Abstract
Nematic droplets are droplets composed of elongated molecules that tend to point in the same direction but do not have any positional order. Such droplets are well known to adopt a spindle shape called tactoid. How such droplets condensate or melt and how the orientational symmetry is broken remains however unclear. Here we use a colloidal system composed of filamentous viruses as model rod–like colloids and pnipam microgel particles to induce thermo–sensitive depletion attraction between the rods. Microscopy experiments coupled to particle tracking reveal that the condensation of a nematic droplet is preceded by the formation of a new phase, an isotropic droplet. As the viruses constitute an excellent experimental realization of hard rods, it follows that the phenomenology we describe should be relevant to diverse micro- and nano-sized rods that interact through excluded volume interactions. This transition between isotropic and nematic droplets provides a new and reversible pathway to break the symmetry and order colloidal rods within a droplet with an external stimulus, and could constitute a benchmark experiment for a variety of technologies relying on reconfigurable control of rods.
Highlights
Reconfigurable self-assembly of rod-like molecules leads both to a better fundamental understanding of phase transition[1,2] and a variety of technologies which rely and urge for switchable molecular structures such as electro-optical modulators, optical switches, light shutters[3,4] or biosensors[5]
As the viruses used in our studies are an excellent experimental realization of hard rods[15], it follows that the phenomenology we describe should be relevant to diverse micro- and nano-sized rods that interact through excluded volume interactions
First, we present the characteristics of our system, namely colloidal rods dispersed in buffer with thermo–responsive microgel particles
Summary
Reconfigurable self-assembly of rod-like molecules leads both to a better fundamental understanding of phase transition[1,2] and a variety of technologies which rely and urge for switchable molecular structures such as electro-optical modulators, optical switches, light shutters[3,4] or biosensors[5]. They eventually coalesce to minimize the interface between the isotropic and the nematic phases to achieve the thermodynamically stable state: two homogeneous phases, the isotropic and nematic phase separated by a single interface This transition has been observed in many experimental systems, tobacco mosaic viruses (TMV)[14], fd–viruses[15], F–actin[16], bohemite rods[17], carbon nanotubes[18,19,20], liquid crystal molecules[21,22] and computer simulations[23,24]. The loss in entropy due to the orientation ordering in the nematic phase is over compensated by the increase in translational entropy of the system: the available space for any one rod increases as the rods become more aligned This conventional pathway is inadequate to study tactoid formation; it necessitates a concentration quench which is tricky to realize. Direct visualization of rods orientation with single particle resolution fluorescence microscopy allows us to track the motions of individual rods and to determine the translational and orientation dynamics of the rods and the order parameter
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