Investigation of liquid droplet and liposome spreading on smooth and patterned substrates

Abstract

In this project, the spreading of liquid droplets and giant liposomes is investigated experimentally and numerically within the scope of continuum physics. Five liquids of different properties in their surface tension and viscosity are used, including deionized (DI) water, SDS (at the critical micelle concentration (CMC)), SDS (10% CMC), glycerol, and water-glycerol mixture (volume ratio 1:1). Liposomes are prepared from DOPC and the mixture of DOPA and DOPC (1:9). Isotropically patterned substrates and anisotropically patterned substrates are prepared to study the effect of surface patterns on spreading. The substrates for droplet spreading are fabricated on shape memory polymer through wrinkling, while those for liposome spreading are on silicon wafers by photolithography and dry etching. The wetting and spreading of liquid droplets and giant liposomes is characterized by the equilibrium contact angles, the equilibrium droplet/liposome dimensions, the dynamic change of contact angles, and the dynamic evolution of droplet/liposome dimensions. On the smooth substrates, liquid droplets and giant liposomes exhibit the shape of a spherical cap. The contact angles and the contact radii are the same along the contact line. Less viscous liquids spread faster than more viscous liquids. After adding surfactant into DI water, SDS (CMC) droplets need more time than DI water to reach the equilibrium states. The spreading speed decreases with the liquid viscosity and increases with the surface tension. On the isotropically patterned substrates, water droplets have similar equilibrium contact angles as on the smooth substrates, showing that the small surface roughness does not significantly affect droplet spreading. On the anisotropically patterned substrates, First and foremost, I would like to express my sincere and warmest thanks to my supervisors, Prof. Chen Yan and Prof. Fan Hui, for their guidance, encouragement, and support during my PhD period. I appreciate their patience and help. Besides, I would like to thank Prof. Huang Weimin, Prof. Yang Chun and Prof. Duan Fei for their support. I am grateful to Dr. Guillaume Tresset for his help in my first year of PhD candidature and his kindness to give me valuable suggestions. I greatly appreciate the help from Dr. Kaori Kuribayashi who shared with me her experience in electroformation of giant liposomes. Deep appreciation and gratitude go to the technical staffs at Robotics Research Center and Center of Mechanics of Micro-Systems. I enjoy the time spent at these two laboratories. I am also grateful to the technical staffs at Materials Laboratory 1