We study numerically the role of hydrodynamics in the liquid-hexatic transition of active colloid... more We study numerically the role of hydrodynamics in the liquid-hexatic transition of active colloids at intermediate activity, where motility induced phase separation (MIPS) does not occur. We show that in the case of active Brownian particles (ABP), the critical density of the transition decreases upon increasing the particle’s mass, enhancing ordering, while self-propulsion has the opposite effect in the activity regime considered. Active hydrodynamic particles (AHP), instead, undergo the liquid-hexatic transition at higher values of packing fraction $$\phi $$ ϕ than the corresponding ABP, suggesting that hydrodynamics have the net effect of disordering the system. At increasing densities, close to the hexatic-liquid transition, we found in the case of AHP the appearance of self-sustained organized motion with clusters of particles moving coherently.
Yield-stress materials, which require a sufficiently large forcing to flow, are currently ill-und... more Yield-stress materials, which require a sufficiently large forcing to flow, are currently ill-understood theoretically. To gain insight into their yielding transition, here we study numerically the rheology of a suspension of deformable droplets under pressure-driven flow. We show that the suspension displays yield-stress behaviour, with the droplets remaining motionless when the applied body-force is below a critical value. In the non-flowing phase, droplets jam to form an amorphous structure, whereas they order in the flowing phase. Yielding is linked to a percolation transition in the contacts of droplet-droplet overlaps, and requires suitable wetting boundary conditions and strict conservation of the droplet area to exist. Close to the yielding transition, we find strong oscillations in the droplet motion which closely resemble those found experimentally in confined colloidal glasses under flow. We show that even when droplets are static the underlying solvent moves by permeation, so that the viscosity of the composite system is never truly infinite, and, as we discuss, its precise value ceases to be a bulk material property of the system.
We study the phase behaviour of a quasi-two dimensional cholesteric liquid crystal shell. We char... more We study the phase behaviour of a quasi-two dimensional cholesteric liquid crystal shell. We characterise the topological phases arising close to the isotropic-cholesteric transition, and show that they differ in a fundamental way from those observed on a flat geometry. For spherical shells, we discover two types of quasi-two dimensional topological phases: finite quasicrystals and amorphous structures, both made up by mixtures of polygonal tessellations of half-skyrmions. These structures generically emerge instead of regular double twist lattices because of geometric frustration, which disallows a regular hexagonal tiling of curved space. For toroidal shells, the variations in the local curvature of the surface stabilises heterogeneous phases where cholesteric patterns coexist with hexagonal lattices of half-skyrmions. Quasicrystals, amorphous and heterogeneous structures could be sought experimentally by self assembling cholesteric shells on the surface of emulsion droplets.
We numerically study by lattice Boltzmann simulations the rheological properties of an active emu... more We numerically study by lattice Boltzmann simulations the rheological properties of an active emulsion made of a suspension of an active polar gel embedded in an isotropic passive background. We find that the hexatic equilibrium configuration of polar droplets is highly sensitive to both active injection and external forcing and may either lead to asymmetric unidirectional states which break top-bottom symmetry or symmetric ones. In this latter case, for large enough activity, the system develops a shear thickening regime at low shear rates. Importantly, for larger external forcing a regime with stable negative effective viscosity is found. Moreover, at intermediate activity a region of multistability is encountered and we show that a maximum entropy production principle holds in selecting the most favorable state.
Proceedings of the National Academy of Sciences, 2019
Significance Chirality plays an important role in many biological systems. Biomolecules, such as ... more Significance Chirality plays an important role in many biological systems. Biomolecules, such as DNA, actin, or microtubules, form helical structures, which at sufficiently high density and in the absence of active forces, tend to self-assemble into twisted cholesteric phases. We study the effect of activity on a droplet of chiral matter, finding a surprisingly rich range of dynamical behaviors ranging from spontaneous rotations to screw-like motion. The phenomena that we uncover require both thermodynamic chirality and activity and are linked to the nontrivial topology of the defects that form by necessity at the droplet surface. It would be of interest to look for analogues of the motility modes found here in chiral microorganisms occurring in nature or in synthetic active matter.
