Thermophoretic microswimmers: Interplay of phoresis, geometry and hydrodynamics

The term swimmer refers to biological or artificial structures that are capable of self-propel by drawing energy from the surrounding environment. The typical size of a swimmer ranges orders of magnitude, from the macroscopic world of a blue whale in the ocean, to the microscopic of a bacteria. Microscopic swimmers, or microswimmers, live in an environment where the viscosity of the fluid dominates their motion, suppressing the inertia that we are so familiar with. Phoresis refers to the physical mechanism in which colloidal particles migrate due to the presence of a solvent gradient, such as thermal, chemical or magnetic. Phoretic colloids have recently emerged as a promising avenue for the design of artificial microswimmers. Thermophoretic colloids are partially coated with a high heat conductivity material, such as gold, which heats faster under laser illumination, creating then a local thermal gradient. The non-coated surface reacts to the difference in temperatures and displays the thermophoretic response to it, driving the motion of the swimmer. The motion of colloids immersed in fluid produce long-ranged flows, which can infere in the motion of further colloids. These fluid-mediated interactions are known as hydrodynamic interactions. Since the colloid is found in solvent, phoresis and propulsion are linked to a hydrodynamic flow field. These fluid-mediated interactions are deeply influenced by the geometry and surface properties of the colloid, and play a major role in the interaction between swimmers. This dissertation addresses the study of self-thermophoretic dimeric and trimeric colloidal swimmers by means of mesoscale computer simulations. In order to precisely understand the debated role of hydrodynamic interactions in these systems, two computational approaches are hereby presented. We use a full hydrodynamic approach, which includes thermophoresis, and a second method which neglects fluid-mediated effects while accounting for thermophoretic interactions. Hydrodynamic simulations are performed via the combination of molecular dynamics (MD) and multi-particle collision dynamics (MPC), which has proven to include hydrodynamics and heat transport. We furthermore propose a thermophoretic Brownian dynamics method for the dry systems, with phoresis implemented by means of pair potential interactions. The advantage of this method respect to regular Brownian methods is that the propulsion mechanism and intermolecular phoretic interactions are described by the same potential. This ensures a correct description of the thermophoretic interactions, disregarding only the hydrodynamics in the system, thus allowing for a fair comparative study. In this thesis we have dealt with multimeric structures made of one heated bead, which creates a local thermal gradient, adjacent to one or two thermophoretic beads. Dimeric swimmers only have one phoretic bead, whereas trimers are build with two phoretic beads. The first trimers have been constructed with all the beads placed rod-like, with the heated bead in the middle. In order to achieve ballistic motion, the two phoretic beads need to have different phoretic responses, thus the phoretic forces sharing orientation. Trimers can moreover be built with two phoretic beads of the same nature, in which the linear structure is no longer useful for propulsion. The three beads are then placed in a triangular lattice, recovering the ballistic propulsion. Changing one of the phoretic beads of this structure to the opposite phoretic behaviour leads to a rotational behaviour due to a phoretic torque, thus to a rotor swimmer. Ensembles of dimeric and trimeric swimmers are studied besides the single swimmer properties, showing that the interplay of phoresis, hydrodynamics and geometry is key to the correct understanding of their collective behaviour. Thermophoresis either helps aggregation, as in the case of thermophilic swimmers, in which phoretic beads try to cluster around heat sources; or strongly prevents clustering when thermophobic beads get repelled by heat sources. These dimers and trimers change their hydrodynamic behaviour when varying the aspect ratio between the phoretic and heated beads’ radii. Interestingly, the effects of hydrodynamics have shown to have distinct impacts on different systems. In some cases, hydrodynamics enhances aggregation, like it is the case for symmetric thermophilic dimers. In other cases, clustering is penalised due to repulsive fluid-mediated interactions, as it is the case for thermophobic dimers and triangular trimers. These effects are further enhanced or diminished by phoretic and steric effects. Furthermore, steric effects lead to alignment or aggregation on the system. The results presented in this dissertation contribute to the understanding of thermophoretically driven artificial microswimmers. The large variety of behaviours which we have seen is which may offer more versatile tools in various systems such as micro-fluidic systems, or even devices with applications in medicine given the biocompatibility of small thermal gradients with most organisms

