Rheo-acoustic gels: Tuning mechanical and flow properties of colloidal gels with ultrasonic vibrations

Colloidal gels, where nanoscale particles aggregate into an elastic yet fragile network, are at the heart of materials that combine specific optical, electrical and mechanical properties. Tailoring the viscoelastic features of colloidal gels in real-time thanks to an external stimulus currently appears as a major challenge in the design of "smart" soft materials. Here we introduce "rheo-acoustic" gels, a class of materials that are sensitive to ultrasonic vibrations. By using a combination of rheological and structural characterization, we evidence and quantify a strong softening in three widely different colloidal gels submitted to ultrasonic vibrations (with submicron amplitude and frequency 20-500 kHz). This softening is attributed to micron-sized cracks within the gel network that may or may not fully heal once vibrations are turned off depending on the acoustic intensity. Ultrasonic vibrations are further shown to dramatically decrease the gel yield stress and accelerate shear-induced fluidization. Ultrasound-assisted fluidization dynamics appear to be governed by an effective temperature that depends on the acoustic intensity. Our work opens the way to a full control of elastic and flow properties by ultrasonic vibrations as well as to future theoretical and numerical modeling of such rheo-acoustic gels.Comment: 21 pages, 14 figure

Interpenetration of fractal clusters drives elasticity in colloidal gels formed upon flow cessation

Colloidal gels are out of equilibrium soft solids composed of attractive Brownian particles that form a space-spanning network at low volume fractions. The elastic properties of these systems result from the network microstructure, which is very sensitive to shear history. Here, we take advantage of such sensitivity to tune the viscoelastic properties of a colloidal gel made of carbon black nanoparticles. Starting from a fluidized state under an applied shear rate $\dot \gamma_0$, we use an abrupt flow cessation to trigger a liquid-to-solid transition. We observe that the resulting gel is all the more elastic when the shear rate $\dot \gamma_0$ is low and that the viscoelastic spectra can be mapped on a master curve. Moreover, coupling rheometry to small angle X-ray scattering allows us to show that the gel microstructure is different from gels solely formed by thermal agitation where only two length scales are observed: the dimension of the colloidal and the dimension the fractal aggregates. Competition between shear and thermal energy leads to gels with three characteristic length scales. Such gels structure in a percolated network of fractal clusters that interpenetrate each other. Experiments on gels prepared with various shear histories reveal that cluster interpenetration increases with decreasing values of the shear rate $\dot \gamma_0$ applied before flow cessation. These observations strongly suggest that cluster interpenetration drives the gel elasticity, which we confirm using a structural model. Our results, which are in stark contrast with previous literature, where gel elasticity was either linked to cluster connectivity or to bending modes, highlight a novel local parameter controlling the macroscopic viscoelastic properties of colloidal gels

Colloidal transport in bacteria suspensions: from bacteria collision to anomalous and enhanced diffusion

International audienceSwimming microorganisms interact and alter the dynamics of Brownian particles and modify their transport properties

Mechanics and structure of carbon black gels under high-power ultrasound

Colloidal gels made of carbon black particles are "rheo-acoustic" materials: their mechanical and structural properties can be tuned using high-power ultrasound, sound waves with submicron amplitude and frequency larger than 20 kHz. The effect is demonstrated using two experiments: rheology coupled to ultrasound to test for the gel mechanical response and an ultra small-angle X-ray scattering experiment coupled to ultrasound to test for structural changes within the gel. We show that high-power ultrasound above a critical amplitude softens carbon black gels at rest due to the formation of micro-cracks in the bulk. The gel softens exponentially with a characteristic time of a few seconds. High-power ultrasound also eases the flow of the gel thanks again to the formation of micro-cracks. High shear rates however tend to select a gel structure that is less sensitive to ultrasound