Sow-Hsin Chen (1935–2021): A Pioneer of Neutron Scattering and the Mysteries of Water
Sow-Hsin Chen was a towering figure in the world of condensed matter physics and nuclear engineering. Over a career spanning more than half a century, primarily at the Massachusetts Institute of Technology (MIT), Chen transformed our understanding of "soft matter"—complex liquids, polymers, and biological molecules. He is perhaps best known for his provocative and groundbreaking research into the anomalous behavior of water, particularly in its "supercooled" state.
1. Biography: From Taiwan to the Frontiers of Physics
Sow-Hsin Chen was born on March 5, 1935, in Chiayi, Taiwan. His academic journey was international and prestigious, reflecting the global nature of 20th-century physics.
- Early Education: He earned his B.S. in Physics from National Taiwan University (1956) and an M.S. from National Tsing Hua University (1958).
- North American Transition: Chen moved to the United States to earn a second M.S. from the University of Michigan (1962), followed by a Ph.D. in Physics from McMaster University in Canada (1964).
- The Brockhouse Influence: At McMaster, Chen studied under Bertram Brockhouse, who would later win the Nobel Prize for developing neutron spectroscopy. This mentorship was pivotal, as it grounded Chen in the techniques of neutron scattering that would define his life's work.
- Academic Career: After postdoctoral fellowships at the Atomic Energy Research Establishment in Harwell, UK, and at Harvard University, Chen joined the faculty of the MIT Department of Nuclear Science and Engineering in 1968. He remained at MIT for the rest of his career, becoming a Professor Emeritus in 2012.
2. Major Contributions: The "Fragile-to-Strong" Transition
Chen’s work focused on using neutron and X-ray scattering to probe the microscopic dynamics of liquids and complex fluids.
The Mystery of Supercooled Water
Chen’s most celebrated contribution is his investigation into the "Fragile-to-Strong" (FTS) dynamic crossover in deeply supercooled water. Most liquids become more viscous as they cool. However, Chen and his team provided experimental evidence that when water is "confined" (trapped in microscopic pores to prevent freezing), it undergoes a fundamental change in its molecular behavior at approximately 225 Kelvin (-48°C).
- The Theory: He argued that water shifts from a "fragile" liquid (where properties change rapidly with temperature) to a "strong" liquid (where properties change more linearly). This supported the controversial "Second Critical Point" hypothesis, suggesting that water can exist in two different liquid phases at very low temperatures.
Soft Matter and Complex Fluids
Chen was a pioneer in Soft Condensed Matter Physics. He developed mathematical models and experimental protocols to study:
- Micelles and Microemulsions: Understanding how surfactants organize in solution.
- Protein Hydration: He showed that the layer of water surrounding a protein (the hydration shell) undergoes a dynamic transition that is essential for the protein’s biological function—essentially, the "glass transition" of the water "turns on" the protein's activity.
3. Notable Publications
Chen was a prolific author with over 500 scientific papers. His work is characterized by a blend of rigorous experimental data and sophisticated theoretical modeling.
- "Observation of a fragile-to-strong dynamic crossover in deeply supercooled confined water" (Proceedings of the National Academy of Sciences, 2005): His most cited and influential paper regarding the phase behavior of water.
- "The violation of the Stokes-Einstein relation in supercooled water" (Nature, 2006): This work explored how water molecules move differently than standard liquid theories predict when temperatures drop.
- Foundations of Neutron Transport Theory (1971): An early, definitive textbook that became a staple for nuclear engineering students.
- Scattering Methods in Complex Fluids (2015): A comprehensive book summarizing his decades of expertise in using scattering to understand soft matter.
4. Awards & Recognition
Chen’s peers recognized him as one of the most innovative experimentalists of his generation.
- Clifford G. Shull Prize (2008): Awarded by the Neutron Scattering Society of America, this is the highest honor in the field of neutron science.
- Guinier Prize (2015): Awarded by the International Union of Crystallography for his lifetime contributions to small-angle scattering.
- Priz de la Recherche (2006): A prestigious French award for his work on the properties of water.
- Fellowships: He was an elected Fellow of the American Physical Society (APS), the American Association for the Advancement of Science (AAAS), and the Neutron Scattering Society of America.
- Academician: He was elected as a member of the Academia Sinica (Taiwan’s national academy) in 2006.
5. Impact & Legacy
Sow-Hsin Chen did more than just publish papers; he built the infrastructure for modern liquid physics.
- The "Water Problem": Before Chen, the behavior of water at low temperatures was largely speculative. His use of "confined water" (putting water in silica pores) allowed scientists to study the liquid state at temperatures previously impossible to reach due to ice formation.
- Mentorship: Chen supervised over 50 Ph.D. students, many of whom now lead research departments worldwide. He was known for his "tough but fair" mentorship style, demanding rigorous mathematical proofs for experimental observations.
- Bridging Disciplines: He successfully bridged the gap between Nuclear Engineering (the tools) and Condensed Matter Physics (the questions), showing how neutron beams could answer fundamental questions about life and matter.
6. Collaborations
Chen was a highly collaborative researcher who thrived on the exchange of ideas:
- H. Eugene Stanley (Boston University): A long-term collaborator and theoretical physicist. Together, they formed a powerhouse duo—Stanley providing the theoretical frameworks for water's phase transitions and Chen providing the experimental proof via neutron scattering.
- Piero Tartaglia (University of Rome): Collaborated extensively on the theory of slow dynamics in glass-forming liquids.
- National Synchrotron Radiation Research Center (NSRRC): Chen was instrumental in advising the development of Taiwan's synchrotron facilities, ensuring his home country remained at the cutting edge of structural biology and materials science.
7. Lesser-Known Facts
- The First Reactor: In the late 1950s, Chen was part of the small team that helped install and operate the first nuclear research reactor in Taiwan at National Tsing Hua University.
- A "Neutron Giant": Despite his soft-spoken demeanor, Chen was known in the community as a "Neutron Giant." He was famous for spending weeks at a time at national laboratories (like Oak Ridge or NIST), personally overseeing experiments until the early hours of the morning.
- Cultural Bridge: Chen remained deeply connected to his roots. He was a key figure in the "Reverse Brain Drain," spending his later years helping Taiwan establish itself as a global hub for neutron and X-ray science.
Sow-Hsin Chen passed away on June 26, 2021. He leaves behind a legacy defined by a relentless curiosity about the most common substance on Earth—water—and the sophisticated tools he perfected to see the invisible dance of its molecules.