May 7, 2021
Professor Thomas Mason

Chemist and physicist Professor Thomas Mason is one of five scientists from around the nation selected as a 2021 Fellow of the Society of Rheology (SOR).  

A professor of physical chemistry and of physics at UCLA, Mason was selected for his work in complex fluids including the creation, demonstration, and development of thermal-entropic 'passive' microrheology. He is the first UCLA faculty member selected as a SOR fellow. Mason leads an interdisciplinary research group in pre- and self-assembled soft matter, nanoemulsions, and microrheology. For more information about research in Mason's group, visit this website.  This news was recently featured in UCLA Newsroom.

SOR Fellowship status recognizes a history of distinguished scientific achievement, a significant technological accomplishment, and/or outstanding scholarship in the field of rheology, the study of deformation and flow of materials. The SOR Fellowships will be presented at the 92nd Society of Rheology Annual Meeting, to be held October 10-14, 2021 in Bangor, Maine. 

In his nomination letter, Professor H. Henning Winter of the University of Massachusetts Amherst, said of Mason: “Over his career so far, Tom has greatly impacted the fields of rheology and soft matter, enlightening many through his mentoring, service, and outreach. The level of his accomplishment in the field of rheology clearly merits fellowship in the Society of Rheology.”

In his letter, Prof. Winter explained that Mason has over 130 peer-reviewed journal publications, the majority of which are related to rheology in one manner or another. Most importantly, he is the inventor of thermal-entropic passive microrheology (PM), an elegant miniature experimental technique to deduce linear viscoelastic properties of a test material from the time-dependent mean displacement of a small Brownian probe suspended in this material. As a major discovery, Mason had realized that the probe’s Brownian motion directly depends on the material’s viscoelasticity, and that the connection is made through a generalized Stokes-Einstein relation. In this way, Brownian probes can serve as a miniature measuring tool for rheology. Mason’s PM approach has become well known, and many groups worldwide have used Mason's approach directly; others have made variations and refinements related to PM. His work also sparked renewed interest in active microrheology. It has been one of the most important and exciting advances in the field of rheology over the last several decades. It has become a classic.

Mason not only created the main approach that involves scattering from an ensemble of probe particles in non-heterogeneous materials, but as a postdoc, he also extended this to local microrheology measurements, based on time-averaged measurements of a single particle made using optical microscopy. This work, done at Johns Hopkins University with collaborators, led to the concept of region-specific local particle tracking microrheology, reported only two years after his light scattering experiments that relied on non-local ensemble averaging over the motion of many probes. Mason reported trajectories of an individual probe sphere in a concentrated DNA solution using 2D quadrant photodiode detection of laser light scattered from the sphere as well as the first sub-pixel video particle tracking microrheology results. Mason also developed an estimation method for rapidly and simply extracting frequency-dependent viscoelastic moduli G' and G'' from time-dependent mean square displacements of probe spheres.

Mason's fundamental work in microrheology has been validated independently by many different groups for a wide range of soft materials and probes that satisfy the clearly stated assumptions of the generalized Stokes-Einstein relation. Recently, as reported in a 2019 PNAS article, Mason’s UCLA group did the primary experiments that show how to correct mean square displacements for collective scattering effects in diffusing wave spectroscopy measurements of dense emulsions. This advance has further broadened the applications of PM to very highly scattering viscoelastic systems having high probe densities. His innovation of PM has been transformational for the field of rheology both experimentally and theoretically. His PM approach is now used in commercial optical devices made by several different companies.

Later work by Mason and his postdoc collaborator Dr. Zhengdong Cheng on PM at ExxonMobil Research introduced rotational passive microrheology of non-spherical wax microdisks, interpreted using a generalized Stokes-Einstein-Debye relation. In this work, Mason also included the technological improvement of a fast video camera into microscopic imaging and high-speed particle tracking for microrheology of soft matter. At UCLA, Mason and his graduate student Jim Wilking created an active rotational microrheometer by applying optical torques using polarized and focused laser light to birefringent wax microdisks in gelatin, showing local yielding behavior which correlated with macroscopic yielding. This extended rotational active microrheology to the non-linear regime. His work in biomicrorheology with UCLA collaborators has also been important, since it clearly revealed that consequences of active driving by molecular motors in living cells could be mis-interpreted if the GSER of passive microrheology was applied inappropriately to active systems.

In addition to creating passive microrheology, Mason also made his mark on the materials-specific area of emulsion rheology, emulsification, and nanoemulsification. Mason's measurements of the linear shear viscoelastic properties, as well as yield and steady-shear behavior, of disordered monodisperse emulsions have been as important to the field of emulsion rheology, in a manner similar to earlier work by well-known rheologists on polymer rheology of monodisperse model polymers. Mason used Dr. Jerome Bibette's depletion fractionation method to obtain monodisperse sub-micron emulsions, and made a comprehensive set of rheological measurements as a function of droplet size and volume fraction. By reducing polydispersity to low levels, Mason showed the connection between the rise in the shear elasticity of the emulsion as the droplet volume fraction was increased through an entropic glass transition and then through a jamming transition of droplets, resulting in droplet deformation. This work, which he published with Bibette and Prof. David Weitz, has served as a simple basis for understanding jamming in a wide variety of soft-matter systems composed of more complex compact dispersed soft objects that can jam, including microgels, clays, and other materials. At UCLA, Mason and his group developed an ultracentrifugal method of size-fractionating nanoemulsions, which have droplet sizes that are too small to fractionate easily using depletion. So, Mason's group also extended rheological measurements of monodisperse emulsions into the nanoscale range, down to droplet diameters as small as around 50 nm. Their work also revealed irreversible elastic vitrification of viscous microscale emulsions into highly elastic nanoemulsions caused by a history of high flow homogenization without any change in the emulsion's composition. Mason’s foundational nanoemulsification method enabled high-throughput production of nanoemulsions with highly controlled size distributions suitable for practical commercial purposes; the main paper on this work has been cited more than 1,000 times and is likely to eventually overtake PM as Mason’s most-cited work. Mason's group is also well known for their small angle neutron scattering (SANS) work on the structure of fractionated nanoemulsions as a function of droplet size and volume fraction, and they used Rheo-SANS to examine disruption of attractive nanoemulsion gels, too.

As a PI at ExxonMobil Research and as a professor at UCLA, Mason has mentored over a dozen graduate students and postdocs in topics related to rheology. He has given more than a hundred talks worldwide about his work on passive microrheology, emulsion rheology, and nanoemulsions, including lectures at summer schools for soft matter in Boulder in the U.S. and Les Houches and Cargese in Europe. Mason's research is well-known and well-regarded internationally. He has previously been honored as a fellow of the American Physical Society (APS) and as a fellow of the American Association for the Advancement of Science (AAAS).

Penny Jennings, UCLA Department of Chemistry & Biochemistry, penny@chem.ucla.edu.