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Abstract When a water drop is placed onto a soft polymer network, a wetting ridge develops at the drop periphery. The height of this wetting ridge is typically governed by the drop surface tension balanced by elastic restoring forces of the polymer network. However, the situation is more complex when the network is swollen with fluid, because the fluid may separate from the network at the contact line. Here we study the fluid separation and network deformation at the contact line of a soft polydimethylsiloxane (PDMS) network, swollen with silicone oil. By controlling both the degrees of crosslinking and swelling, we find that more fluid separates from the network with increasing swelling. Above a certain swelling, network deformation decreases while fluid separation increases, demonstrating synergy between network deformation and fluid separation. When the PDMS network is swollen with a fluid having a negative spreading parameter, such as hexadecane, no fluid separation is observed. A simple balance of interfacial, elastic, and mixing energies can describe this fluid separation behavior. Our results reveal that a swelling fluid, commonly found in soft networks, plays a critical role in a wetting ridge. 

Abstract In fully transient, mussel‐inspired hydrogels, metal‐coordinate complexes form supramolecular crosslinks, which offer tunable viscoelastic properties and mechanical reversibility. The metal‐coordination complexation that comprises the crosslinks can take on tris‐, bis‐, mono‐, and free‐state modalities (3, 2, 1, or 0 ligands per ion, respectively). Although prior work has established relationships between network crosslinking and mechanical properties, the effect of crosslink and ligand modalities on gel‐surface adhesion is not well understood for fully transient hydrogels. Using glass and nickel‐coated spherical probes, the effect of network crosslinking modalities on the adhesive strength of hydrogels based on histidine‐Ni²⁺and nitrodopamine‐Fe³⁺ion crosslinks is investigated. Since crosslink modalities have a strong impact on the mechanical properties of the bulk network, it is first determined how adhesion relates to the mechanical properties, regardless of the distribution of crosslinking modalities and ligand type. It is ultimately found that the peak adhesive stress increases with decreasing percentage of ligands in tris‐crosslinks. 

ABSTRACT Commercial silicone elastomers are commonly used in soft materials research due to their easily tunable mechanical properties. However, conventional polydimethylsiloxane (PDMS) elastomers with moduli below ∼100 kPa contain uncrosslinked free molecules, which play a significant role in their behavior. To utilize these materials, it is important to quantify what role these free molecules play in the mechanical response before and after their removal. We present a simple and inexpensive extraction method that enables the removal of free molecules from a lightly crosslinked sheet of Sylgard 184, a commercially available PDMS elastomer. The materials can contain a majority of free molecules yet maintain a thin and flat geometry without fractures after extraction. Subsequently, we compare the modulus, maximum stretchability, and hysteresis behavior with mixing ratios ranging from 60:1 to 30:1, before and after extraction. We show that the modulus, maximum stretchability, and dissipation increase upon extraction. Moreover, our approach offers a route to prepare crosslinked silicone elastomers with a modulus as low as ∼20 kPa without free molecules from a commercially available kit. © 2020 Wiley Periodicals, Inc. J. Polym. Sci.2020,58, 343–351