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We study the properties of a solid-solid close-packed to body-centered tetragonal transition in a colloidal suspension via fluorescence confocal laser scanning microscopy, in three dimensions and in real space. This structural transformation is driven by a subtle competition between gravitational and electric dipolar field energy, the latter being systematically varied via an external electric field. The transition threshold depends on the local depth in the colloidal sediment. Structures with order intermediate between close-packed and body-centered tetragonal were observed, with these intermediate structures also being stable and long lived. This is essentially a colloidal analogue of an ''atomiclevel'' interfacial structure. We find qualitative agreement with theory (based purely on energetics). Quantitative differences can be attributed to the importance of entropic effects. DOI: 10.1103/PhysRevLett.92.058301 PACS numbers: 82.70.Dd, 05.70.Fh, 64.70.Kb, 83.80.Gv The solid state exhibits a rich variety of crystalline symmetries. Elemental solids are often body-centered cubic close to the melting point, and close packed at low temperatures. On the other hand, alloys of two or more elements display an astounding variety of crystal structures. Many solids undergo crystal-crystal transitions that can take place only under the change of an external parameter, with thermal fluctuations not being predominant. One such transition is a diffusionless solid-solid transformation known as a martensitic transition [1][2][3][4].We report here on a 3D real-space study in a colloidal model system where we use an external electric field as a control parameter to study a martensitic crystal transformation in the presence of a second competing external field: gravity. Phenomenological modeling of martensitic transitions has indeed been shown to involve terms that act as effective long-range anisotropic interactions [5]. While the gravitational interaction is not relevant in atomic and molecular crystalline solids, it could well be that the complex phase behavior in alloys arises from a similar subtle competition of interactions. The only example we know where a colloidal model system was used to study a martensitic crystal transition focused on the effects of confining walls on crystal symmetry [6]. Brownian motion ensures that colloidal dispersions have a well-defined thermodynamic temperature and exhibit phases analogous to those seen in atomic systems; microscopy enables the quantitative study [7][8][9] of structure (including defects and grain boundaries) and dynamics on a single-particle level in the bulk: this is difficult in studies of atomic systems. In addition, controlled switching between two colloidal crystal structures can be used to make photonic band gap materials [10 -12].Our colloidal system is composed of silica spheres (radius, R 692 nm) suspended in a (20:80 volume ratio) water-dimethylsulfoxide (DMSO) liquid mixture, chosen to match the refractive index of the silica spheres. It has been previously establ...

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We investigate the liquid-vapor interface of the restricted primitive model (RPM) for an ionic fluid using a density-functional approximation based on correlation functions of the homogeneous fluid as obtained from the meanspherical approximation (MSA). In the limit of a homogeneous fluid our approach yields the well-known MSA (energy) equation of state. The ionic interfacial density profiles, which for the RPM are identical for both species, have a shape similar to those of simple atomic fluids in that the decay towards the bulk values is more rapid on the vapor side than on the liquid side. This is the opposite asymmetry of the decay to that found in earlier calculations for the RPM based on a square-gradient theory. The width of the interface is, for a wide range of temperatures, approximately four times the second moment correlation length of the liquid phase. We discuss the magnitude and temperature dependence of the surface tension, and argue that for temperatures near the triple point the ratio of the dimensionless surface tension and critical 1 temperature is much smaller for the RPM than for simple atomic fluids.

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