Frontiers of Materials Award Symposium: Functional Composition Control of Surface Mechanics in Soft, Water-swollen Gels: Functional Composition Control of Surface Mechanics in Soft, Water-Swollen Gels
Sponsored by: TMS Functional Materials Division, TMS Structural Materials Division, TMS: Biomaterials Committee
Program Organizers: Alison Dunn, University of Illinois Urbana-Champaign
Wednesday 2:00 PM
March 22, 2023
Room: Sapphire D
Location: Hilton
Session Chair: Alison Dunn, University of Illinois Urbana-Champaign
2:00 PM Keynote
Soft Surface Layers on Swollen Gels Mediate Their Contact and Sliding Mechanics: Alison Dunn1; 1University of Illinois Urbana-Champaign
Water content and mobility determine the properties and performance of hydrogels. At the surface of a hydrogel, the swelling causes a soft surface gradient layer due to the boundary with the open bath. We have developed methods to estimate the thickness and character of the gradient layers using instrumented probe techniques at the nanoscale and microscale. In this talk, I will highlight the ways in which this soft gradient layer redistributes the force applied in contacting situations, facilitates robust low friction, and allows for unique adhesive properties.
2:40 PM Invited
Contact Mechanics of Soft Hydrated Materials: Yang Lai1; Dongjing He1; Yuhang Hu1; 1Georgia Institute of Technology
Soft gels and biological tissues resemble similar molecular structures - polymeric networks plus solvents. Mechanical characterization of these materials is challenging because of the extreme softness and complex time-dependent behavior. Comparatively, indentation is practically simple. In this work, we identify the underlying physics that governs the time-dependent behavior of gels and soft tissues and find scaling relations that allow for a unified solution and master curve independent of indenter geometries to be obtained. Besides, we also use indentation to study the adhesion hysteresis of these materials. Combining a unique multi-length scale indentation technique and rigorous theoretical analysis, we can decouple the time-dependent adhesion from the time-dependent bulk behaviors and extract the intrinsic interfacial properties including adhesion energy, separation distance, and cohesive strength. We also show the transition of mechanism from uniform bond breaking to Griffith type of fracture as the contact size goes across a big range of length scales.
3:20 PM Invited
Hydrogel Structure and Surface Physics: Angela Pitenis1; 1University of California, Santa Barbara
Hydrogels are a class of materials broadly defined as three-dimensional crosslinked networks of hydrophilic polymer chains swollen with water. The mesh size of hydrogel networks (similar to the average distance between crosslinks) is thought to control their mechanical and transport properties. Small-angle X-ray scattering (SAXS) measurements of bulk polyacrylamide (PAAm) hydrogels have given ranges of mesh size between 1 and 10 nm, corresponding to water contents from roughly 88 to 98%. However, recent investigations have suggested that it is the superficial structure of hydrogels that controls surface physics (i.e., adhesion, friction) during contact and sliding. Here, we have shown that PAAm hydrogels may exhibit depth-wise gradients in crosslink density following free-radical polymerization across air-water interfaces which is highly sensitive to environmental oxygen.
3:40 PM Break
4:00 PM Invited
Controlling Lubricity of Interfaces with Charged Hydrogels: Rosa Espinosa-Marzal1; 1University of Illinois Urbana-Champaign
With recent developments in soft robotics, novel stimuli-responsive materials are desired for controlling interfacial forces. Hydrogels have been a topic of interest in biotribology for decades due to their biocompatibility, extremely low coefficients of friction, and chemical functional groups similar to those present in biological tribosystems. This work describes several strategies for modulating the balance between physical interactions in the polymer network –via pH, temperature, electrostatic potential, or salt concentration– and thereby, to control the microstructure. Microstructural variations alter the gel’s contact mechanics and thereby friction and/or adhesion in situ.
4:40 PM Invited
Friction of Thin Hydrogel Films: Disentangling the Contributions of Poroelastic Flow and Interface Molecular Interactions: Antoine Chateauminois1; 1ESPCI, CNRS UMR 7615, Sorbonne Université
Friction of water-swollen hydrogels involves a complex interplay between poroelastic flow, elasto-hydrodynamic lubrication and molecular interactions at the contact interface. Here, these issues will be discussed from contacts experiments between model hydrogel thin (~ µm) films grafted on silicon substrates and silanized glass probes, in situations where elasto-hydrodynamic lubrication is prevented. From a combination of dedicated linear sliding and rotational friction experiments with in situ contact visualization, we show that the main contribution to friction of poroelastic flow within the bulk polymer network arises from velocity-dependent changes in the contact geometry. Most of the frictional energy dissipation is found to result from interface effects which are discussed using a modified form of the Schallamach’s model taking into account molecular pinning-depinning events at the glass/gel interface. This approach provides a consistent description of the friction force when the velocity, the normal force and the physical chemistry of the glass probe are varied.