Self Organisation : Clouds to Crystals
(a) Silicone oil drop in immersion oil in zero field. (b–h) Structures seen as a function of frequency spanning hydrodynamic and dipolar regimes. The electric field is perpendicular to the plane of the page.
Self organization of large-scale structures in nature - either coherent structures like crystals, or incoherent dynamic structures like clouds - is governed by long-range interactions. In many problems, hydrodynamics and electrostatics are the source of such long-range interactions. The tuning of electrostatic interactions has helped to elucidate when coherent crystalline structures or incoherent amorphous structures form in colloidal systems. However, there is little understanding of self organization in situations where both electrostatic and hydrodynamic interactions are present. This work shows a minimal two-component oil-in-oil model system where the strength and lengthscale of the electrohydrodynamic interactions can be controlled by tuning the amplitude and frequency of the imposed electric field. A rich phenomenology of exotic structure and dynamics ranging from incoherent cloud-like structures to coherent droplet arrays is observed as a function of the hydrodynamic lengthscale.