Rheoacoustic Gels: Tuning Mechanical and Flow Properties of Colloidal Gels with Ultrasonic Vibrations

Abstract

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 show that ultrasound allows one to achieve this goal in well-controlled conditions. 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 the fragmentation of the gel network into large clusters that may or may not fully re-aggregate once ultrasound is turned off depending on the acoustic intensity. Ultrasound is further shown to dramatically decrease the gel yield stress and accelerate shear-induced fluidization, thus opening the way to a full control of elastic and flow properties by ultrasonic vibrations.Colloidal gels constitute a class soft materials with a huge spectrum of applications, ranging from paints, oil extraction and construction to pharmaceuticals and food products [1,2]. These gels are typically formed from the aggregation of attractive nanoscale particles that arrange into a space-spanning network which strength provides the system with elastic properties at rest. The particle network is, however, fragile enough that rather weak external forces disrupt the gel structure and induce flow above a critical yield strain or stress [3]. Such unique mechanical and flow properties are key to applications that require an interplay between solid and fluid behaviour, including cement placement, ink-jet printing or flow-cell batteries. In this context, decades of research have investigated the influence of physico-chemical parameters such as temperature, pH and ionic strength on the aggregation of attractive particles into colloidal gels [4][5][6][7][8][9]. While electrostatic repulsion, van der Waals attraction and steric interaction, combined in the well-known DLVO interparticle potential, generically lead to the formation of fractal networks through diffusion-or reaction-limited cluster aggregation, additional chemical effects such as hydration layers or particle bridging generate a wealth of more complex gel microstructures [10]. In the quest of smart, responsive materials, tuning the gel architecture in realtime and reversibly is a key challenge. However, playing on the above physico-chemical parameters turns out to be difficult, if not impossible when the chemistry of the material is strongly constrained. Electro-or magnetorheological materials allow for such tuning through the use of electrical and magnetic fields [11][12][13][14]. Yet, applications are obviously restricted to materials with specific compositions [15][16][17].The exquisite sensitivity of colloidal gels to external mechanical perturbations provides an...

Criterion for Fingering Instabilities in Colloidal Gels

“…Moreover, the observation that the fingering characteristics are set solely by the properties of the fully fluidized state of the sample might shed light on discrepancies pointed out in [32], where the finger width observed in hair gel solutions is independent of their yield stress. More generally, the local fluidization scenario described here is in line with recent rheological work on stress-induced failure in gels [56], suggesting that our critical energy criterion could be relevant for predicting the outcome of delayed failure in colloidal gels [57,58]. However, our results strongly contrast with experiments on jammed assemblies of soft particles such as dense microgels for which the wavelength of the pattern is set by the yield stress of the material [29].…”

Active biomaterials for mechanobiology

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Acoustic monitoring of the gelation of a colloidal suspension

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