Confinement-induced Transition Research Articles

Confinement-Induced Transition between Wavelike Collective Cell Migration Modes.

The structural and functional organization of biological tissues relies on the intricate interplay between chemical and mechanical signaling. Whereas the role of constant and transient mechanical perturbations is generally accepted, several studies recently highlighted the existence of long-range mechanical excitations (i.e., waves) at the supracellular level. Here, we confine epithelial cell monolayers to quasi-one-dimensional geometries, to force the establishment of tissue-level waves of well-defined wavelength and period. Numerical simulations based on a self-propelled Voronoi model reproduce the observed waves and exhibit a phase transition between a global and a multinodal wave, controlled by the confinement size. We confirm experimentally the existence of such a phase transition, and show that wavelength and period are independent of the confinement length. Together, these results demonstrate the intrinsic origin of tissue oscillations, which could provide cells with a mechanism to accurately measure distances at the supracellular level.

Quantum manipulation of electronic phase separation in complex oxides

Complex oxides system displays exotic properties such as high temperature superconductivity, colossal magnetoresistance and multiferroics. Owing to the strong correlation between lattice, spin, charge and orbital degrees of freedom, competing electronic states in complex oxides system often have close energy scales leading to rich phase diagrams and spatial coexistence of different electronic phases known as electronic phase separation (EPS). When the dimension of complex oxides system is reduced to the length scale of the correlation length of the EPS, one would expect fundamental changes of the correlated behavior. This offers a way to control the physical properties in the EPS system. In this paper, we review our recent works on electronic phase separation in complex oxide systems. We discovered a pronounced ferromagnetic edge state in manganite strips; by using lithographic techniques, we also fabricated antidot arrays in manganite, which show strongly enhanced metal-insulator transition temperature and reduced resistance. Moreover, we discovered a spatial confinement-induced transition from an EPS state featuring coexistence of ferromagnetic metallic and charge order insulating phases to a single ferromagnetic metallic state in manganite. In addition, by using unit cell by unit cell superlattice growth technique, we determined the role of chemical ordering of the dopant in manganite. We show that spatial distribution of the chemical dopants has strong influence on their EPS and physical properties. These works open a new way to manipulate EPS and thus the global physical properties of the complex oxides systems, which is potentially useful for oxides electronic and spintronic device applications.

Direct Nanomechanical Measurement of an Anchoring Transition in a Nematic Liquid Crystal Subject to Hybrid Anchoring Conditions

We have used a surface forces apparatus to measure the normal force between two solid curved surfaces confining a film of nematic liquid crystal (5CB, 4'-n-pentyl-4-cyanobiphenyl) under hybrid planar-homeotropic anchoring conditions. Upon reduction of the surface separation D, we measured an increasingly repulsive force in the range D = 35-80 nm, reaching a plateau in the range D = 10-35 nm, followed by a short-range oscillatory force at D < 5 nm. The oscillation period was comparable to the cross-sectional diameter of the liquid crystal molecule and characteristic of a configuration with the molecules parallel to the surfaces. These results show that the director field underwent a confinement-induced transition from a splay-bend distorted configuration at large D, which produces elastic repulsive forces, to a uniform planar nondegenerate configuration with broken homeotropic anchoring, which does not produce additional elastic forces as D is decreased. These findings, supported by measurements of the birefringence of the confined film at different film thicknesses, provide the first direct observation of an anchoring transition on the nanometer scale.

Confinement-Induced Transition of Topological Defects in Smectic Liquid Crystals: From a Point to a Line and Pearls

We report a study on the confinement-induced transition of the topological defects of liquid crystals (LCs) using smectic-A LCs confined in prolate spheroids with homeotropic anchoring. Upon increasing the aspect ratio of a LC droplet, dispersed in a stretched elastomer film, the topological defect undergoes a transition from a point to a line of which the length is a function of the aspect ratio. Additionally, when the size of a droplet is larger than a certain value, the defect has a pearl-necklace-like texture. We propose a simple model to understand the formation of these defects in terms of the misorientation and undulation instability of the smectic layers.

Surfactant Two-Dimensional Self-Assembly under Confinement

Confinement-induced structural rearrangements in supported self-assembled surfactant layers in aqueous salt solutions are investigated using classical density functional theory. The systematic study of the influence of the nature of electrolyte revealed that 2:1 electrolyte stabilizes the hemicylindrical configuration of ionic surfactant layers, while a confinement-induced transition to a tilted monolayer configuration was found in symmetric 1:1 and 2:2 electrolytes. On the basis of this study, we formulate a general model for the energetics of structural rearrangements in supported surfactant layers. This model provides a basis for directed self-assembly of surfactant templates with desired structure and stability for scalable synthesis of nanocomposite functional materials, templated crystal growth, and biomolecule adsorption.