Date Available

1-19-2023

Year of Publication

2023

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Engineering

Department/School/Program

Materials Science and Engineering

Advisor

Dr. Jonathan T. Pham

Co-Director of Graduate Studies

Dr. Matthew J. Beck

Abstract

Soft, slippery surfaces have gained increasing attention due to their wide range of potential applications, for example in self-cleaning, anti-fouling, liquid collection, and more. One design approach in creating slippery surfaces is using a swollen elastomer, which is a polymer network swollen with a lubricant. This type of surface may be beneficial for longer-term use than standard lubricant-infused surfaces, and provides a versatile surface with tunable mechanical properties. Hence, understanding the physics of soft surface interactions is important for fundamental soft matter physics, biomaterials, adhesives, and coatings. This research experimentally investigates wetting on soft infused networks, with the aim of providing insight towards a physical description of wetting on a low-modulus, swollen elastomer.

In principle, when a water drop is deposited on a soft surface, the surface tension of water can pull up the surface and form a wetting ridge at the drop periphery. However, when the soft material surfaces are also swollen with a fluid, the imbibed fluid may separate from the surface at the drop periphery to minimize the elastic energy that is caused by network stretching. Although significant efforts have been put into understanding the wetting ridge formation on soft crosslinked solids, as well as designing slippery, lubricant-infused porous surfaces (SLIPS), there is limited knowledge on soft and swollen surfaces. We bridge soft crosslinked solids and SLIPS by studying how infusing a soft crosslinked network with lubricant affects drop interactions, with a focus on low modulus elastomers (e.g. ~100 kPa or less). Specifically, by using silicone oil–swollen polydimethylsiloxane (PDMS) elastomers, we investigate situations of both static and dynamic wetting. My research is mainly divided into two integrated sections, briefly described below.

In the first part of this work, we present an approach to visualize a crosslinked network and its swelling fluid separately by employing fluorescent molecules and confocal microscopy. We can vividly see the PDMS network and the oil phases separately at the contact line. We systematically investigate how the degrees of crosslinking and swelling affect fluid separation and network deformation during wetting on swollen networks. The degree of swelling is varied from đť‘„=1 (dry) to saturation (maximum swelling) for four different degrees of crosslinking. Upon swelling the network to lower swelling ratios, the height of the wetting ridge increases due to the decreased modulus from swelling. As the degree of swelling increases further however, fluid clearly separates from the network. By continuing to increase the degree of swelling towards saturation, the amount of fluid separation increases. Qualitatively, the general trend of an initially increasing wetting ridge height followed by fluid separation is universal, regardless of the degree of crosslinking. Our experiments reveal that the static wetting ridge of a soft and swollen network can comprise both a region of network pullup and a region of pure fluid; this suggests that the swelling fluid, commonly found in soft networks, can play a critical role on surface wetting properties.

In the second part, we transfer our focus from static wetting to dynamic wetting conditions to investigate the slippery properties of soft, swollen elastomers. In the dynamic wetting state, we study how the polymer network density and degree of swelling related to the pinning force and friction of a water drop. We first study when a water drop sticks or slides on a vertical, silicone oil–swollen PDMS elastomer, where gravity drives the drop to slide down while surface interactions promote drop sticking. Hence, a critical drop volume exists directly when gravity overcomes drop-surface adhesion forces. We find that the critical water drop volume for sliding decreases when increasing the degree of swelling, nearly independently from crosslinking for very soft elastomers. This is likely associated with oil separating from the bulk substrate when it encounters a water drop, lubricating the surface and decreasing the pinning force. Additionally, we develop a cantilever-based approach to measure lateral friction forces between drops and soft surfaces, while observing the water-elastomer contact line with confocal microscopy. Results show the friction force decreases when increasing the degree of swelling, which is a function of velocity.

In summary, the results demonstrate the importance of considering the fluid inside of gels when investigating wetting of soft surfaces, offering fundamental insight into soft wetting, and into the slippery properties of soft, oil-swollen elastomers.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2023.033

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