Projecte d'urbanització del sector nº 5 d'Esparraguera

El projecte desenvolupa un sector classificat com a sól urbanitzable al municipi d'Esparreguer

Anàlisis del consum de combustible i emissions de sofre en un vaixell de cabotatge que consumeixi fuel-oil : Comparació de les possibles solucions per a les emissions

L’objectiu d’aquest treball és aprofundir en el coneixement d’un motor marí real cremant fuel-oil, les normatives que els regeixen i calcular les principals característiques d’aquest referents a la generació de gasos d’escapament. Realitzar un estudi sobre l’abocament del diòxid de sofre a l’atmosfera provocat per la combustió de combustibles fòssils, les normatives que ho regulen, dimensionament d’aquests i les repercussions que aquest té sobre el medi ambient, la naturalesa i les persones. Un cop aquests factors són conegut amb claredat, trobar les principals solucions que existeixen en l’actualitat per a resoldre o minimitzar aquest problema. L’estructura del treball ha seguit un ordre lògic on s’ha estudiat des del punt més bàsic i general i s’ha anat aprofundint. Inicialment es presenta l’embarcació del treball i poc a poc es va concretant en tots els temes relacionats per a acabar descrivint els principals mètodes per reduir les emissions de sofre a l’atmosfera i proposant una solució real

Estructura tarifària òptima del sector del taxi

El sistema tarifari que fa servir el sector del taxi actualment a Barcelona és un sistema que depèn de la distància recorreguda i el temps de viatge de cada trajecte. Òbviament, com qualsevol altre sistema tarifari, depèn dels punts d’origen i destí, però la tarifa a pagar es veu influenciada per les condicions de trànsit. Amb l’objectiu de trobar una tarifa més entenedora per l’usuari, però que segueixi sent rendible pel taxista, es proposa un sistema tarifari amb preus tancats, és a dir, que l’usuari pugui conèixer el preu del trajecte coneixent origen i destí. En aquest tesina es desenvolupa l’estudi per obtenir aquest sistema pels trajectes que uneixen la ciutat de Barcelona i l’Aeroport del Prat, en ambdós sentits. El primer punt imprescindible per poder fer l’estudi és el càlcul dels costos que té un taxista al llarg de l’any. Aquests costos es divideixen en directes (aquells que tenen relació directa amb l'exercici de taxista) i en indirectes (costos no relacionats directament amb la professió). Un cop obtinguts aquests costos, el pas següent és el càlcul de la generació de viatges de taxi a l'Aeroport del Prat, així com l'atracció de les diferents zones en que es divideix la ciutat de Barcelona per tal de dur a terme l'estudi. Per això, es creen dos models matemàtics capaços de predir aquestes dades. Amb costos i demandes, el següent pas és fer una proposta de tarifes raonables. És a dir, trobar quin preu ha de pagar l'usuari per un trajecte en taxi entre Barcelona i l'aeroport, de manera que li surti rendible al taxista, sense ser abusiu per l'usuari. Es proposen, doncs, dos tipus de sistemes tarifaris: una tarifa plana per a tots els viatges, i una zonificació de Barcelona amb un preu per a cadascuna d’aquestes zones. L'aplicació d’un nou sistema tarifari diferenciat del que hi ha actualment implica una variació en la demanda. Coneixent el preu mitjà actual dels viatges entre l'aeroport i cadascuna de les zones d'estudi, i coneixent les tarifes proposades, es fa una aproximació de la demanda que hi haurà un cop instaurats els nous preus

Evaluation of the binding energy of viral capsid proteins using a Virtual AFM

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2016, Tutor: David Reguera LópezViruses are biological agents with great potential in the field of nanotechnology. Nowadays, their amazing properties are starting to be unveiled using physical techniques like the atomic force microscope. In this study, we have used a Virtual AFM, i.e., a simulator that mimics a typical AFM nanoindentation experiment, to study the mechanical properties of viral shells. In particular, with our VAFM, we have analysed the possibility of evaluating the binding energy between proteins using rupture experiments and how that estimate of the energy depends on experimental parameters such as the loading rate or the size of the ti