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Flow of flexible matter through complex environments
Publication . Silva, Danilo; Gama, Margarida Telo da; Araújo, Nuno
In this thesis we investigated the flow of flexible particles in complex environments, with a focus
on droplet-based emulsions driven by flow and the sedimentation of deformable capsules and
droplets in confined geometries. We used the lattice Boltzmann method (LBM) for fluid modelling
and employed a combination of intrinsic LB methods and coupling with other techniques
to simulate multicomponent droplets and flexible capsules. We conducted a comprehensive
review, summarising different approaches utilising LBM in simulating fluid-filled soft structures.
We highlight the relevance of these models in fields such as droplet microfluidics, drug
delivery, and microparticle synthesis, while categorising the methods into fluid-structure and
fluid-fluid methods, which consider interfacial boundaries and hydrodynamic interactions. We
emphasise the versatility of the lattice Boltzmann method in handling complex boundary conditions
and incorporating physical models. Additionally, we discussed benchmark tests for model
validation. In further studies, we extended a multicomponent LB method to 3D geometries and
simulated droplets flowing in a wetting channel. The results revealed a discontinuous shear
thinning transition as the external force increased. We examined the effect of surface tension,
directly related to droplet deformability, demonstrating that higher surface tension led to less
deformable droplets and thus require larger forces for shear thinning to occur. In the next study,
we looked at the shape transitions of sedimenting capsules and droplets. In the confined regime,
we found a transition to bullet shape consistent with experiments. Interestingly, we find that the
transition from oblate to bullet shaped droplets and capsules consistently occurs at a specific ratio
between the capsule size and confinement, regardless of the flexibility. A detailed analysis of
hydrodynamic stresses and forces provides valuable insights into the mechanisms driving these
shape transitions. Overall, the application of the lattice Boltzmann method, and the combination
of computational and experimental approaches (conducted by the Oppenheimer Group for Soft
Matter Physics at Tel Aviv University), sheds light into the dynamics of droplet-based systems
and deformable capsules. These findings have implications for a wide range of fields involving
soft matter systems, opening up new possibilities for designing and optimising processes in
droplet microfluidics, drug delivery, food & cosmetic industry and beyond.
Dynamics of mixtures under confinement
Publication . Nunes, André; Gama, Margarida Telo da
We study the self-organization of mixtures of colloidal particles in the presence of external fields and explore design strategies to obtain targeted structures. We consider two types of binary mixtures.
In the first, the particles differ in their Stokes coefficient and in the second, the particles differ in their response to external fields. While for the first type the thermodynamic equilibrium state is completely mixed, for the second one, segregation is expected at equilibrium. Both types are characterized by having a complex energy landscape and, as a result, during the collective dynamics the systems get frequently trapped in local minima for long periods of time and so they rarely reach thermodynamic equilibrium within the timescale of relevance. The specific state in which the systems are kinetically arrested will depend strongly on the history. Thus, in order to properly simulate and characterize the dynamics of these systems, we need new methods and techniques. We employ particle based simulations such as Brownian Dynamics (BD) to probe the time evolution of the systems and Monte Carlo (MC) simulations to study the properties of the equilibrium states. We also use continuum models, including dynamical density functional theory of fluids (DDFT), to access long time scales that are not reachable with direct numerical simulations. We report the behavior in the presence of different spatially dependent external fields. We also show how mixtures of the first type can segregate while they are sedimenting under gravity. Finally, we propose a way to control the number of structural defects in crystals using random potentials in mixtures of the second type. Implications of our findings in the field of Soft Condensed Matter Physics are also discussed.
Collective dynamics of flexible active particles on substrates : from cells to tissues
Publication . Estevão Pereira Pinto, Diogo; Araújo, Nuno; Gama, Margarida Telo da
We study the effects of disorder in epithelial confluent tissues through the Voronoi model for dense tissues. The modeling of epithelial tissues relies on three different mechanisms: cell-cell and cell-medium interactions, and propulsion or activity. First, we focus on the role of cell-cell interaction in this model by exploring, in the athermal limit, its anomalous jamming behavior. We introduce a new metric that allows us to find a hierarchical structure in its energy landscape similar to colloidal particle systems. We then introduce a cell-medium interaction by explicitly considering an interaction between the cells and their underlying substrate. We consider that the targeted geometry of the cells changes according to their spatial position and in turn affects the cells motility. We show that when the characteristic length scale of the disorder is smaller than the cell size, the cell motility increases when compared to its homogeneous counterpart. This result is in sharp contrast to what has been reported for tissues with heterogeneity in the mechanical properties of the individual cells, where the disorder favors rigidity. Due to the internal biological complexity of the cells, changes to the cell-substrate interaction should trigger a hierarchy of biochemical responses in the cell that lead to its adaptation to the new substrate region. As such, the process of cell adaptation to its underlying structure is not instantaneous but requires a finite time that in many cases competes with other relevant timescales for the dynamics such as, for example, the diffusion timescale. With this in mind, we then introduce a characteristic adaptation time of the cells to the cell-substrate interaction changes. We study how the competition between the adaptation of the cells and their mobility can compromise the fidelity of the substrate and by relating this with the previous disordered substrate propose a typical time scale for the adaptation of cells that is relevant for experiments. Lastly, we consider non-confluent tissues by allowing the cells to break from one another and create empty spaces. This change opens the door to the study of the surface properties of cell colonies and it is a first step towards the study of the transition from a single cell to confluent tissue. Implications of our findings in the field of Soft Condensed Matter Physics are discussed.
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Entidade financiadora
Fundação para a Ciência e a Tecnologia
Programa de financiamento
9471 - RIDTI
Número da atribuição
PTDC/FIS-MAC/28146/2017