Active Emulsions: Physicochemical Hydrodynamics and Collective Behavior

Abstract

Compartmentalization is critical to separate the components of the living system from the environment. It spatially concentrates reactants and chemical networks and confines the genetic material of the cell. In cells, this semi-permeable membrane is responsible for compartmentalisation. It regulates the communication of the cell with the environment enabling the cell to create spatiotemporal chemical gradients that are critical for functioning out of equilibrium.U and characteristic length scale L, the Reynolds number is defined as the ratio of the inertial term, ∼ ρu • ∇u, to the viscous term, ∼ η∇ 2 u, in the Navier-Stokes equation [30]. Hence, Micro-swimmers with or without moving partsArtificial microswimmers that are driven mechanically use moving parts that mimic biological counterparts in performing non-reciprocal movements. The first example of such systems was the linear chains of DNA-linked magnetic colloids that were attached to a red blood cell [42]. The flexible synthetic flagellum was actuated by an external oscillating magnetic field. The manufacturing of such designs is challenging and they are prone to fatigue failure. Intrinsic symmetry breakingIntrinsic symmetry breaking means the asymmetry is imposed on the systems, typically geometrically, and is not a result of dynamical instability. Thus, it is useful for the diffusive regime where P e ≪ 1. The broadly-adopted approach for creating such an asymmetry is to spatially vary the surface activity. Janus particles are well-known Active dropletsThe framework discussed in Section 1.3.1 is applicable to any autophoretic particle which undergoes chemical activity at its surface such as active droplets. Active droplets, in particular, are known as the experimental realization of isotropic autophoretic particles. Estimates from scaling analysis suggest that the dominant driving force for active droplets is provided by Marangoni stresses (see Chapter 3). Thus, active droplets have to induce a self-sustaining fore-aft gradient in interfacial tension to achieve continuous propulsion. This gradient generally originates from a chemical activity at the interface which drives a fluid dynamical advection-diffusion instability. Based on the way the energy for activity is obtained, the experimental systems fall into three distinct categories: i) a chemical reaction taking place inside the droplet, ii) phase separation, and iii) micellar solubilization. i. Internal chemical reactionSince the chemical reaction takes place inside the droplet, this type of active droplet, compared to its counterparts, has a shorter lifetime and thereby shorter cruising range, i.e. the maximum distance the swimmer can self-propel before stopping. The mechanism depends on a specific chemical reaction, therefore, it is challenging to tune, modify, or improve. This limits the applications as well as further studies where for Collective dynamics governed by long-range interactionsLong-range inter-particle interactions in phoretic suspensions include [10] (i) Chemical interactions. The che...