Helicopter rotation and smectic-isotropic coexistence of strongly attractive rods

Abstract

Hydrodynamic simulations of strongly attractive rodlike colloids are performed with and without shear flow. In the absence of flow, the isotropic-nematic coexistence becomes isotropic-smectic A, and the interfacial properties clearly vary with increasing attraction strength. In the presence of shear flow, a new collective rotation appears in which the director rotates in the vorticity-flow plane in a similar fashion to the movement of the rotor of a helicopter. DOI: 10.1103/PhysRevE.83.040701 PACS number(s): 64.70.M−, 82.70.Dd, 64.60.−i, 83.50.−v Liquid crystals in equilibrium, and in the presence of flow fields, show a very rich structural and dynamical behavior. Equilibrium phase diagrams display coexistence between isotropic (I), where rods are randomly oriented, nematic (N), where rods have a orientational preference, smectic (Sm), with both orientational and positional order, and crystalline phases, where the positional order is long ranged [1]. Attraction induced by depletion interactions has been considered in analytical calculations and experiments [2,3] of rods with large aspect ratios (rod length L over rod diameter σ ). The broadening of existing coexistence regions like I-N and N-Sm has been determined, together with the appearance of new ones as I-I, N-N, and eventually also I-Sm. Computer simulations are a powerful tool that can provide a detailed description of these systems. Numerous simulations with repulsive rods of small aspect ratios, typically from 2 to 6, have studied phases with positional order (see, e.g., Ref.[4]). Simulations of thermotropic systems of Gay-Berne colloidal rods [5] with increasing attraction have reported the appearance of different phases as a function of the potential asymmetry. Gay-Berne forces become unrealistically thin at the ends with increasing aspect ratio. I-Sm coexistence has been obtained with such rods by keeping the two phases at different temperatures [6]. In the presence of shear flow, the phase diagram is fundamentally altered [7]. Isotropic phases tend to align with flow, and oriented phases may undergo orchestrated motions that have been characterized about two decades ago [8]. Nematic phases have shown rotations in the flow-gradient plane (similar to the blades of a mill) or with an angle tilted in the vorticity direction (similar to the paddle in a kayak). These rotations are known as tumbling and kayaking [9]. Emerging questions are the effect that shear has when interfaces of varying softness are present or when applied to phases with positional order. Examples of attractive colloids where shear alignment is interesting for industrial applications are carbon nanotubes [10], wormlike micelles [11], or polymers.In this Rapid Communication, we investigate by means of computer simulations the effect of large attractions on the I-N coexistence of systems of rods with aspect ratio 20. At rest, increasing the attraction transforms the nematic phase into the smectic and linearly decreases the interface width. In the presence of shear flow, a qualitativ...

Filamentous Viruses Grafted with Thermoresponsive Block Polymers: Liquid Crystal Behaviors of a Rodlike Colloidal Model with “True” Attractive Interactions

“…The degree of shifting of C N to high concentrations is much higher than that of C I . Such classic widening behaviors of the I–N coexisting concentrations at each temperature qualitatively agree with the prediction of many theories dealing with rods with attractions. ,, The enhanced widening of the I–N coexisting concentrations upon increasing temperature suggests that the strength of interparticle attraction increases with increasing temperature. Therefore, we realize a system with pairwise interparticle attractions on a single particle level, the strength of which can be conveniently regulated in a wide range of temperatures.…”

“…As stated in the Introduction, there are many theoretical predictions about the liquid crystal behavior of rodlike particles or rigid polymers with attractive interactions, a few of which can accommodate certain experimental data with limited success. ,− The challenge in theoretical dealing of such cases is the detailed and accurate definition of the attractive potentials that is often dependent on the orientation of the particle. , Flory’s classic lattice theory of rigid polymers can account for interpolymer attractions using the parameter of the Flory–Huggins parameter (χ) between the LC mesogens and the solvent, which has been later extended to rigid polymers grafted with side polymers that are highly related to our system. , However, the lattice model involves many stringent assumptions in the treatment of a system of ordered rigid rods, and such theory can only qualitatively compare with experimental results. In the case of colloidal rods with effective attractions realized by the depletion effect, detailed theoretical phase diagrams have been constructed but pronounced discrepancy was found when comparing with experimental data …”

“…Inspired by the abnormal LC behaviors of rodlike particles with attractive interactions, various kinds of theories dealing with such situations have appeared and been tested with simulations, leading to a consensus that the isotropic (I)–nematic (N) coexisting concentrations in the phase diagram will widen with increasing attractive strength: i.e., the number density of the rod in the isotropic liquid phase ( p i ) will decrease while that of the nematic LC phase ( p n ) will increase. − Compared to such theoretical progress, experimental investigations of real systems with attractive interactions have lagged far behind, mainly due to the shortage of well-defined system with controllable attractive interactions. Early efforts have been dedicated to some systems like poly­(γ-benzyl l -glutamate) (PBLG), rodlike mineral particles, and so on, which have attractive interactions of unknown origins. − However, the attraction in these cases is often uncontrollable and leads to gelation or other dynamically arrested states that defies systematic and unambiguous investigation of the equilibrium phase separation.…”

Temperature inhomogeneities simulated with multiparticle-collision dynamics

“…The MPC method has already been extensively tested, showing, for example, to reproduce the Navier-Stokes equation, 44 to include hydrodynamic interactions in polymer solutions, 50,54 or more recently to fulfil the fluctuation theorem. 55 In isothermal conditions, MPC has already been applied to a large number of systems like colloidal solutions, 56,57 rod-like colloids, 58,59 polyelectrolyte solutions, 60 or nanoswimmers.…”