In-situ Raman spectroscopy for soft contacts

Contacting surfaces often experience deformations across multiple length scales. These deformations become particularly complex when one of the materials is soft, such as elastomers.Elastomers are widely used in flexible electronics and intelligent interactive systems, where their surface deformability facilitates the acquisition of multimodal physical information. Therefore, precise identification and effective monitoring of soft contact deformation are crucial. Despite the significant physical insights provided by existing imaging and theoretical tools, inherent approximations affect their precision and granularity. We hypothesize that mapping chemical (molecular) signatures could be a promising approach, as these deformations often originate from molecular properties.

In this work, we aim to uncover the physical and chemical signatures of soft contact deformation using in-situ confocal Raman spectroscopy. We first assembled a calibration experimental setup consisting of five spherical glass probes of equal radius that are fixed onto a glass substrate, with an additional glass substrate placed on top. The four glass spheres placed at the corners provide support, while the contact of the central glass sphere with the upper glass substrate is used for measurements. This calibration experiment established the workflow for subsequent Raman mapping measurements. Next, we replaced the upper glass substrate with a PDMS-coated glass substrate, allowing the PDMS layer to contact the central glass sphere and undergo contact deformation. Due to the optical path of a typical Raman spectroscope, an inverted contact configuration was necessary. We then conducted systematic Raman mapping measurements in the x, y, and z directions in and around the contact region of interest. By analyzing the intensity of the 2905 cm^-1 Raman peak of PDMS, we developed a framework to generate Raman contour maps at different imaging planes. Such Raman mapping embeds crucial chemical signatures of the system that translates into tangible physical insights.

Tracking the variation in the relative intensity of the 2905 cm^-1 Raman peak of PDMS within the contour maps enabled us to monitor the spatial variation of the contacting interfaces at different vertical planes. In doing so, it allowed us to extract physical parameters like contact radius and indentation depths as well as confirm the non-conformal nature of contact deformation. Our experimental findings showed good agreement with the Hertz theory, although we observed subtle deviations indicating localized inhomogeneities at the contacting interfaces.