Effects of particle roughness on the rheology and structure of capillary suspensions*

Capillary suspensions are three-phase mixtures consisting of particles in a bulk liquid, that are connected by liquid menisci of a secondary liquid. These capillary suspensions have promising applications as precursors for ceramic materials or in the development of new low-fat foods [1].

Since particle roughness has been shown to drastically influence the rheological properties of concentrated suspensions due to interlocking asperities, as well as the wetting behavior of Pickering emulsions by altering the three-phase contact angle or by introducing contact line pinning [2], we study here the effects of particle roughness on capillary suspensions, which combine the themes of particles in contact with capillary bridges. For this study, silica nanoparticles with sizes between 40 nm and 200 nm are electrostatically adsorbed onto silica microparticles to create fluorescently labeled, raspberry-like particles with varying roughness [2]. Using both confocal microscopy and rheology, the network microstructures and liquid bridge sizes are linked to the linear viscoelastic response of the suspensions.

With increasing roughness, more secondary liquid is trapped in the particle asperities and rendered unavailable for liquid bridge formation, leading to a decrease in network clustering and viscoelastic moduli. By increasing the relative amount of secondary liquid for more rough particles, the same viscoelastic properties as compared to the smooth particles are reproduced, but with slightly lower values for coordination number and clustering coefficient. Furthermore, using an asymptotically nonlinear oscillatory rheology measurement protocol, similar to Natalia et al. [3], we investigate the non-cubical power law scaling of the third harmonic in the shear stress response that results from both Hertzian contacts and friction between particles connected by capillary bridges. A scaling of the power law exponents according to load-controlled friction is obtained, contrary to the adhesive-controlled friction found by Natalia et al. [3].

References
[1] E. Koos, Current Opinion in Colloid & Interface Science, 2014, 19, 575–584.
[2] M. Hu, C.-P. Hsu, and L. Isa, Langmuir, 2020, 36, 11171-11182
[3] I. Natalia, R.H. Ewoldt, and E. Koos, Soft Matter, arXiv:2104.05678