Observation of the Pinning-Induced Crystal-Hexatic-Glass Transition in Two-Dimensional Colloidal Suspensions

Identification of the glass formation process in various conditions is of importance for fundamental understanding of the mechanism of glass transitions as well as for developments and applications of glassy materials. We investigate the role of pinning in driving the transformation of crystal into glass in two-dimensional colloidal suspensions of monodisperse microspheres. The pinning is produced by immobilizing a fraction of microspheres on the substrate of sample cells where the mobile microspheres sediment. Structurally, the crystal-hexatic-glass transition occurs with increasing the number fraction of pinning ρ pinning, and the orientational correlation exhibits a change from quasi-long-range to short-range order at ρ pinning = 0.02. Interestingly, the dynamics shows a non-monotonic change with increasing the fraction of pinning. This is due to the competition between the disorder that enhances the dynamics and the pinning that hinders the particle motions. Our work highlights the important role of the pinning on the colloidal glass transition, which not only provides a new strategy to prevent crystallization forming glass, but also is helpful for understanding of the vitrification in colloidal systems.

On the orientational correlations in the supercooled chloride lithium aqueous solution using the hybrid reverse Monte Carlo simulation

Employing the Hybrid Reverse Monte Carlo (HRMC) simulation, we compute, using the obtained three-dimensional configurations, the orientational correlations of water molecules in the supercooled 9.26 molal LiCl aqueous solution. This study aims to add relevant structural properties to those obtained in our latest studies and further support our findings. The Li/Cl pair ions hydration shells and the water molecules distribution studied using the Radial Pair Distribution Functions (RPDF), ([Formula: see text]) and ([Formula: see text]) are further described using the Orientational Pair Correlation Functions (OPCF), [Formula: see text] which describes the probability of a water molecule oriented by the Euler angles [Formula: see text], being located at the position [Formula: see text], with respect to another water molecule oriented [Formula: see text] placed at the origin. The high dimensionality of the orientational correlation functions has not presented a calculation disability, as known with several simulations, in the face of the efficiency of the HRMC and the water–water orientational correlation functions showed the dominant impact of ions on the water molecular dipole orientations within the hydration shells and in the hydrogen bonded molecules network.

Magnetic anisotropic colloids in nematic liquid crystals: Fluctuating dynamics simulated by multi-particle collision dynamics

Elongated colloids with a permanent magnetic moment and immersed in a nematic liquid crystal are studied numerically using a mesoscopic scheme that supports fluctuations, hydrodynamics, and topological defects. Colloids are accompanied by disclinations curves and subjected to an effective torque caused by the nematic environment and to a magnetic torque due to an external magnetic field. The case is analyzed where these torques compete to bring colloids to two different mutually perpendicular equilibrium states. The fluctuating dynamics of the colloid-defect pair is studied in terms of orientational correlation functions. Analytical expressions for these correlations are derived on the basis of an approximated planar lineal model. A good agreement is found between the numerical and analytical methods when magnetic torques are much larger than nematic torques, while for smaller magnetic torques nonlinear effects are demonstrated to be important. As conclusion, the numerical technique could be considered a reliable approach to the rotational motion of polar nanoparticles in liquid crystals.

Mechanics of Random Discontinuous Long-Fiber Thermoplastics—Part I: Generation and Characterization of Initial Geometry

The deformation mechanics of dry networks of large-aspect-ratio fibers with random orientation controls the processing of long-fiber thermoplastics (LFTs) and greatly affects the mechanical properties of the final composites. Here, we generate initial geometries of fiber networks in a cubic unit cell with a fiber aspect ratio of l/d = 100 and fully periodic boundary conditions for later numerical simulation. The irreversible random sequential adsorption (RSA) process is first used to generate a quasi-random structure due to the excluded-volume requirements. In order to investigate the nonequilibrium character of the RSA, a second method, which is similar to the mechanical contraction method (MCM) (Williams and Philipse, 2003, “Random Packings of Spheres and Spherocylinders Simulated by Mechanical Contraction,” Phys. Rev. E, 67, pp. 1–9) and based on a simplified Metropolis Monte Carlo (MC) simulation is then developed to produce quasi-equilibrium fiber geometries. The RSA packing results (ϕ ≈ 4.423% when using a fiber aspect ratio of 100) are in good agreement with the maximum unforced random packing limits (Evans and Gibson, 1986, “Prediction of the Maximum Packing Fraction Achievable in Randomly Oriented Short-Fibre Composites,” Compos. Sci. Technol., 25, pp. 149–162). The fiber structures were characterized by several distribution functions, including pair-spatial and pair-orientation distributions, based on either the center-to-center distance or the shortest distance between the particles. The results show that the structures generated by the RSA have an easily-detectable long-range spatial correlation but very little orientational correlation. In contrast, the quasi-equilibrium structures have reduced spatial correlation but increased short-range orientational correlation.