The aggregation of dense colloidal solutions has been investigated by means of low-angle static light scattering. We show that the scattered pattern exhibits a finite-^-vector peak, whose intensity and position q m change with time. We find that the intensity distributions scale according to S(q/q m ,t) -q m (t) ~dF(q/q m )y in agreement with the scaling law for spinodal decomposition. While -3 for spinodal decomposition, here scaling requires that d =df, the fractal dimension of the clusters. PACS numbers: 64.75.+g, 05.40,+j, 64.60.Ht, 82.70.Dd The spinodal decomposition (SD) is a phase-separation process that has been investigated in a large number of quite dissimilar systems like small-molecule liquid mixtures [1-4], metallic alloys [5,6], polymer blends [7,8], inorganic glasses [9], and thermodynamically unstable colloidal systems [10]. The peculiar feature of SD is that the long-wavelength diffusion coefficient becomes negative so that fluctuations grow instead of decaying. The fastest-growing fluctuation occurs at a finite wave vector and this gives rise to the well-known ring in the pattern of scattered radiation. The intensity and radius of the ring change in time as the thermodynamically stable state is approached. In spite of the diversity of the physical systems, universal features in the dynamics are observed. It should also be pointed out that other phase-separating systems, although not exactly falling in the class of SD, exhibit the same behavior [11,12].Colloidal aggregation is another area where a substantial amount of work has been produced in recent years [13]. In this case also diffusion plays an essential role. Monomers diffuse to form fractal clusters, and the clusters themselves diffuse to coalesce into even larger clusters and so on.In this paper we will show that colloidal aggregation exhibits the same features of SD in spite of the fact that nothing anomalous occurs to the diffusion coefficient of the monomers and clusters. By using very-low-angle static light scattering and high monomer concentration, we present for the first time clear evidence of a ring in the scattered intensity pattern. We will show that during the later stages the dynamics is in agreement with the scaling predictions of Marro, Lebowitz, and Kalos [14], also put forward by Furukawa [15] and by Binder and Stauffer [16].The position of the scattered peak q m and the scaled structure factor S(q/q m ,t) are related by the equation S(qlq m .t)=q m (t)-d F{qlq m ),where F(q/q m ) is a time-independent scaling function. For ordinary spinodal decomposition, rf -3, while here we find that the relation holds if we take d=d/, where dj is the fractal dimension of the clusters. The surprising similarities between these results and those related to the spinodal decomposition are likely to suggest some underlying common mechanism in the dynamics of these irreversible processes. The measurements have been performed on a solution of polystyrene spheres 0.0190 jim in diameter in a water-heavy-water mixture. The mixture was adjuste...
Spatial scale invariance represents a remarkable feature of natural phenomena. A ubiquitous example is represented by miscible liquid phases undergoing diffusion. Theory and simulations predict that in the absence of gravity diffusion is characterized by long-ranged algebraic correlations. Experimental evidence of scale invariance generated by diffusion has been limited, because on Earth the development of long-range correlations is suppressed by gravity. Here we report experimental results obtained in microgravity during the flight of the FOTON M3 satellite. We find that during a diffusion process a dilute polymer solution exhibits scale-invariant concentration fluctuations with sizes ranging up to millimetres, and relaxation times as large as 1,000 s. The scale invariance is limited only by the finite size of the sample, in agreement with recent theoretical predictions. The presence of such fluctuations could possibly impact the growth of materials in microgravity.
A fluctuating hydrodynamics approach is presented for the calculation of the structure factor for timedependent nonequilibrium diffusive processes in binary liquid mixtures. The hydrodynamic equations are linearized around the time-dependent macroscopic state given by the usual phenomenological diffusion equation. The cases of free diffusion, thermal diffusion, and barodiffusion are considered in detail. The results are used to describe the low-angle scattered intensity distributions from the time-dependent concentration profiles during the approach to steady state. The theoretical predictions are found to be in agreement with experimental data from thermal diffusion and free diffusion experiments. It is shown that in general the presence of nonequilibrium concentration fluctuations yields a substantial increase in the static structure factor over the equilibrium value, at least for the cases of free diffusion and thermal diffusion. As in the case of nonequilibrium fluctuations at steady state, the static structure factor displays a fast k Ϫ4 divergence at larger wave vectors k, and saturation to a constant value for k smaller than a critical wave vector k RO. It is also shown that the static structure factor from a sedimenting mixture is actually temporarily lowered below the equilibrium value for k smaller than k RO. As the steady state is approached, the structure factor loses any k dependence and it attains the equilibrium value. ͓S1063-651X͑98͒14910-3͔
We have investigated with small angle light scattering and optical microscopy transient gelation phenomena which occur in phase-separating colloid-polymer mixtures. The scattering intensity distribution shows a peak at non-zero wave vector and satisfies the asymptotic q-4 Porod behaviour. Consistent with these observations, optical micrographs show an alternating pattern of dark and bright domains. These findings suggest that the polymer-induced depletion forces lead to the formation of a bicontinuous network of colloid-rich and colloid-poor domains, via a spinodal decomposition process. This bicontinuous network rapidly attains a gellike character as indicated by the arrest of speckle fluctuations. The occurrence of the gel is ascribed to polymer-induced aggregation between the colloids in the colloid-rich phase. Due to the reversible nature of the aggregation the network restructures and eventually the gel collapses, as is manifested by the rapid separation of the colloid-rich phase from the colloid-poor phase.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.