##### Staff member publications

De Corato, Marco, Arqué, Xavier, Patiño, Tania, Arroyo, Marino, Sánchez, Samuel, Pagonabarraga, Ignacio, (2020). Self-propulsion of active colloids via ion release: Theory and experiments *Physical Review Letters* 124, (10), 108001

We study the self-propulsion of a charged colloidal particle that releases ionic species using theory and experiments. We relax the assumptions of thin Debye length and weak nonequilibrium effects assumed in classical phoretic models. This leads to a number of unexpected features that cannot be rationalized considering the classic phoretic framework: an active particle can reverse the direction of motion by increasing the rate of ion release and can propel even with zero surface charge. Our theory predicts that there are optimal conditions for self-propulsion and a novel regime in which the velocity is insensitive to the background electrolyte concentration. The theoretical results quantitatively capture the salt-dependent velocity measured in our experiments using active colloids that propel by decomposing urea via a surface enzymatic reaction.

Cacace, T., Memmolo, P., Villone, M. M., De Corato, M., Mugnano, M., Paturzo, M., Ferraro, P., Maffettone, P. L., (2019). Assembling and rotating erythrocyte aggregates by acoustofluidic pressure enabling full phase-contrast tomography *Lab on a Chip* 19, (18), 3123-3132

The combined use of ultrasound radiation and microfluidics is a promising tool for aiding the development of lab-on-a-chip devices. In this study, we show that the rotation of linear aggregates of micro-particles can be achieved under the action of acoustic field pressure. This novel manipulation is investigated by tracking polystyrene beads of different sizes through the 3D imaging features of digital holography (DH). From our analysis it is understood that the positioning of the micro-particles and their aggregations are associated with the effect of bulk acoustic radiation forces. The observed rotation is instead found to be compatible with the presence of acoustic streaming patterns as evidenced by our modelling and the resulting numerical simulation. Furthermore, the rotation frequency is shown to depend on the input voltage applied on the acoustic device. Finally, we demonstrate that we can take full advantage of such rotation by combining it with quantitative phase imaging of DH for a significant lab-on-a-chip biomedical application. In fact, we demonstrate that it is possible to put in rotation a linear aggregate of erythrocytes and rely on holographic imaging to achieve a full phase-contrast tomography of the aforementioned aggregate.

De Corato, M., Dimakopoulos, Y., Tsamopoulos, J., (2019). The rising velocity of a slowly pulsating bubble in a shear-thinning fluid *Physics of Fluids* 31, (8), 083103

We study the rising motion of small bubbles that undergo contraction, expansion, or oscillation in a shear-thinning fluid. We model the non-Newtonian response of the fluid using the Carreau-Yasuda constitutive equation, under the assumptions that the inertia of the fluid and the bubble is negligible and that the bubble remains spherical. These assumptions imply that the rising velocity of the bubble is instantaneously proportional to the buoyancy force, with the proportionality constant given by the inverse of the friction coefficient. Instead of computing the rising velocity for a particular radial dynamics of the bubble, we evaluate its friction coefficient as a function of the rheological parameters and of the instantaneous expansion/contraction rate. To compute the friction coefficient, we impose a translational motion and we linearize the governing equations around the expansion/contraction dynamics of the bubble, which we solve using a perturbation expansion along with the finite element method. Our results show that the radial motion of the bubble reduces the viscosity of the surrounding fluid and may thus markedly decrease the friction coefficient of the bubble. We use the friction coefficient to compute the average rise velocity of a bubble with periodic variations of its radius, which we find to be strongly increased by the radial pulsations. Finally, we compare our predictions with the experiments performed by Iwata et al. [“Pressure-oscillation defoaming for viscoelastic fluid,” J. Non-Newtonian Fluid Mech. 151(1-3), 30–37 (2008)], who found that the rise velocity of bubbles that undergo radial pulsations is increased by orders of magnitude compared to the case of bubbles that do not pulsate. Our results shed light on the mechanism responsible for enhanced bubble release in shear-thinning fluids, which has implications for bubble removal from complex fluids.