An experimental and theoretical view of autocatalytic colloids interacting with interfases

Abstract

This thesis contains my doctoral research in condensed soft matter physics prepared at the University of Barcelona. It covers the interaction of active Janus particles with liquid-liquid and liquid-solid interfaces, both experimentally and computationally. The structure of this thesis is made up of three parts.

In the first part I address the need to obtain methods and devices for experimental analysis. This discussion is divided into 3 chapters. In the first I show how to create active Janus particles and new microfluidic devices necessary for the development of the thesis. Various techniques are detailed, such as drop casting, sputtering, plasma cleaner, PDMS printing by 3D printer, lithography or spin coating. In the second chapter I show how to correctly record, store and compress the experimental videos, in order to obtain values such as the velocity, the rotational Brownian time or the translational diffusion constant of these particles. These measurements also depend on the duration of the recorded videos, and on how many frames per second are used. Thus, in this chapter I also analyse what effect these parameters have for the above calculation, and what parameters are optimal to achieve lighter videos. I also offer a software solution to compress captured videos further, if possible. Finally, in the third chapter I show how to capture the movement of the particles, using both traditional and novel techniques based on neural networks. The chapter shows how to manipulate particle trajectories using image processing algorithms and ends with a neural network-based particle tracking program capable of detecting hundreds of these colliding particles without losing their identity.

In the second part of this thesis I study the interaction of these particles at the individual level with liquid- liquid interfaces. This part begins with an experimental chapter, where the study of the behaviour of particles that move on a solid surface and contact these liquid interfaces. To do this, I analyse the time they are in the interface after contacting it, the distance they travel along them and the orientation they have in them. The particles acquire a defined orientation that allows them to travel longer distances and for a longer time than expected, so these interfaces are useful to guide particles. These experiments are complemented with a small theoretical model, but it is insufficient to explain the results and therefore in the next chapter I develop an advanced model based on a hybrid Lattice-Boltzmann-Finite Element algorithm, where I exploit a model of active self- diffusiophoretic particles that interact with a dissolved medium asymmetrically between both sides of the interface. This model is capable of reproducing the behaviour of the particles in the interface and allows to study in detail the innate effect of the capillarity present in inactive particles and the extra component added in the active ones due to its attraction and phoretic production.

The third and last part of this thesis, composed of a single chapter, explores how we can take advantage of the interaction of active particles with interfaces to obtain an accumulation system. This system is based on the use of solid-liquid interfaces with a complex geometry where the unit cells of the system do not break the detailed balance. The change in the type of interfaces was because active particles behaved in a similar way in the presence of liquid-liquid and solid-liquid interfaces, but they are simple to create a complex geometry. The chapter includes a model based on the theory of topological insulators that can predict the experimental behaviour, and with it we experimentally demonstrate behaviours predicted in the theory of topological insulators.

Esta tesis contiene mi investigación doctoral en física de la materia blanda condensada elaborada en la Universidad de Barcelona. Cubre la interacción de las partículas Janus activas con interfases líquido-líquido y líquido-sólido, tanto experimentalmente como computacionalmente. La estructura de esta tesis se compone de tres partes.

En la primera parte abordo la necesidad de obtener métodos y dispositivos para el análisis experimental. Esta discusión se divide en 3 capítulos. En el primero muestro cómo crear partículas Janus activas y nuevos dispositivos microfluídicos necesarios para el desarrollo de la tesis. En el segundo muestro cómo grabar, almacenar y comprimir correctamente los vídeos experimentales, optimizando los parámetros característicos del sistema. En el tercero muestro cómo capturar el movimiento de las partículas, usando tanto técnicas tradicionales como novedosas basadas en redes neuronales.

En la segunda parte de esta tesis estudio la interacción de estas partículas a nivel individual con interfases líquido-líquido. En el primer capítulo de esta parte explico mis estudios desde un punto de vista experimental. La necesidad de una mejor explicación de los experimentos me hizo considerar desarrollar un modelo avanzado basado en un algoritmo Lattice-Boltzmann, que muestro en el siguiente capítulo. En cualquier caso, en ambos capítulos vemos cómo se pueden guiar las partículas activas utilizando interfases líquido-líquido.

La tercera y última parte de esta tesis, compuesta de un único capítulo, explora cómo podemos aprovechar la interacción de partículas activas con interfases para obtener un sistema de acumulación. Este sistema se basa en el uso de interfases sólido-líquido con una geometría compleja donde las celdas unitarias del sistema no rompen el equilibrio detallado. El cambio del tipo de interfases fue debido a que las partículas activas se comportaron de manera similar en presencia de interfases líquido-líquido y sólido-líquido, pero éstas últimas son sencillas de crear para una geometría compleja.