Spherically Anisotropic Colloids

Most molecules in nature are spherically anisotropic, which gives rise to unique types of collective behavior both in the bulk material and within the various interfacial phases. In bulk phases, the anisotropy may give rise to liquid crystalline phases; at the interfaces, the anisotropy is suggested to be responsible the formation of quasi-two-dimensional surface-frozen phases. The formation of liquid-crystalline phases and the surface freezing transitions were intensively studied during the last decades, yet the conventional techniques do not allow the individual particles to be imaged in real-time, during the kinetics of these phase transitions. Thus, the physical mechanism driving these phase transitions is obscure.

We employ colloidal spheroids (ellipsoids of revolution) to mimic the behavior of spherically anisotropic atoms and molecules. These particles undergo Brownian motion, thus tending to minimize their free energy, akin to atoms and molecules; yet, these particles are visible for optical microscopy. We track these particles in real time, in three dimensions, employing confocal microscopy. We fine-tune the interactions between these particles, which is impossible with the conventional molecules. These studies shed light onto the role of rotational degrees of freedom in fluids and reveal an unexpected phase-transition to exist in these fluids at the ‘sphere point’, where the particle aspect ratio is unity. A similar phase transition has recently been observed in random packings of granular matter, suggesting that studies of fluids may possibly open new exciting directions towards the development of the ‘thermodynamics of granular matter’.