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Dissipation in active matter systems: self-organization and transport

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Universitat de Barcelona. Facultat de Física

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[eng] This thesis delves into the study of catalytic Janus particles (AP) that convert chemical energy into mechanical motion, resulting in energy dissipation. Understanding this dissipation is challenging due to the complex interplay between chemical and mechanical processes. Traditional thermodynamic models often fail to fully capture the dynamics of the system, as they tend to treat active Brownian particles as minimally interactive with their environement and overlook entropy production and energy dissipation in non-equilibrium conditions. To address this gap, the thesis introduces a new model that considers thermodynamic constraints related to dissipation and entropy production, providing a deeper understanding of energy dissipation in active systems. Expanding on this framework, the research investigates how assemblies of Janus particles behave when exposed to varying concentrations of fuel in an inhomogeneous medium. The study reveals a non-linear relationship between energy dissipation and the fraction of particles that assemble, leading to a new thermodynamic criterion for self-assembly based on the behavior of chemical potentials. This offers a clearer understanding of how microscopic interactions drive larger-scale self-organization. Environmental factors such as concentration gradients and fluid flows significantly affect the formation and stability of these active matter structures. Hydrodynamic interactions (HI) increase the mobility of catalytic Janus particle aggregates, enabling the formation of more complex structures. However, while these interactions can reduce the efficiency of energy conversion, they create feedback loops between particle activity and the surround-ing medium. In these loops, changes in substrate consumption and fluid flow affect both the speed of chemical reactions and the resulting structural configurations of the particles. Managing these interactions is crucial for optimizing the performance and assembly of the particles. In confined environments, active particles have various applications, such as drug delivery, in situ cancer treatments, and environmental cleanup. However, effective particle transport in these settings remains challenging. Studies on particle transport in porous media show that oscillating forces in channels with exible walls can boost transport efficiency through enhanced stochastic resonance. Further optimization occurs when channel oscillations are synchronized with transverse forces, improving the particles' ability to navigate complex biological and environmental settings. A notable phenomenon identified in this research is the presence of a stochastic resonance regime for active particles under confinement. In this regime, periodically adding substrate improves transport efficiency at specific noise levels, enabling the particles to travel longer distances while consuming less fuel. This has practical implications for medical applications, such as transporting particles through cell membranes and tissues, and for environmental applications like soil remediation. In summary, this thesis develops a comprehensive framework that integrates entropy production, energy dissipation, chemical reactions, hydrodynamic interactions, and concen- tration gradients in non-equilibrium systems. It addresses gaps in current thermodynamic models, which typically focus on isolated aspects of these processes. Through its investiga- tion of the self-assembly and transport of catalytic active particles, this research uncovers key mechanisms that govern particle behavior, structure, and transport efficiency. The findings provide valuable insights for the design of advanced materials and devices that require controlled self-assembly and transport properties.

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[spa] El estudio explora además cómo los ensamblajes de partículas de Janus son influenciados por la concentración de combustible en un medio inhomogéneo. Se revela una relación no lineal entre la disipación de energía y la fracción de partículas ensambladas, introduciendo un criterio termodinámico para el autoensamblaje basado en el potencial químico. Este hallazgo profundiza nuestro conocimiento sobre cómo las interacciones microscópicas impulsan la organización macroscópica. Factores ambientales como los gradientes de concentración y los flujos de fluidos afectan significativamente la estabilidad de las estructuras de materia activa. Las interacciones hidrodinámicas (HI) aumentan la movilidad de las partículas catalíticas de Janus, promoviendo la formación de estructuras complejas. Aunque las HI pueden reducir la eficiencia energética, crean bucles de retroalimentación entre la actividad de las partículas y el medio circundante, afectando la cinética y las configuraciones estructurales. En entornos confinados, como en la administración de medicamentos o la remediación ambiental, el transporte eficiente de partículas es un desafío. Los estudios muestran que las fuerzas oscilantes en canales de paredes deformables mejoran la eficiencia del transporte a través de efectos intensificados de resonancia estocástica. La sincronización de estas oscilaciones con fuerzas transversales optimiza el transporte, ofreciendo estrategias para navegar en sistemas biológicos o ambientales complejos.

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Dissipation in active matter systems: self-organization and transport