Title: Insights of Evaporating Dynamics of a Sessile Droplet on a Chemically Patterned Surface
Speaker: Prof. Huihe Qiu，The Hong Kong University of Science & Technology
Time: Wednesday, Jan 17, 2018, 16：00~17：30
Venue: Meeting room 625, Hongsheng Technology Building, Wushan Campus
Recently, chemically patterned surfaces has attracted a lot of attention owing to their special features in controlling contact line dynamics and evaporation of sessile droplets. Though significant progress has been made in studying contact line dynamics and evaporation of a sessile droplet on homogeneous surfaces, many unanswered questions remain for chemically patterned surfaces. Furthermore, the evaporation of a multicomponent droplet on a chemically patterned surface is more complex due to local Marangoni force variations. In fact, the pinning and depinning of a sessile droplet on chemically patterned surfaces are determined by the interplay of the interfacial free energies of the droplet and the wettability pattern of the surfaces. The interplay can give rise to morphological transitions upon changing the volume/concentration of the droplet or the wettability contrast and thus the contact angle on the substrate.
In this talk, the questions of the effects of chemically patterned surfaces on the evaporation of single/multi-component sessile droplets are addressed. We first report the evaporation dynamics of droplets on hydrophobic-network surfaces and found out that the processes can be divided into different stages: constant contact line (CCL), constant contact angle (CCA), pattern-pinning (PP) and moving contact line (MCL) stages. We developed analytical models for accurately predicting the transition of evaporation stage (critical receding contact angle). It is shown that when the contact line is pinned on a hydrophobic−hydrophilic boundary, different contact angles can be interpreted as the variation of the length of the contact line occupied by each component. Furthermore, each evaporation stage can be successfully predicted by using this newly developed models. The effective contact line length that affects the evaporation rate is analyzed taking the influence of chemically patterned surfaces into consideration. The contact line dynamics, fluid flow and species dynamics of an evaporating multicomponent droplet on chemically patterned surfaces are reported utilizing a novel AIEgen-based direct visualization, high speed visualization, micro resolution PIV and ray tracing techniques. Utilizing the experimental and analytical results, the mechanisms behind contact line pinning/depinning and the critical receding contact angles under different species concentrations on chemically patterned surfaces are discussed in details on the microscale.
Professor Huihe Qiu is currently Professor and Associate Head in the Department of Mechanical and Aerospace Engineering at The Hong Kong University of Science & Technology. Professor Qiu received his Ph.D. degree from Institute of Fluid Mechanics, LSTM, at the University of Erlangen, Germany in 1994. Professor Qiu’s research areas are in, multiphase flow and heat transfer, fluid dynamics, optical diagnostics, nano- and microfluids and flapping wing aerodynamics. Professor Qiu is Editor-in-Chief/Editor/Associate Editor of four international journals and a member of the editorial board for more than 10 international journals, such as the members of Editorial Advisory Board of Experiments in Fluids. He has been invited to give 21 plenary and keynote speeches in International Conferences. He is the recipient of the Best Paper Award of Institute of Physics (IOP) in 1994, Philips Outstanding Paper Award in the International Conference on Electronic Packaging Technology and High Density Packaging (2012), ASME Best Poster Award (2010), Best Paper Award, 2nd World Congress on Mechanical, Chemical, Material Engineering (2016), Best Paper Award, 4th International Conference on Heat Transfer and Fluid Flow (2017), The State Scientific and Technological Progress Award (SSTPA) and the Scientific and Technological Achievement Award from the State Education Commission.