography.

The award is part of the NSF Faculty Career Development Program which encourages young faculty to pursue cutting-edge research and advance educational excellence.

“I am grateful as a junior faculty member to receive support from NSF to continue my proposed research,” said Zhu, who joined UCSB’s mechanical engineering department in 2019, after completing his doctorate. . 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 direction of research,” she said.

“I sincerely congratulate Professor Zhu on receiving this highly esteemed award,” said Rod Alferness, Dean of the College of Engineering at UCSB. “This reflects its enormous potential for creating new knowledge related to heat transfer and for designing new phase change technologies.

“It also recognizes his motivation to apply his findings to prevent future pandemics and to broaden our understanding of respiratory disease transmission. “

Zhu plans to continue his work related to respiration by focusing on phase change, or the process by which matter changes from a solid, liquid, or gas state to a different state. A phase change takes place due to the transfer of heat, or the exchange of thermal energy between physical systems.

Liquids evaporate when sufficient thermal energy is supplied to break the intermolecular bonds between molecules, allowing them to turn into gas. When a vapor comes into contact with the surface of matter at a lower temperature, it condenses into a liquid releasing heat.

Evaporation of a respiratory droplet in an aerosol is an example of the phase change from liquid to vapor, just like boiling and condensation, which engineers have used to generate electricity, control building temperature, desalinate the water and cool the electronics. Technologies with superior heat transfer performance have made the above mentioned applications possible.

Zhu says understanding phase shift and heat transfer on a microscopic scale will reveal secrets that can lead to next-generation technologies and increased energy efficiency at very large scales, while also providing insight. on the spread of respiratory droplets at the scale of viral transfer.

She proposed a project to develop a temperature measurement technique capable of directly probing the three-phase region, which was very difficult to achieve before. The three-phase region refers to where liquid, vapor, and solid meet during 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 in the three-phase region, which typically has a length scale of several hundred nanometers or a few micrometers,” Zhu said. .

“Although we can measure the macroscopic heat transfer performance, it has been very difficult to understand what is happening at the boundary of the phase change at the microscopic scale, which is necessary if we are to design devices for next-generation engineering for better thermal transport control, “she said.

In his NSF-funded project, Zhu seeks to develop an innovative platform to measure, with unprecedented precision and spatial resolution, temperature near the solid-liquid-vapor contact line for evaporation processes and of condensation.

Conventional methods use resistance temperature detectors and infrared cameras but they are limited either by their spatial resolution or by their distance 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 the phase change in an environmental chamber.

Micro-Raman spectroscopy is the use of scattered light to measure the vibrational energy modes of a microscopic sample, providing both chemical and structural information. Zhu, who says the technique has never been used before to study phase change heat transfer, believes the ability to take high spatial resolution measurements in situ, where the process actually takes place, will provide a exciting opportunity to acquire new knowledge. .

Zhu also plans to study heat transfer during phase change in nanostructures of semiconductor materials. Taking the temperature of nanostructures will allow him to determine which materials and conditions cause the lowest temperature rise during phase changes.

This idea, in turn, should enable him to design devices with high critical heat flux (CHF) and high heat transfer coefficient (HFC). CHF refers to the maximum temperature reached during a phase change, while HFC refers to 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,” Zhu said.

“The fundamental knowledge gained through this work will potentially lead to highly efficient and improved phase change devices that can save energy and reduce freshwater withdrawals for power plants, as well as provide thermal desalination to power plants. high energy efficiency, efficient heat dissipation for high temperatures. power density electronics and more energy efficient building thermal control, ”she said.

As for the relevance of his work to COVID-19, Zhu will apply his findings to the transmission of respiratory diseases from the perspective of heat and mass transfer. This could lead to better strategies for ensuring airflow and ventilation to prevent virus build-up inside, or guidelines for face masks that offer optimal filtering.

His findings will also be incorporated into his undergraduate and graduate courses through laboratory experiments, lectures, and projects to further demonstrate heat transfer and phase changes.

“The COVID-19 pandemic has prompted me to think about how, as a thermal fluid engineer, I can help solve real-world problems and demonstrate to the younger generation how science and engineering can. help fight and control the disease, ”she said. “This NSF-funded project will allow me to accomplish both. “

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