Crystalline multilayers of charged colloids in soft confinement: experiment versus theory

Abstract

We combine real-space experiments and lattice sum calculations to investigate the phase diagram of charged colloidal particles under soft confinement. In the experiments we explore the equilibrium phase diagram of charged colloidal spheres in aqueous suspensions confined between two parallel charged walls at low background salt concentrations. Motivated by the experiments, we perform lattice sum minimizations to predict the crystalline ground state of point-like Yukawa particles which are exposed to a soft confining wall potential. In the multilayered crystalline regime, we obtain good agreement between the experimental and numerical findings: upon increasing the density we recover the sequence [structure: see text].

Confinement Induced Plastic Crystal-to-Crystal Transitions in Rodlike Particles with Long-Ranged Repulsion

“…Using colloids with κa ¼ 0.79 and 0.37 (where κ is the inverse Debye screening length characterizing the range of the repulsive interactions, and a is the particle radius) we found a phase sequence very similar to Ref. [16]: 1△ → 2□ → 2R → 2△ → 3□ → 3R → 3△ → 4□ → 4R → 4△ (R denotes a rhombic phase) [17]. These observations imply that long-ranged electrostatic interactions play an important additional role in the packing of charged particles.…”

“…More detailed investigations since then disclosed that many intermediate phases exist, such as a buckling phase in between 1△ → 2□, a rhombic phase in between n□ → n△, and prismatic, hexagonal close packing (hcp)-like, hcp(100), hcp⊥, and pre-square phases in between n△ → ðn þ 1Þ□ [3,[5][6][7]. All these structures have been verified by computer simulations [8][9][10][11][12][13], and the consistency between experiments and simulations gives us confidence in our understanding of the nature of the transitions for hard spheres.Fewer intermediate phases are found for charged spheres, which interact via long-ranged electrostatic interactions, both with each other and with the walls [14][15][16][17]. Using colloids with κa ¼ 0.79 and 0.37 (where κ is the inverse Debye screening length characterizing the range of the repulsive interactions, and a is the particle radius) we found a phase sequence very similar to Ref.…”

“…Fewer intermediate phases are found for charged spheres, which interact via long-ranged electrostatic interactions, both with each other and with the walls [14][15][16][17]. Using colloids with κa ¼ 0.79 and 0.37 (where κ is the inverse Debye screening length characterizing the range of the repulsive interactions, and a is the particle radius) we found a phase sequence very similar to Ref.…”

“…Because of the long-range repulsive forces and weak orientational correlation between the rods, the sequence of lattice transitions observed is well approximated by recent theories and simulations for spheres [8][9][10][11][12][13]16]. However, the nature of the preferred orientations of rods in this quasi-2D confinement was found to be a subtle competition between repulsive rod-wall interactions, which favor the rods to be parallel to the wall, and repulsive rod-rod interactions, which favor the rods to be parallel to each other and perpendicular to the wall [26].…”

Confined colloidal crystals in and out of equilibrium

“…Soft confinement, on the other hand, is described by a smooth external potential. An important example are charged glass plates which act as a soft confinement to charged colloids as described by an exponentially screened Coulomb interaction [31][32][33].…”