Sow-Hsin Chen

Sow-Hsin Chen, a professor emeritus at MIT, passed away on 26 June 2021. As a soft-matter physicist, he pioneered the study of structure and dynamics of macromolecules using small angle neutron and x-ray scattering, and contributed to the understanding of the properties of water in the supercooled state. Even though he was an experimentalist, he had a deep interest in theoretical models and an enthusiastic appreciation of molecular dynamics simulation.

Born in Taiwan in 1935, he was determined to become a physicist early on. He majored in physics at the National Taiwan University and obtained a master’s degree in the first class of the graduate school of nuclear science in the National Tsinghua University. Supported by an International Atomic Energy Agency fellowship, he attended the International School of Nuclear Science and Technology at the Argonne National Laboratory in 1958. In 1962 he obtained another master’s degree, in nuclear engineering, from the University of Michigan. In 1964 he obtained his PhD; he was the first PhD student of future Nobel laureate Bertram N. Brockhouse at McMaster University in Canada. After that, he worked with Peter Egelstaff at Harwell and then became a postdoctoral fellow with future Nobel laureate Nicolaas Bloembergen at Harvard. Then he joined the Department of Nuclear Science and Engineering at MIT in 1968. In spite of a long and difficult period of failing health in the last 21 years, he still managed to stay actively engaged in research until the day of his passing.

Although he worked mostly with neutrons, he was also an expert in light and x-ray scattering. At Harvard he built a successful instrument to measure scattered intensity correlation functions, the digital correlator. His early studies included binary liquids near their demixing critical point that later were extended to macromolecular systems including proteins and micellar systems. Broadly speaking, he was interested in clarifying the nature of slow dynamics of materials, especially in supercooled states, from the integrated perspective of laboratory experiments, theoretical modeling, and molecular simulations.

Among all his research interests, water stands out as the overarching focus. He first studied water through Brillouin light scattering in the late 1960s and then moved to use neutron scattering beginning in the 1970s. His contributions pertained to supercooled water, water under pressure, and confined water. In the 1990s he showed that the slow dynamics of water, a molecular and network forming liquid, follows the prediction of mode-coupling theory, and he demonstrated how this behavior is strongly coupled to the thermodynamic anomalies of water, and moreover, to the possible existence of a second liquid–liquid critical point in the supercooled region. He observed a crossover of the water dynamics that help pinpoint the possible location of this second critical point.

Just as significant as his scientific talents were his human gifts. Besides being a teacher, he was also a friend and a brother to his students and collaborators. None of those who knew him will forget his ever-present smile even during difficult moments. He was open to all cultures brought to him by his students and collaborators worldwide. Together with his family, he loved art and cultural heritage. Perhaps for this reason, he had many collaborators in Italian universities such as Rome, Florence, and Messina.

As an MIT professor with decades of experience using neutrons, his advice was much sought after by universities and government labs abroad that were expanding their scientific programs and neutron facilities. He played an early role in introducing nuclear power technology to Taiwan. Soon after the oil embargo crisis in the US in 1974, he organized an international conference in Taipei, where several of his departmental colleagues at MIT lectured on key aspects of fission power generation under development at that time. In addition to offering his own assessments, he organized topical workshops to glean expert opinions from international communities. These included workshops on reactor-based neutron science in Taiwan, two pioneering US–China workshops on neutron-scattering science and technology, as well as others held at National Tsinghua University in Taipei, Tsinghua University in Beijing, and Sun Yat-Sen University in Guangzhou. Over the years, he hosted numerous short-term visits by foreign scientists at MIT.

In this tribute we speak for many of his former students, postdocs, departmental and external colleagues, friends, and collaborators in expressing our admiration of his unwavering faith in the scientific power of neutron scattering and an insatiable curiosity of molecular mechanisms in the materials world. Collectively we all cherish our memories of time spent with him.