UROP Proceedings 2020-21

School of Science Department of Physics 75 Experimental Study of The Fast Dynamics in Soft Matter Physics Supervisor: XU Qin / PHYS Student: PENG Huaiyue / PHYS Course: UROP1100, Summer Soft materials often exhibit strong mechanical response to external perturbation in a short period. Many dynamic processes, including droplets impact, pinch-off, and coalescence involve complex mechanics in a time period of a few milliseconds. When a solvent enters the network of crosslinked polymer chains in the gel by osmosis, the gel would experience volumetric growth. Due to the constraint of containers, it will lead to creases and wrinkles on the surface of soft materials. We try to explore the basic nature of instabilities, including their length and distribution. In this program, we will study the characteristics of surface instabilities and use fast camera to study the droplets impact on these instabilities. Interfacial Instabilities of Soft Material Supervisor: XU Qin / PHYS Student: GURUNG Nishan / MATH-GM Course: UROP1100, Summer Swelling Polydimethysiloxane (PDMS) gels can affect their surface properties substantially, which can be of significant research interest given the vast applicability of PDMS. Swelling under surface constraints gives rise to surface instabilities thus, forming micro-patterns. These surface patterns are not well- understood so in this project, we vary parameters such as solvent swelling power, swelling duration, sample stiffness and thickness, and investigate their effects on the observed patterns, including their peak heights. We also report a surprisingly large mass extraction rate via swelling, with the extracted material having a viscosity 6x higher than that of the uncross-linked polymer. This indicates that the swelling process not only extracts uncrosslinked polymers, like we normally expect, but also some shorter cross-linked polymer chains. Excitation and imaging of Terahertz phonon-polaritons Supervisor: ZHANG Jingdi / PHYS Student: CHENG Chun Hei / SSCI Course: UROP1000, Summer The goal of this project is to study the effect of a sample placed on a crystal on the propagation of phononpolariton in the crystal using FDTD (finite difference time domain) simulation which is helpful for the development of phononpolariton spectroscopy. In this UROP 1000 programme, the dispersion relations of LiNbO3 in ordinary and extraordinary polarizations are obtained from the 2D FDTD simulation of phononpolaritons in the crystal. It is found that using excitation pulses with duration of 1020fs can give the best result for dispersion relation in LiNbO3. The results in this report narrows down the pulse duration range for future development of the simulation and allows comparison with theoretical results to examine the accuracy of simulation.

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