In this work we have characterized the phase behaviour and the dynamics of bidimensional mixtures of active and passive Brownian particles.

A stochastic model is used to assess the effect of external parameters on the development of submerged biofilms on smooth and rough surfaces. The model includes basic cellular mechanisms, such as division and spreading, together with an elementary description of the interaction with the surrounding flow and probabilistic rules for extracellular polymeric substance matrix generation, cell decay, and adhesion. Insight into the interplay of competing mechanisms such as the flow or the nutrient concentration change is gained. Erosion and growth processes combined produce biofilm structures moving downstream. A rich variety of patterns are generated: shrinking biofilms, patches, ripplelike structures traveling downstream, fingers, mounds, streamerlike patterns, flat layers, and porous and dendritic structures. The observed regimes depend on the carbon source and the type of bacteria.

In our work we have studied a two-dimensional suspension of finite-size Vicsek hard disks, whose time evolution follows an event-driven dynamics between subsequent time steps. Having compared its collective behaviour with the one expected for a system of scalar Vicsek point-like particles, we have analysed the effect of considering two possible bouncing rules between the disks: a Vicsek-like rule and a pseudo-elastic one, focusing on the order-disorder transition. Next, we have added to the two-dimensional suspension of hard-disk Vicsek particles disk-like passive obstacles of two types: either fixed in space or moving according to the same event-driven dynamics. We have performed a detailed analysis of the particles' collective behaviour observed for both fixed and moving obstacles. In the fixed obstacles case, we have observed formation of clusters at low noise, in agreement with previous studies. When using moving passive obstacles, we found that that order of active particles is better destroyed as the drag of obstacles increases. In the no drag limit an interesting result was found: introduction of low drag passive particles can lead in some cases to a more ordered state of active flocking particles than what they show in bulk.

Abstract. Biofilms are aggregates of bacteria attached to surfaces, which are very adaptable to changes in the environment and survive under extreme conditions. These living structures are behind many problems in research and industry. Therefore there is an increasing interest in improving our understanding on biofilms to be able to control them, either designing protocols to destroy them when harmful or promoting their growth when beneficial.A bidimensional cellular automata model for biofilm development is proposed to study the biofilm behaviour as its key growth parameters vary. The model includes several metabolic and spreading mechanisms typical of bacteria: cell division and spreading, detachment mechanisms adapted to the flow and probabilistic rules for EPS matrix generation.Numerical simulations of the model reproduce a number of biofilm patterns observed in real experiments: ripples, streamers, mushroom networks and patches. The influence of the nutrient concentration and the type of flow on the evolution of the bacterial community is monitorized.Biofilm tends to cover the whole surface when enough nutrients are available. Erosion enhances the creation of holes in this cover and promotes a variety of geometric patterns. The survival of the colony and its final shape will depend on the balance between the main growth parameters.Large Reynolds numbers and poor nutrient sources promote the formation of flat, and thin biofilms. Decreasing the Reynolds number or increasing the nutrient and oxygen concentration enhance pattern formation.

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