Lamellar ordering, droplet formation and phase inversion in exotic active emulsions

Abstract

We study numerically the behaviour of a two-dimensional mixture of a passive isotropic fluid and an active polar gel, in the presence of a surfactant favouring emulsification. Focussing on parameters for which the underlying free energy favours the lamellar phase in the passive limit, we show that the interplay between nonequilibrium and thermodynamic forces creates a range of multifarious exotic emulsions. When the active component is contractile (e.g., an actomyosin solution), moderate activity enhances the efficiency of lamellar ordering, whereas strong activity favours the creation of passive droplets within an active matrix. For extensile activity (occurring, e.g., in microtubule-motor suspensions), instead, we observe an emulsion of spontaneously rotating droplets of different size. By tuning the overall composition, we can create high internal phase emulsions, which undergo sudden phase inversion when activity is switched off. Therefore, we find that activity provides a single control parameter to design composite materials with a strikingly rich range of morphologies.

Rheology of active polar emulsions: from linear to unidirectional and inviscid flow, and intermittent viscosity

“…The system undergoes a morphological transition from the lamellar phase towards an emulsion of active material in a passive background, a behaviour also found by Bonelli et al 43 Fig. 8) may occur with almost everywhere vanishing gradient of v x , and in this case it will be called superfluidic 32,65 (see light grey profile in Fig.…”

“…The system that we consider 43,44 has the property that a tunable amount of active material can be homogeneously dispersed in an emulsion. With respect to the single-component active gel theory, this approach has the great advantage that the typical lengthscale of active injection can be kept under control by setting opportunely the parameters of the model.…”

Soft channel formation and symmetry breaking in exotic active emulsions

“…This is energetically favoured, with respect to the lamellar configuration, as it allows for relieving elastic stresses. More interestingly, this phenomenology-which was already observed in unconfined systems 34 -is accompanied by symmetry breaking along the gradient direction. While an active matrix interspersed with passive droplets gathers and grows on one side of the channel, the rest of the system is left in a state poorly populated of active material.…”

“…The equilibrium properties of the system are encoded in a modified Landau-Brazovskii [34][35][36]39,40 free-energy functional F [φ, P] that can be expressed as the first contribution defines the thermodynamic property of the binary mixture:…”

Morphology and flow patterns in highly asymmetric active emulsions

“…Binary fluids with an active component have been considered in [17,18] showing that the active part may cause instabilities on active passive interface. Recently [19] a model has been introduced where emulsification of the active component is favored by the presence of surfactant added to the mixture. This model generalizes the aforementioned active gel theory to describe the behavior of a mixture of isotropic passive and polar active fluids.…”