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Projeto de investigação
Center for Theoretical and Computational Physics
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Publicações
Solute transport and absorption in porous media
Publication . Ramos, José Miguel Condeço; Araújo, Nuno Miguel Azevedo Machado de; Coelho, Rodrigo Carlos Viana
The transport of a solute in a solvent is an advection-diffusion problem that is widespread in both
nature and industry. Many porous materials, such as soils and rocks, are intrinsically disordered at the
pore scale which results in complex flow patterns. In this work, the dynamics of the fluid and solute was
simulated using the lattice-Boltzmann method (LBM) to investigate how solute dispersion and absorption
are affected by the porous medium properties. We simulate systems with circular obstacles to represent
the pore scale and obtain the macroscopic source term for different microscopic properties. We validated
our code for advection, diffusion, and absorption in porous media. First, we validated the model for the
dispersion in a Poiseille flow by confirming that the theoretical prediction between the effective diffusion
and Peclet number holds true in our simulations. Similarly, we simulated the solute dispersion in a ´
regular grid of circular objects to analyze the relationship between these two quantities. Additionally,
we considered a system with no advection, only diffusion and absorption at a constant rate our results
were according with the theoretical expectation and we verified that the relative error becomes smaller
for larger obstacles (finer discretization). We also studied a regular porous medium with and without
saturation for advection, diffusion, and absorption. In systems with saturation, we found out that the
solute concentration decays exponentially while in the case of constant absorption, solute concentrations
decrease linearly. For the same conditions we also discovered that the amount of solute lost per obstacle
is constant for systems without saturation, whereas the proportion of solute lost is not. We found that the
exponential decay of the concentration as a function of distance in disordered porous media is similar to
that of regular porous media with the same simulation parameters.
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.
Active nematic interfaces on substrates
Publication . Figueiredo, Hélio Rafael de Jesus da Cruz; Coelho, Rodrigo C. V.; Gama, Margarida Telo da
We explore the dynamics of active nematics droplets on flat surfaces, utilizing lattice Boltzmann simulations of the continuum hydrodynamic theory.
Our investigation covers a wide range of dynamical regimes, examining how
these dynamics vary with the activity intensity and droplet size on surfaces
with strong anchoring and varying equilibrium contact angles. The activity
is revealed to govern a spectrum of dynamical behaviors, encompassing selfpropulsion, scission, active wetting, and droplet evaporation. Additionally,
we observe that, on a specific surface (defined by anchoring and equilibrium
contact angle), the dynamical regimes can be characterized by the active
capillary number of suspended droplets. Furthermore, the concentration of
active nematics in the droplets changes with activity, weakly impacting wetting behavior but significantly influencing droplet evaporation. Our analysis
offers a comprehensive overview of the diverse dynamical regimes observed
in active nematics droplets, proposing a unified description of their behavior
on surfaces. We emphasize the crucial role of droplet size and discuss the
diminishing influence of these regimes in the infinite-size limit, where active
nematics turbulence prevails across all activity levels.
Flow through time-evolving porous media
Publication . Matias, André F. V.; Araújo, Nuno A. M.; Coelho, Rodrigo C. V.
In several problems of interest, a fluid flows through a porous medium modifying its structure.
The dynamics of this fluid-structure interaction is a problem of practical interest that encompasses
several fundamental questions in Soft Matter Physics related to complex flows, instabilities,
and solute transport. In this thesis, we extended the theories for fluid flow in porous
media to account for different phenomena such as swelling, erosion, and deposition. We also
extend the continuum descriptions of the fluid flow in porous media and of the dispersion and
dissolution of solute. We start by considering changes in the medium due to swelling and erosion
and extend existing pore scale lattice Boltzmann models to include both. We analyze their
competition and identify a transition between regimes where either swelling or erosion dominate.
Next, we propose a continuum description for erosion and deposition that couples the
velocity and porosity fields. The proposed model, based on the capillary model, is validated
using pore scale simulations. The simulation of media with mild spatial inequalities in porosity,
and erosion resistance is now possible. These inequalities over time get amplified, leading to
the formation of main streamlines. We show that, even for uniform erosion resistance, a weak
disorder in porosity suffices to trigger permanent channelization. The same is observed with
uneven erosion resistance. We finish with a continuum equation to model solute transport and
dissolution, parametrized by the P´eclet number and the rate of mass transfer between the solid
and the fluid. We study the time dependence of the extracted mass for different values of the
parameter space. The continuum description is validated by combining extraction experiments
with coffee and computational fluid dynamics. An analytical solution is derived for the limit of
slow mass transfer, which is corroborated by numerical simulations
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
6817 - DCRRNI ID
Número da atribuição
UIDB/00618/2020