We study soft materials physics in experiments, with a special focus on the emergent flow behaviors of soft materials and their mesoscopic structural origins. Research in soft materials has had a profound impact on our society. The invention of liquid crystal displays (LCD), the development of e-ink used in Kindles via charged colloidal particles, and the potential use of shear-thickening suspensions to “top kill” disastrous blowout in oil wells are just a few examples that highlight the fascinating applications of soft materials. In each case, the underlying mechanism is governed by the unique structures of soft materials on mesoscopic length scales between the molecular and the macroscopic. These mesoscopic structures can readily respond to stimuli such as weak electromagnetic fields, shear, acoustic vibrations or even thermal fluctuations. Thus, understanding the interplay between mesoscopic structures of a soft material and its bulk material properties, as well as controlling structural transitions through external forces, becomes the key for advancing our soft material research.
Our study concentrates on three forefront areas of soft material research. (1) Sheared colloidal suspensions and biological fluids. By employing various methods such as confocal microscopy, light scattering and rheometry, we study flow properties of suspensions composed of micron/nano-scale particles; as an extension, we also investigate the shear-induced dynamics of biological fluids such as suspensions of red blood cells or swimming bacteria. (2) Granular flows with applications to geological problems. Using the state-of-art high-speed imaging techniques for discrete granular particles and fluid flows, we investigate granular flows in avalanches and under shear. We apply our lab-scale experiments to the understanding of emergent patterns in the geo-scale of natural granular flows. (3) Glass/jamming transition of soft materials. Large classes of materials ranging from simple glass-forming liquids, polymer melts, to even shaving foams, show a similar transition between a flowing fluid-like state and a disordered solid state. These transitions in different systems can be unified with the generic jamming phase diagram. By developing a novel setup, “confocal rheoscope”, which couples a confocal microscope to a mechanically deformable cell, we investigate the effect of mechanical perturbations on the jamming transition. The goal of our research is to control and furthermore to design soft materials with desirable material properties, based on the understanding of their mesoscopic or microscopic structures.
- DARPA Young Faculty Award, 2016
- 3M Non-Tenured Faculty Award, 2016
- McKnight Land-Grant Professorship, 2016-2018
- Packard Fellowship, 2015
- NSF CAREER Award, 2015
- Grainger Fellowship, 2006
- Gaurang and Kanwal Yodh Prize, 2006
- B. Zhang and X. Cheng, “Structures and dynamics of glass-forming colloidal liquids under spherical confinement”, Phys. Rev. Lett. 116, 098302 (2016). (Selected as “Editors’ Suggestion” with Synopsis)
- Y. Peng, L. Lai, Y.-S. Tai, K. Zhang, X. Xu, and X. Cheng, “Diffusion of ellipsoids in bacterial suspensions”, Phys. Rev. Lett. 116, 068303 (2016). (Selected as “Editors’ Suggestion”)
- R. Zhao, Q. Zhang, H. Tjugito, and X. Cheng, "Granular impact cratering by liquid drops: Understanding raindrop imprints through an analogy to asteroid strikes", Proc. Natl. Acad. Sci. USA 112, 342 (2015).
- B. D. Leahy, X. Cheng, D. C. Ong, C. Liddell-Watson, and I. Cohen, “Enhancing Rotational Diffusion Using Oscillatory Shear”, Phys. Rev. Lett. 110, 228301 (2013).
- X. Cheng, X.-L. Xu, A. R. Dinner S. A. Rice, and I. Cohen, “Assembly of vorticity- aligned hard-sphere colloidal strings in a simple shear flow”, Proc. Natl. Acad. Sci. USA 109, 63 (2012)
- X. Cheng, J. H. McCoy, J. N. Israelachvili, and I. Cohen, “Imaging the Microscopic Structure of Shear Thinning and Thickening Colloidal Suspensions”, Science 333, 1276 (2011)
- X. Cheng, “Packing structure of a two-dimensional granular system through the jamming transition”, Soft Matter 6, 2931 (2010)
- X. Cheng, “Experimental study of the jamming transition at zero temperature”, Phys. Rev. E 81, 031301 (2010)
- L.-N. Zou, X. Cheng, M. L. Rivers, H. M. Jaeger, and S. R. Nagel, “The Packing of Granular Polymer Chains”, Science 326, 408 (2009)
- X. Cheng, L. Xu, A. Patterson, H. M. Jaeger, and S. R. Nagel, “Towards the zero-surface- tension limit in granular fingering instability”, Nature Phys. 4, 234 (2008)
- X. Cheng, G. Varas, D. Citron, H. M. Jaeger, and S. R. Nagel, “Collective Behavior in a Granular Jet: Emergence of a Liquid with Zero Surface Tension”, Phys. Rev. Lett. 99, 188001 (2007)
- X. Cheng, J. B. Lechman, A. F. Barbero et al., “Three-Dimensional Shear in Granular Flow”, Phys. Rev. Lett. 96, 038001 (2006).