Two-Dimensional Heterogeneous Dynamics at the Surface of a Colloidal Suspension

Abstract

We report on an x-ray photon correlation spectroscopy experiment investigating the surface structure and dynamics of colloidal particles suspended in a supercooled viscous liquid. The static structure factor in the direction parallel and perpendicular to the surface reveals a more disordered structure at the surface as compared to the bulk. The particles display heterogeneous ballistic dynamics parallel to the surface. The particle dynamics in the direction perpendicular to the surface is much slower and does not show the hallmarks of ballistic motion.

Beyond simple exponential correlation functions and equilibrium dynamics in x-ray photon correlation spectroscopy

“…The positions of the peak τ * as a function of Q and as a function of waiting time are shown in figures 5 and 6, respectively, illustrating this close correspondence between the maximum of the temporal heterogeneity and the correlation time. Similar correspondence has been observed for χ (Q, t) for several systems [31], [37]- [39], [49,73] and also for χ 4 , where the peak position is considered a measure of the persistence of spatial heterogeneities [63]- [66]. While the position of the peak in χ(Q, t) unsurprisingly follows this common trend, the magnitude of χ (Q, t) for the sheared depletion gel is significantly smaller than the variance characterizing the hyper-diffusive dynamics in dilute gels measured with DLS [31], reflecting the relatively smooth evolution in dynamics observed in figure 3.…”

“…An accompanying feature of these dynamics is a relaxation rate that varies approximately linearly with wave vector, ∼ Q, implying hyper-diffusive, convectivelike motion. The wide assortment of disordered soft materials displaying these dynamics, as well as their observation with XPCS in a variety of polymeric systems [40]- [47] and in colloidal motion in glassy solvents [46,48,49], suggests a generic underlying mechanism; however, no clear consensus about their microscopic origin has emerged. As mentioned above, the continuous time Lévy flight model with a power-law distribution of waiting times can account for equation (8) with either γ < 1 or γ > 1.…”

Correlated heterogeneous dynamics in glass-forming polymers

“…The dominating feature is the peaking of χ T around the inflection point t = τ c /e of g 2 . The relatively high peak value on the order of 10 −2 was also observed in colloidal gels and for surface dynamics of colloidal particles suspended in viscous liquids [53,54]. The height of the χ T peak has a distinct q-related scaling proportional to q p at small q [53] that is connected to the q dependence of τ c (T ,q).…”

Microscopic dynamics in nanocomposite photosensitive films studied by X-ray photon correlation spectroscopy

“…Similar strategies have already been adopted in previous experiments [9,24]. The sample, mounted vertically in the vacuum chamber, was aligned forming an angle of 0.15° with respect to the incident beam, well below the critical angle for total external reflection from the silicon-air interface at 8.06 keV (α c =0.22°).…”