In recent years, the study self-propelled particles in liquids has come under the spotlight of the physical and biophysical research communities. These are biological or artificial microscopic or nanoscopic objects immersed in a liquid which, by the effect of one of several possible mechanisms (e.g. mechanical deformations, chemical reactions at their surface, gradients in their vicinity), are capable of selfpropulsion. The dynamics of systems of self-propelled particles is determined by the interplay between Brownian fluctuations and selfpropulsion which brings them into an out-of-equilibrium state with distinctive and interesting properties. In addition, achieving motion control of such particles is of great interest due to their disruptive potential applications in different fields, including medicine or biotechnology, since they could be used as efficient drug-delivery vectors, pumps for lab-on-a-chip thechnologies or as non-invasive microsurgey vehicles.

The mechanisms of self-propulsion investigated to date have been found to be operational up to a minimal scale of about a micrometer. Nevertheless, scaling down self-propelling particles to the nanoscale is an important open scientific and technological challenge in the field. However, going from micro to nano is not just a matter of scale-down: the factors that determine transport on the nanometer scale are very different from the ones controlling the macroscopic laws of hydrodynamics, and the strategies to induce self-propulsion of nanoparticles must rely on different physical mechanisms from the ones at the microscale.

In this project we explore novel propulsion mechanisms effective in the nanometer range based on self-thermophoresis, i.e. motion induced by thermal gradients created by the swimmer itself in its immediate environment. Such gradients around the propelling particle can be created by the non-isotropic dissipation of energy absorbed from an externally controlled electromagnetic field. The methodology used is based on the performance and analysis of all-atom molecular dynamics simulations of realistic models of vibrationally excited molecules and their surrounding solvent. Such description allows us to track the energy flux into the solvent and determine the conditions for self-thermophoresis. In addition, the analysis of the dynamics of solvent molecules permits to test in the nanoscale the validity of mesoscopic models of self-thermophoresis based on a continuous hydrodynamic description.

Depletion forces are effective interactions of entropic origin ubiquitous in colloidal systems when they are immersed in a solution of smaller non-adsorbing particles or polymers (depletants). Depletion interactions in colloids are attractive and arise due to the osmotic pressure difference when depletants are expelled from the region between the colloidal particles. These effective interactions are important to understand the aggregation and stability of colloidal suspensions, especially in biological systems, where the medium is often crowded with different types of polymers. Depletion forces have also been used as a driving force to induce self-assembly of colloids into predesigned structures in the quest to synthesize new functional materials. Although in its origin the understanding of depletion interactions is based on thermodynamic equilibrium ideas, in recent years there is an important effort to characterize this interaction and its effects in conditions out of equilibrium.

In this project we investigate the aggregation of discoidal particles immersed in a suspension of smaller spherical particles. We developed a thermodynamic model to describe the aggregation in equilibrium, which compares well with numerical simulations of discs and spheres interacting through repulsive excluded volume interactions where depletion forces emerge naturally [1,2]. Now we are investigating the departure from the equilibrium distributions of aggregates in the case where the spherical solutes are self-propelling particles. Although there exist a number of studies on the emergent interactions between fixed passive particles due to the presence of active particles, we present here a novel approach based on the analysis of the stationary distributions of the aggregates formed due to the presence of the active depletant. 

[1] C. Calero, M. A. Morata, I. Pagonabarraga, “Aggregation of discoidal particles due to depletion interaction” The Journal of Chemical Physics 155.7 (2021): 074904. 

[2] C. Calero, I. Pagonabarraga. "Self-Assembly of Microscopic Rods Due to Depletion Interaction." Entropy 22.10 (2020): 1114.

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