Cell motility is crucial to biological functions ranging from wound healing to immune response. T... more Cell motility is crucial to biological functions ranging from wound healing to immune response. The physics of cell crawling on a substrate is by now well understood, whilst cell motion in bulk (cell swimming) is far from being completely characterized. We present here a minimal model for pattern formation within a compressible actomyosin gel, in both 2D and 3D, which shows that contractility leads to the emergence of an actomyosin droplet within a low density background. This droplet then becomes self-motile for sufficiently large motor contractility. These results may be relevant to understand the essential physics at play in 3D cell swimming within compressible fluids. We report results of both 2D and 3D numerical simulations, and show that the compressibility of actomyosin plays an important role in the transition to motility.
The rheological behaviour of an emulsion made of an extensile active polar component and an isotr... more The rheological behaviour of an emulsion made of an extensile active polar component and an isotropic passive fluid is studied by lattice Boltzmann method. Different flow regimes, such as intermittent viscosity and unidirectional motion, are found.
We review the state of the art of active fluids with particular attention to hydrodynamic continu... more We review the state of the art of active fluids with particular attention to hydrodynamic continuous models and to the use of Lattice Boltzmann Methods (LBM) in this field. We present the thermodynamics of active fluids, in terms of liquid crystals modelling adapted to describe large-scale organization of active systems, as well as other effective phenomenological models. We discuss how LBM can be implemented to solve the hydrodynamics of active matter, starting from the case of a simple fluid, for which we explicitly recover the continuous equations by means of Chapman-Enskog expansion. Going beyond this simple case, we summarize how LBM can be used to treat complex and active fluids. We then review recent developments concerning some relevant topics in active matter that have been studied by means of LBM: spontaneous flow, self-propelled droplets, active emulsions, rheology, active turbulence, and active colloids.
We present here a comparison between collision-streaming and finite-difference lattice Boltzmann ... more We present here a comparison between collision-streaming and finite-difference lattice Boltzmann (LB) models. This study provides a derivation of useful formulae which help one to properly compare the simulation results obtained with both LB models. We consider three physical problems: the shock wave propagation, the damping of shear waves, and the decay of Taylor–Green vortices, often used as benchmark tests. Despite the different mathematical and computational complexity of the two methods, we show how the physical results can be related to obtain relevant quantities.
The morphology of a mixture made of a polar active gel immersed in an isotropic passive fluid is ... more The morphology of a mixture made of a polar active gel immersed in an isotropic passive fluid is studied numerically. Lattice Boltzmann method is adopted to solve the Navier–Stokes equation and coupled to a finite-difference scheme used to integrate the dynamic equations of the concentration and of the polarization of the active component. By varying the relative amounts of the mixture phases, different structures can be observed. In the contractile case, at moderate values of activity, elongated structures are formed when the active component is less abundant, while a dynamic emulsion of passive droplets in an active matrix is obtained for symmetric composition. When the active component is extensile, aster-like rotating droplets and a phase-separated pattern appear for asymmetric and symmetric mixtures, respectively. The relevance of space dimensions in the overall morphology is shown by studying the system in three dimensions in the case of extensile asymmetric mixtures where int...
Physica A: Statistical Mechanics and its Applications, 2018
We investigate numerically, by a hybrid lattice Boltzmann method, the morphology and the dynamics... more We investigate numerically, by a hybrid lattice Boltzmann method, the morphology and the dynamics of an emulsion made of a polar active gel, contractile or extensile, and an isotropic passive fluid. We focus on the case of a highly off-symmetric ratio between the active and passive components. In absence of any activity we observe an hexatic-ordered droplets phase, with some defects in the layout. We study how the morphology of the system is affected by activity both in the contractile and extensile case. In the extensile case a small amount of activity favors the elimination of defects in the array of droplets, while at higher activities, first aster-like rotating droplets appear, and then a disordered pattern occurs. In the contractile case, at sufficiently high values of activity, elongated structures are formed. Energy and enstrophy behavior mark the transitions between the different regimes.
We study numerically the role of hydrodynamics in the liquid-hexatic transition of active colloid... more We study numerically the role of hydrodynamics in the liquid-hexatic transition of active colloids at intermediate activity, where motility induced phase separation (MIPS) does not occur. We show that in the case of active Brownian particles (ABP), the critical density of the transition decreases upon increasing the particle's mass, enhancing ordering, while self-propulsion has the opposite effect in the activity regime considered. Active hydrodynamic particles (AHP), instead, undergo the liquid-hexatic transition at higher values of packing fraction φ than the corresponding ABP, suggesting that hydrodynamics have the net effect of disordering the system. At increasing densities, close to the hexatic-liquid transition, we found in the case of AHP the appearance of self-sustained organized motion with clusters of particles moving coherently.
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Papers by giuseppe negro