Article

# Shear-induced dynamic polarization and mesoscopic structure in suspensions of polar nanorods.

Institut für Theoretische Physik, Technische Universität Berlin, Hardenbergstrasse 36, D-10623 Berlin, Germany.

Physical Review Letters (Impact Factor: 7.73). 02/2009; 102(2):028301. DOI: 10.1103/PhysRevLett.102.028301 Source: PubMed

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**ABSTRACT:**Based on a continuum theory, we investigate the manipulation of the non-equilibrium behavior of a sheared liquid crystal via closed-loop feedback control. Our goal is to stabilize a specific dynamical state, that is, the stationary "flow-alignment", under conditions where the uncontrolled system displays oscillatory director dynamics with in-plane symmetry. To this end we employ time-delayed feedback control (TDFC), where the equation of motion for the ith component, q_i(t), of the order parameter tensor is supplemented by a control term involving the difference q_i(t)-q_i(t-\tau). In this diagonal scheme, \tau is the delay time. We demonstrate that the TDFC method successfully stabilizes flow alignment for suitable values of the control strength, K, and \tau; these values are determined by solving an exact eigenvalue equation. Moreover, our results show that only small values of K are needed when the system is sheared from an isotropic equilibrium state, contrary to the case where the equilibrium state is nematic.Physical Review E 12/2013; 88:062509. · 2.31 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**We study the role of the control parameter triggering nematic order (temperature or concentration) on the dynamical behavior of a system of nanorods under shear. Our study is based on a set of mesoscopic equations of motion for the components of the tensorial orientational order parameter. We investigating these equations via a systematic bifurcation analysis based on a numerical continuation technique, focusing on spatially homogeneous states. Exploring a wide range of parameters we find, unexpectedly, that states with oscillatory motion can exist even under conditions where the equilibrium system is isotropic. These oscillatory states are characterized by wagging motion of the paranematic director, and they occur if the tumbling parameter is sufficiently small. We also present full non-equilibrium phase diagrams, in the plane spanned by the concentration and the shear rate.Physical Review E 07/2013; 88(1):012505. · 2.33 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Combining computer simulations and experiments we investigate the role of particle charges on the layering behavior of confined silica colloids (slit-pore geometry). To this end we perform colloidal-probe atomic-force-microscope measurements of the oscillatory solvation force of confined suspensions involving three different particle sizes (and resulting total charges). For all systems, the wavelength of the oscillations as function of volume fraction is found to display a power-law behavior characterized by similar exponents. However, the results for different particle sizes cannot be simply mapped onto each other, which demonstrates the importance of correlation effects due to the different Coulomb coupling. Our experimental findings are consistent with the results from parallel Monte Carlo computer simulations of a (DLVO) model system.Soft Matter 03/2010; 6:2330-2336. · 4.15 Impact Factor

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