Influence of the surface charge on the homogeneity of colloidal crystals

Abstract

Five groups of suspensions composed of polystyrene particles, having similar size but different effective surface charge, were adopted to investigate the effects of surface charge and volume fraction on the homogeneity of colloidal crystals through checking the difference between D exp and D uni by reflection spectroscopy method (D exp , D uni are the experimental and the expected value of the average nearest neighbor interparticle distance by assuming a uniform structure, respectively). We found volume fractions (ranging from 0.006 to 0.02) and structure types basically have no influence on the values of D exp /D uni . Moreover, for crystals formed by lowly charged particles, D exp /D uni is approximately equal to 1, implying the crystals are homogeneous. With the increase of effective surface charge, D exp gradually deviates from D uni and the formed crystals become inhomogeneous. Our experimental observations are in accordance with the previous simulation results. Additionally, we also found D exp /D uni initially drops quickly with increasing effective surface charge and then it tends to an asymptotic value (∼0.85), it is supposedly due to the saturation of effective charge. Our relevant computer simulations confirmed that the study scheme that using D exp /D uni as an indicator to assess the homogeneity of crystal structure is tenable and the simulation results are consistent with experiments.

Effect of void structures in crystalline structure on the shear moduli of charged colloidal crystals

“…The Yukawa potential V Yukawa (r) which considers Coulomb repulsions only was usually used for V (r) in this shear modulus-potential model before [11,29,30]. However, experimental observations indicated that there are voids appeared in highly-charged colloidal crystals [12,31,32]. So in addition to the repulsion, inclusion of the conterions-mediated long-range attraction was suggested and then the inter-particle interaction potential was replaced by SI potential [33,34].…”

“…where n is the particle number density of the suspension, ε 0 ε r the dielectric permittivity of the suspension, k B T the thermal energy, and n salt = 2000N A c [12,15] the number density of background electrolyte with N A being Avogadro constant and c the molar concentration of foreign salt (mol L −1 ).…”

“…For instance, it was shown [24] that in the bcc-fcc transition process of colloidal crystallization, the shear moduli are much smaller than theoretical expectations and this can be chalked up to crystalline imperfection due to not-well-equilibrated structure during the transition state. Another example demonstrating the structural imperfection is that there exist some void structures in the crystal region of crystals formed by highly-charged colloidal suspensions [12,13,23].…”

“…In this respect the elasticity of the colloidal crystals has been studied in recent years both experimentally and theoretically [10][11][12][13][14]. Experimentally, shear elastic moduli of the colloidal crystals are usually measured through the method of torsional resonance spectroscopy (TRS) in which standing waves are excited when rotary oscillations are imposed on the colloidal crystals [15][16][17].…”

Shear moduli in bcc-fcc structure transition of colloidal crystals

“…The concentration of the particles can be conveniently adjusted by adding latex or pure water, and the exact concentration can be evaluated through the conductivity measurements. 30,33 …”