Reducing drop friction on liquid repellent polydimethylsiloxane-coated surfaces
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Abstract
Liquid repellent surfaces are a kind of surface on which liquid drops can detach or shed from them without residue. Based on such properties, they are promising materials to be used for self-cleaning, enhancing condensation, reducing ice adhesion, thermal cooling and water harvesting. Extra low drop friction is necessary for this surface to have a good liquid repellency in these applications. Polydimethylsiloxane (PDMS) is one kind of environmentally friendly polymerized organosilicon. They are usually used as a coating to enhance the liquid repellency of the hydrophilic substrate. Due to its low glass transition temperature (Tg), it is usually considered to be a soft and flexible material at room temperature.
In this thesis, we focus on the open question on three different PDMS-coated surfaces:
1. A partially cross-linked PDMS coating is the one most common coating with a weak liquid repellency. A ridge forms when drops are placed on it due to the high softness of PDMS. It results in discontinuous stick-slip motion at the three-phase contact line. After being deformed, the formed cracks will also result in a high resistance for drop moving. How to endow a better liquid repellency to it with low drop friction is still a challenge.
2. Liquid PDMS can be used as a lubricant to enhance the liquid repellency on porous surfaces. It is used to fabricate liquid repellent oil-infused surfaces. However, this infusing PDMS layer is usually thick, and it will result in a ridge formation at the three-phase contact line. Combined with the viscous flow near the liquid-liquid interface, moving drops on this oil layer will dissipate more energy than moving at an ideal low-friction surface. Reducing the friction from such dissipation is still a problem.
3. Linear or approximately linear PDMS chain can be anchored on a flat surface to have a nano-scale PDMS coating. They are called PDMS brush or pseudo-brush. Due to its high mobility at room temperature, the surface shows repellency to both water and low-surface tension liquids. Nowadays, there is a lack of work to prepare a highly linear PDMS chain by a fast grafting-from method. Moreover, even though such surfaces prepared from different methods or precursors are reported all to be liquid repellent, the layer properties will vary. It will result in dynamic friction difference when drop sliding off them. Key questions are: on which sample the drop will have the lowest friction? How does the PDMS layer affect the drop friction?
In this thesis, Chapter 1 will give a brief introduction to the basic wetting theory on both rigid and soft surfaces, and the background and concepts in liquid repellent surface design.
Chapter 2.1 is related to the first kind of PDMS. We try to improve the liquid repellency on a cross-linked PDMS coated highly stretchable substrate by adding a rough structure on top of it. In our case, PDMS is coated on a polyisoprene substrate to have a high stretchability (tension strain can be larger than 300 %). The PDMS layer is used as an adhesion layer to fix two-level fluorinated nanofilament structures on it. Water and low-surface-tension liquids can be quickly shed from the surface even though the surface is stretched to a tensile strain of ~ 225 %.
Chapter 2.2 investigates water drop behavior on the liquid PDMS infused surface. Liquid-infused surfaces are one kind of liquid repellent surface which highly reduces static friction. Drops can start to slide off the surface with low extra traction. However, a large liquid ridge formed on the thick infusing liquid PDMS layer and the viscous flow near the liquid-liquid interface is high. It increases the dynamic friction force during the drop sliding process. We highly decrease the oil layer on top of the structure by infusing a volatile PDMS/solvent mixture into a porous PDMS modified structure. Since the lubricant layer is thin and fixed by the anchored pre-coated PDMS, ridge formation and interfacial flow are suppressed. It leads to low drop sliding friction and fast drop shedding.
In Chapter 2.3 and 2.4, we focus on nano-scale PDMS coatings. We first report a one-step grafting-from approach to rapidly fabricate linear PDMS brushes on surfaces through spontaneous polymerization of dichlorodimethylsilane. It shows low values of contact angle hysteresis (less than 5º) with most liquids. Then, I tried to study the drop motion on these nano-scale PDMS brush surfaces which are prepared by both the grafting-from and grafting-to methods reported before. The difference in coating thickness resulting from the preparation is found to be the main factor that determines the drop dynamic friction. 4-5 nm thick PDMS layers showed the lowest dynamic friction. Layer inhomogeneities and the PDMS chain stretch are found to be two main mechanisms to affect the friction with the varied thickness.
In conclusion, the reduction of drop friction on soft PDMS-coated surfaces can be achieved through careful design to control energy dissipation during drop shedding, ultimately enhancing the liquid repellency of these surfaces in practical applications. The findings from these studies will inspire us to develop low-friction PDMS-coated surfaces with improved performance across various industrial applications, including microfluidics, biomedical devices, and heat transfer processes in polymer packaging circuits.