Séminaire régulier IUSTI – 21 sept. 2018 – 11h salle 250

Surface roughness: a key property for the design of soft materials

Lucio Isa – ETH, Zurich

Surface roughness has a strong impact on many aspects of colloids, from affecting depletion and capillary interactions, to controlling their ability to be dispersed in various solvents. Classic studies typically link surface topography to macroscopic material properties, falling short of elucidating the microscopic link between the two. We tackle this problem by synthesizing a broad library of model all-silica colloids with tunable surface roughness and controlled surface chemistry [1]. We then carefully characterize their surface properties and investigate the impact that their roughness has on both adsorption at fluid interfaces and on their flow as dense suspensions. Surface roughness can be engineered to control the discontinuous shear thickening (DST) of dense suspensions. Although macroscopically well characterized, the microscopic origin of DST is still debated, especially in connection to particle contacts. We elucidate here the mechanisms underpinning DST by carrying out nano-tribological measurements of the inter-particle contacts of our model rough colloids. We demonstrate that our rough particles exhibit DST over a broader range of shear rates and for volume fractions much lower (<15%) than for smooth colloids, due to interlocking of surface asperities [2]. Akin to contact-line pinning for macroscopic droplets on structured surfaces, we then also demonstrate that surface roughness strongly pins particle contact lines and arrests their adsorption at fluid interface in long-lived metastable positions. Such pinning imparts tremendous contact angle hysteresis, which can practically invert the particle wettability, irrespective of their chemical nature, and leads to a new route for emulsion stabilization [1]. These results demonstrate that our approach to elucidate the effect of general system properties rather than focusing on a specific class of soft materials is a powerful route to connect seemingly distinct phenomena that share a common core. Following the example of roughness, similar paths can be followed for other general properties, such as softness or asymmetry, where a broad range of new discoveries awaits. References [1] M Zanini, C Marschelke, S Anachkov, E Marini, A Synytska and L. Isa “Universal emulsion stabilization from the arrested adsorption of rough particles at liquid-liquid interfaces”, Nature Communications, 8, 15701, 2017 [2] C-P Hsu, SN Ramakrishna, M Zanini, ND Spencer and L Isa “Roughness-dependent tribology effects on discontinuous shear thickening”, Proc. Natl. Acad. Sci. U.S.A., 115 (20), pp. 5117-5122, 2018