Direct Visualization of Constant Stress Deformation in Viscoelastic Materials -- Speaker: Hubert Chan, Ph.D. Candidate, UCI

Complex materials such as colloidal gels, glasses, and emulsions are traditionally utilized in the food, ceramics, and cosmetics industries as well as a number of emerging technologies including direct-write assembly. The decision to incorporate these soft materials into various applications is often attributed to their complex deformation behavior in response to external forces. However, the underlying origins of these phenomena remain poorly understood without the ability to characterize in detail how the microstructure of these materials evolves during deformation. We introduce a custom-built, stress-controlled shear cell coupled to a fast-scanning confocal microscope for direct visualization of constant-stress shear deformation in soft materials and complex fluids. We model and experimentally demonstrate the performance of our device in both steady state and transient experiments with different viscoelastic materials. Our apparatus can conduct unidirectional constant-stress experiments as accurately as most commercial rheometers, with the capability to directly visualize the flow field. Further, our step-stress experiments on viscoelastic materials are devoid of creep-ringing, which is an advantageous aspect of the viscous coupling between the torque generation mechanism and the viscoelastic sample. Most recently, we utilize the device to capture and contrast the dynamic events that trigger the transition from slow, prolonged creep to liquid-like flow in both repulsive and attractive glasses.

Bio:

Hubert Chan is a fifth-year graduate student working toward a Ph.D. in chemical engineering at UCI under the guidance of Professor Ali Mohraz. His research interests involve uncovering the relationship between the rheology and microstructure of complex colloidal systems. Hubert received his bachelor’s degree from UC Berkeley in 2008 as a double major in chemical engineering and materials science and engineering. Prior to his arrival at UCI, he was an applications engineering intern at KLA-Tencor, where he characterized the functional capabilities of surface profilometers. Hubert also currently holds the position of outreach coordinator in the CHEMS graduate student association.