Xiang Cheng, Associate Professor of Chemical Engineering and Materials Science at University of Minnesota, Twin Cities, is the recipient of the 2019 SOR Metzner Early
Career Award. Cheng's research uses innovative imaging and keen physical insight to unravel the rheology of soft matter. He couples rheological measurements with
state-of-the-art imaging techniques including high-speed photography, fast confocal microscopy, and digital inline holography to investigate the rheology of active fluids,
jammed soft materials, and the impact of liquid drops.
Cheng received his BS in physics from Peking University in China in 2002 and his PhD in 2009, also in physics, at the University of Chicago under the supervision of Sidney
Nagel and Heinrich Jaeger. His thesis focused on the flow of granular materials. This body of 10 publications, including two as a single author, has gained over 500 citations.
Cheng did his postdoctoral work with Itai Cohen at Cornell, where he studied the rheology of colloidal suspensions. He designed and built a planar shear cell integrated with
a fast confocal microscope. He used it to image single particle dynamics as they transition from shear-thinning to shear-thickening in concentrated suspensions. In particular,
he quantified the shear-driven distortion of uniform particle structures and experimentally demonstrated the entropic origin of shear-thinning flows. His results on particle
dynamics in shear-thickening flows stimulate ongoing discussions on the microscopic origin of shear thickening. This work broke new ground in access to structure-property
analysis in soft matter rheology. Published in Science in 2011, it has been cited nearly 300 times.
In 2013 Cheng joined the University of Minnesota as the inaugural Macosko Assistant Professor. He has initiated a very active and diverse research program, which has been recognized
with early-career awards from Packard Foundation, DARPA, NSF, and 3M. He has continued granular-flow studies at Minnesota. Using high-speed photography and laser profilometry he has
discovered a remarkable similarity between liquid drop impacts and asteroid craters; the YouTube video of this work has nearly 900,000 views. In a PNAS publication with three
undergraduate researchers, he modeled crater diameters over 7 orders of magnitude, from raindrops on sand to asteroids on different planetary bodies. This work has important
applications in soil erosion, drip irrigation, and powder coating.
Xiang has also applied his imaging expertise to real time visualization of polymer bigel formation. He showed that, initially, elastic modulus decreases due to coarsening after these
polymers phase separate, but then it increases strongly due to particles jamming in the interface. His group received an NSF grant to apply these techniques to design graphene-stabilized
co-continuous polymer blends. By localizing graphene at interfaces, electrical percolation occurs at as low as 0.025 wt% graphene. Xiang is also applying confocal microscopy to study the
dynamics of colloids in strong confinement, breaking new ground in our understanding of glass-formation processes.
Recently Xiang launched a new research program on the rheology of active fluids using bacterial suspensions as a model system. His work revealed an unusual symmetric shear banding in
concentrated bacterial suspensions and provided new insights on the microscopic dynamics leading to the emergence of active “superfluids.” He also studied the rheology of bacterial
suspensions under confinement and illustrated the influence of bacteria-wall interactions on the flow of suspensions. In addition, he has collaborated with Kevin Dorfman on studying
shear banding in entangled polymer solutions. They have investigated the diffusive modes in entangled DNA and shown the connection to inhomogeneous shear fields.
A goal of the Metzner award is to encourage the best young researchers to be engaged in the rheological community. Xiang is a perfect example of this, coming from just ‘outside’ and
already showing a respect and wish to engage with the rheological community. He will continue to be a strong contributor to the Society for many years.