Authors:	Li, Shuai
Advisor:	Butt, Hans-Jürgen
Title:	Droplet dynamics on polydimethylsiloxane brushes
Online publication date:	14-Aug-2023
Year of first publication:	2023
Language:	english
Abstract:	Surfaces with super liquid-repellency properties are critical in many industrial and domestic applications because of their low lateral adhesion towards water, ice, snow, and microorganisms. Attributed to their excellent liquid repellency, superhydrophobic surfaces and lubricant-infused surfaces (where lubricant liquid is infused into texted substrates) have attracted considerable attention over the past few decades. As an alternative, polymer brushes, which consists of one-end-fixed polymer chains on substrate, are also demonstrated to have low sliding angles. Due to the non-fluorinated and more liquid-like characteristics, polydimethylsiloxane (PDMS) brushes is one promising type of polymer brushes among others, e.g. perfluoropolyether brushes and alkyl brushes. This thesis describes droplet dynamics on PDMS brushes and its potential applications in water harvesting, anti-icing, droplet manipulation, condensation heat transfer, and slide electrification. First, I describe a novel strategy to reduce the water/ice lateral adhesion on PDMS brushes by introducing organic vapor, e.g. toluene vapor, in the surrounding environment. The vapor makes PDMS brushes polymer chains more liquid-like thus it exhibits lower sliding angles for water droplets. The evidence of such flexible layer is obtained using atomic force microscopy. The condensation experiments in the presence of toluene vapor shows the greatly reduced departure droplet sizes, at which sizes water droplets start to slide on the vertically oriented surface. The significantly increased water collection rate indicates its strong potential in the water harvesting applications. Further measurements show that the vapor lubrication can also be used for droplet manipulation and anti-icing applications. To explain the mechanism for vapor lubrication, I conduct contact angle measurements on three different hydrophobic surfaces (including PDMS brushes) in different vapor environment (n-hexane, toluene, cyclohexane, air, dimethyl sulfoxide, tetrahydrofuran, and ethanol). In water-soluble vapors, a substantial decrease is observed in contact angles (θ). The contact angles are explained well by Young’s equation, cos θ = (γSV – γSL) / γLV, where γSV, γSL, γLV denotes the solid-vapor, solid-liquid, and liquid-vapor interfacial surface tension. The decrease of θ is attributed to the vapor induced change in the interfacial tensions. However, the low contact angle hysteresis (difference between advancing and receding contact angle) on PDMS surfaces in saturated n-hexane and toluene vapor cannot be explained by Young’s equation. The observation supports the hypothesis that these vapors adsorb into the PDMS and form a lubricating layer. Next, I investigate the condensation heat transfer performance on PDMS brushes by using ethanol-water mixtures. The purpose of the mixture is to optimize the surface tension and latent heat of vaporization. On PDMS brushes, all mixtures show a low contact angle hysteresis of less than 10°. So it is highly possible that condensate will form discrete droplets (dropwise condensation) instead of a covering liquid film (filmwise condensation) on the surface. The condensation measurements in microscale show that the droplet nucleation and growth are substantially enhanced by mixing water and ethanol. The further heat transfer measurements show that the thermal performance is greatly enhanced by more than one order of magnitude when compared to heat transfer coefficient of filmwise condensation. The transition from dropwise to filmwise condensation is also successfully delayed when compared to that on fluorinated surface. It is well known that water droplets running down a hydrophobic surface become positively charged and leave behind a negative surface charge. Finally, I study this spontaneous charge separation on lubricant-infused PDMS brushes by measuring the charge of continues water drops sliding over the surface. On a fluorinated hydrophobic surface or PDMS brushes the charge per drop decreases with the increased drop number. However, on lubricant-infused PDMS brushes, the charge per drop starts from 0, then it increases. After reaching a maximum value, the charge per drop decreases before reaches a stable value. Such interesting phenomenon is due to the depletion process of the lubricant on the surface, where the surface conductivity is changed during this process. The investigation here provides a fundamental understanding of drop charge on lubricant-infused surfaces. I hope this may contribute to the applications in the further step. The projects highlighted the significant role of droplet dynamics and polydimethylsiloxane brushes surface in applications such as anti-icing, drop manipulation and condensation heat transfer.
DDC:	000 Allgemeines 000 Generalities 540 Chemie 540 Chemistry and allied sciences
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 09 Chemie, Pharmazie u. Geowissensch.
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-9333
URN:	urn:nbn:de:hebis:77-openscience-7e818cc6-8393-440b-8867-49ae3309b4f83
Version:	Original work
Publication type:	Dissertation
License:	CC BY
Information on rights of use:	https://creativecommons.org/licenses/by/4.0/
Extent:	X, 136 Seiten ; Illustrationen, Diagramme
Appears in collections:	JGU-Publikationen