Confinement-controlled phase behavior of charged colloids under gravity.

We investigate the structure of sedimented charged colloidal suspensions under gravitational confinement. Inside a sealed glass sample cell, particles dispersed in a medium accumulate near the upper or the bottom planar wall and form quasi-2D colloidal layers. By adjusting the medium composition and particle electrostatic interactions, we systematically vary the gravitational Peclet number and the dimensionless screening parameter. Using light microscopy imaging, we study the structural development of the sedimented layers at different initial particle concentrations. Our results suggest a complex phase behavior where strong confinement and charge screening regimes favor the formation of ordered triangular lattices stacked in layer sequences, characteristic of face-centered cubic crystal planes. On the contrary, systems with weak confinement present disordered fluid phases. For intermediate parameters, we experimentally identify a coexistence region of triangular and rhombic phases. This study provides an experimental framework for predicting structural transitions in gravitationally confined soft-matter systems.