The state of the art of capillary suspensions was described in a review paper published in Current Opinion in Colloid & Interface Science [1], reprinted below in Section A. Our model system for capillary suspensions consists of fairly monodisperse, spherical, silica particles fluorescently labeled with rhodamine B isothiocyanate in a mixture of 1,2-cyclohexane dicarboxylic acid diisononyl ester (Hexamoll DINCH) and n-dodecane, with added aqueous glycerol. Capillary suspensions are prepared using a two-step ultrasonication: an emulsification of both liquids after which the particles are added and sonicated again. The three components are all index matched and the silica contact angle can be modified [2]. The attractive interaction strength can be modified by tuning the contact angle, and fraction of secondary liquid, and, by changing the particle roughness. This lets us access both granular-like systems with weak interactions and strong attractive gels using the same model system.
During the project, we switched from using porous silica particles to nonporous particles as there were problems noted with adsorption of the secondary liquid into the pores. This means the particles are detected on the images as red rings rather than filled circles, but the particle detection algorithm based on edge detection and Hough transform is able to detect both cases. The algorithm now includes a simple graphical user interface for both local detection and manual addition or removal of missing or misdetected particles to improve the final detection efficiency. Using consecutive images on the rheoconfocal, the displacement of the particles can be detected as a function of the applied shear.
Our initial goal of the project was to track microstructural changes in the network in response to external shear applied via a linear shear cell. These structural changes were correlated with the rheological response of the material. Application of external shear via the linear shear cell, however, was unsuitable. Due to the very low yield strain in capillary suspensions, the applied shear was often above the flow point and specific changes during yielding could not be adequately captured. Furthermore, the prior setup only allowed for the deformation profile to be captured in one shear plane.
While this has provided valuable information, proving that capillary suspensions tend to undergo solid-body movement, where the rotation of particles around their respective bridges is resisted through both the structure of the network and the extra torque provided by the contact angle pinning and/or the contact angle hysteresis, full 3D tracking is necessary. Therefore, a rheometer was mounted onto a highspeed confocal microscope in 2022. The improved setup has allowed us to directly compare bulk, rheological changes with local, microscopic changes to the clusters and network, as discussed further in Section 2.3.