Abstract

Charged surfaces in contact with liquids containing ions are accompanied in equilibrium by an
electric double layer consisting of a layer of electric charge on the surface that is screened by a diffuse ion
cloud in the bulk fluid. This screening cloud determines not only the interactions between charged
colloidal particles or poly- electrolytes and their self-assembly into ordered structures [1], but also on how
the interaction of a charged colloidal particle with an oil-water interface can be tuned from attractive to
repulsive by varying the salt concentration, as we will discuss in this talk [2]. In the second part of this
talk we will discuss to what spatial complexity the electric double layers can be designed. We show that
electric double layers of non-trivial topology including tori, multi-tori and knots can be realised in
charged colloids with complex-shaped particles, using numerical modelling. We show that the topology
of double layers can be defined via a cut-off in the ion concentration without any loss of generality, and
demonstrate that the double layer topology can be tuned by changing the Debye screening length of the
medium, or by changing the shape and topology of the (colloidal) particle [3]. If time permits, we will
finally discuss the coupling of electric double layers to a nematic texture, and show the effects of salt on
the anchoring strength of a charged wall.
[1] J.C. Everts, M.N. van der Linden, A. van Blaaderen and R. van Roij, Soft Matter 12 (2016) 6610-
6620.
[2] J.C. Everts, S. Samin and R. van Roij, Phys. Rev. Lett. 117 (2016) 098002.
[3] J.C. Everts and M. Ravnik, Sci. Rep. 8 (2018) 14119.