From elasticity to capillarity in soft materials indentation

Abstract

For soft materials with Young's moduli below 100 kPa, quantifying mechanical and interfacial properties by small scale indentation is challenging because in addition to adhesion and elasticity, surface tension plays a critical role. Until now, microscale contact of very soft materials has only been studied by static experiments under zero external loading. Here we introduce a combination of the colloidal probe technique and confocal microscopy to characterize the force-indentation and force-contact radius relationships during microindentation of soft silicones. We confirm that the widespread Johnson-Kendall-Roberts theory must be extended to predict the mechanical contact for soft materials. Typically a liquid component is found within very soft materials. With a simple analytical model, we illustrate that accounting for this liquid surface tension can capture the contact behavior. Our results highlight the importance of considering liquid that is often associated with soft materials during small scale contact.

Fractography of poly(N-isopropylacrylamide) hydrogel networks crosslinked with mechanofluorophores using confocal laser scanning microscopy

“…2,3 To overcome these challenges, it is important to develop more sophisticated experimental methodologies and characterization techniques. Conventional measurement methods can be adapted for the mechanical characterization of soft polymer materials to determine their elastic behavior and response, based on the principles of indentation 4,5 or cavitation rheology. 6 Concomitantly, the traditional fractography methods, such as atomic force or electron microscopy probe the fracture surface of materials, but are time-consuming and only work post-mortem.…”

Extracting uncrosslinked material from low modulus sylgard 184 and the effect on mechanical properties

“…For example, studies on soft adhesion, wetting, cavitation, and cell–surface interactions frequently implement PDMS as a model material. PDMS is used because it is commercially available (e.g., Sylgard 184), easy to prepare, and easy to tune the elastic modulus from a few megapascal down to a few kilopascal . This simple modulus control has enabled the growing and vast use of commercial PDMS elastomers with moduli below 100 kPa .…”

“…PDMS is used because it is commercially available (e.g., Sylgard 184), easy to prepare, and easy to tune the elastic modulus from a few megapascal down to a few kilopascal . This simple modulus control has enabled the growing and vast use of commercial PDMS elastomers with moduli below 100 kPa . In order to decrease the modulus, the amount of crosslinking agent is reduced relative to the reactive silicone base in many commercial elastomers, which come as two‐part kits.…”

Surface elastic constants of a soft solid

“…In this regime, the significant force contribution from surface stress, Υ, fundamentally alters experimental behavior, and consequently the interpretation of many materials characterization techniques. Examples where surface stress can play an important role include atomic force microscopy probing of nanowire interfaces [13,14], indentation tests [3,[15][16][17] and wettability measurements of soft solids [18][19][20][21][22][23][24].…”