Fabrication and mounting of near-critical silicone gels for mechanical testing of colloidal solids

We describe a shear cell and associated sample preparation methods to be used to measure mPa-scale shear stresses using a confocal microscope. The shear cell can be mounted on an inverted confocal microscope that is used to image the deformation of a calibrated polymer gel with embedded tracers, from which the shear stress is determined. The gels have shear moduli G ∼ 1-10 Pa, which, when combined with the high strain resolution afforded by single-particle locating of the embedded tracers, are sufficiently compliant to measure mPa stresses. While these exceptionally compliant gels are necessary to achieve mPa stress resolution, they are easily deformed by surface tension, leading to several technical challenges with their fabrication and functionalization. To overcome these challenges, a fabrication technique using sacrificial molds is described, alongside processing steps to functionalize the gels without distorting the surface. While the apparatus presented is designed specifically for simple shear deformation of soft solids, the processing and fabrication techniques can be employed to mold and fabricate compliant gels in other geometries.