Author ORCID Identifier
Year of Publication
Doctor of Philosophy (PhD)
Chemical and Materials Engineering
Dr. Jonathan T. Pham
Dr. Matthew J. Beck
Friction and adhesion of soft materials are important for pressure sensitive adhesives, biomaterials, and soft robotics; however, the behavior on the microscale is not fully understood. When two objects come into contact, their interactions are usually mediated by small contact points due to surface roughness. At the microscale size, surface forces can deform soft materials to minimize energy by increasing the contact area, which is balanced by the elastic deformation of the polymer network. However, for soft, crosslinked materials with a modulus below ~100 kPa, it is challenging to predict the behavior with prior contact and friction models. Additionally, lightly crosslinked, polymeric materials often contain an uncrosslinked liquid portion (free chains), which complicates the contact and friction behavior. In this thesis, we combine confocal microscopy, colloidal probe microscopy, and lightly crosslinked polydimethylsiloxane (PDMS) to investigate microscale contact and friction mechanisms of soft materials.
To better understand the lightly crosslinked PDMS, we first characterize its material properties, with and without free chains. The PDMS is made using a commercially available Sylgard 184 kit with different mixing ratios of the base to curing agent. To test the material without free chains, we develop a simple and inexpensive extraction method that enables the removal of free chains from a lightly crosslinked PDMS sheet, while retaining its geometry. We then compare the modulus, strain at break, and hysteresis response before and after free chain removal. We find that the modulus, maximum stretchability, and dissipation increase upon extraction.
After characterizing the material, we use a confocal microscope to investigate the contact mechanics of a glass microsphere on ~5 kPa PDMS, when a layer of oil is present. Decreasing the amount of oil on the surface increases how far a glass microsphere indents into the substrate, due to a liquid capillary force from the oil. To predict the indentation depth, we propose a simple model that balances the capillary force of the oil layer and the particle-substrate adhesion with the elastic and surface tension forces of the substrate. Both the JKR (Johnson-Kendall-Roberts) and Hertz contact models are compared as the basis to describe contact. Our results show that minimal solid adhesion exists in the presence of the oil layer, and that surface stress needs to be considered to describe the contact.
In the last sections of this dissertation, we investigate friction by moving the PDMS surface laterally while measuring forces in-situ with an atomic force microscope (AFM). We find that folding occurs with a sufficiently high lateral resistance, although the folds themselves do not increase the peak lateral force. Through finite element analysis (FEA) from a collaboration, experimental surface treatments, and in-situ AFM-confocal studies, we show that adhesion energy alone is not sufficient to predict the nucleation of folds or the lateral force. Additionally, we explore the periodicity of the folds and stick-slip response during the kinetic portion of the friction curve. In summary, this dissertation quantifies the effect of free chains on the mechanical properties of lightly crosslinked PDMS, proposes a contact model for when an oil layer is present, and discovers a folding mechanism that enables lateral motion across soft materials, offering new insight into microscale soft contact and friction.
Digital Object Identifier (DOI)
The work in this dissertation was primarily supported by the National Science Foundation (CMMI-1825258) from 2018-2022. Additional support was provided by a subaward through the KY-NSF-EPSCoR (OIA-1355438) research scholars program for some undergraduate efforts in 2018, and startup funds from University of Kentucky from 2017 to 2018.
Glover, Justin, "MICROSCALE CONTACT AND FRICTION OF LOW MODULUS, LIGHTLY CROSSLINKED POLYDIMETHYLSILOXANE" (2022). Theses and Dissertations--Chemical and Materials Engineering. 140.