Abstract
Active fluids comprise a variety of systems composed of elements immersed in a fluid environment which can convert some form of energy into directed motion; as such they are intrinsically out-of-equilibrium in the absence of any external force. A fundamental problem in the physics of active matter concerns the understanding of how the characteristics of autonomous propulsion and agent-agent interactions determine the collective dynamics of the system. We study numerically the suspensions of self-propelled diffusiophoretic colloids, in (quasi)-2d configurations, accounting for both dynamically resolved solute-mediated phoretic interactions and solvent-mediated hydrodynamic interactions. Our results show that the system displays different scenarios at changing the colloid-solute affinity and it develops a cluster phase in the chemoattractive case. We study the statistics of cluster sizes and cluster morphologies for different magnitudes of colloidal activity. Finally, we provide evidences that hydrodynamics plays a relevant role in the aggregation kinetics and cluster morphology, significantly hindering cluster growth.
Highlights
In our simulations solvent and solute hydrodynamics is fully resolved, from the far field down to the distances on the order of the particle size
We deal with spherical particles, which rules out the possibility of alignment-induced collective motion; instead, chemical production and diffusion mediate an effective interaction, analogously to the experimental system studied in ref. 26 and 28
While in the experiments it was surmised that active colloids experienced an attractive interaction, here we can tune the affinity of the particles for the solute via the phoretic mobility m0, which can be regarded as an effective charge,[51] i.e. positive/negative values induce repulsive/attractive interactions, respectively
Summary
2.1 Hydrodynamic model for the solvent–solute mixture The 3d Navier–Stokes equations for the fluid velocity field u. In principle, with arbitrarily patchy[38] (yet axisymmetric) active colloids, in the present study we will specialise in the case of Janus-like particles,[39] that produce solute at a constant rate per unit surface a0, homogeneously over only one hemisphere, i.e.:. ). The velocities attained in our simulations are such kd that the typical particle Reynolds Re = VpR/n and Peclet Pe = VpR/D numbers are always smaller than 10À1 (Vp is the self-propulsion speed), making the advection terms in eqn (1) and (2) negligible. The fluid is confined along the z-direction by two parallel walls, at which a no-slip boundary condition is imposed on the velocity field and a zero-flux condition applies for the equation for C. The boundary conditions at the particle surfaces need a deeper description that will be provided hereafter
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