Real-space observation of salt-dependent aging in Laponite gels

TL;DR

This work addresses how salt concentration governs aging in low-concentration Laponite gels by combining real-space fluorescence imaging, label-free scattering, and particle-tracking microrheology. The authors show that increasing $C_{ ext{NaCl}}$ accelerates structural heterogeneity and reduces aggregate size, while microrheology reveals liquid-like diffusion in Laponite-poor regions and gel-like dynamics in Laponite-rich regions, with van Hove analyses suggesting nanoscale heterogeneities within the rich phase. The results support a nonequilibrium gelation framework in which salt acts as a depthful quench, promoting arrested phase separation and yielding long-lived, finely structured gels at high salt. These insights advance understanding of aging in charged colloidal gels and have implications for fluid transport, drug delivery, and tissue engineering where salt-driven aging controls material performance.

Abstract

Colloidal gels gradually evolve as their structures reorganize, a process known as aging. Understanding this behavior is essential for fundamental science and practical applications such as drug delivery and tissue engineering. This study examines the aging of low-concentration Laponite suspensions with varying salt concentrations using fluorescence microscopy, scattering imaging, and particle tracking microrheology. Structural heterogeneity appeared earlier at higher salt concentrations, and the average size of aggregates decreased as the salt concentration increased further. Fourier transform analysis corroborated these trends, and scattering images showed similar results. Microrheology revealed distinct dynamics in Laponite-rich and Laponite-poor regions: the poor phase exhibited liquid-like behavior, while the rich phase exhibited gel-like properties. Further analysis suggested the presence of submicron or nanoscale structural heterogeneities within the rich phase. These findings provide insight into how aging and salt concentration shape the structure and dynamics of colloidal gels.

Figures (7)

• Figure 1: Preparation of Laponite suspensions.
• Figure 2: Confocal fluorescence images of 1 Laponite suspensions at varying salt concentrations $C_{\mathrm{NaCl}}$ and waiting times $t_{\mathrm{W}}$. The bright areas indicate Laponite-rich regions. Images exhibiting homogeneous appearance are outlined in blue, while those exhibiting heterogeneous appearance are outlined in red. The arrow at $C_{\mathrm{NaCl}} = {}$0, $t_{\mathrm{W}} = {}$174 indicates the crack. The scale bar represents 50. Images were acquired at a height of 50 above the glass bottom to minimize the influence of the glass surface on structure formation. Fluorescence intensity is normalized by remapping the top and bottom 1% of values to the upper and lower bounds, respectively.
• Figure 3: Power spectra of fluorescence images obtained from 1 Laponite samples at different salt concentrations ($C_{\mathrm{NaCl}} = {}$020) at a fixed waiting time of $54\days$. Line colors correspond to different values of $C_{\mathrm{NaCl}}$.
• Figure 4: Scattering images of 1 Laponite suspensions for various salt concentrations ($C_{\mathrm{NaCl}} = {}$020) at a waiting time $t_{\mathrm{W}} = {}$3;110. Blue and red outlines indicate homogeneous and heterogeneous structures, respectively, as in Fig. \ref{['fig:fluor-label']}. The scale bar represents 50.
• Figure 5: Translational diffusion of fluorescent beads dispersed in 1 Laponite suspensions. (a) Representative time-averaged fluorescence image of a heterogeneous Laponite suspension ($C_{\mathrm{NaCl}} = {}$8mM, $t_{\mathrm{W}} = {}$26days), with Laponite-rich and Laponite-poor regions visible as high and low-intensity areas, respectively (see Section 2.6(i); without Gaussian filtering). (b-d) Color-coded time-averaged squared displacement (TAMSD) trajectories show that the fluorescent beads exhibit three different diffusions depending on the Laponite structure (outlined in blue, green, and red; see Fig. S9 and Section 2.6(ii) for the classification). (b) Normal Brownian diffusion in a homogeneous structure. (c) spatially uniform restricted diffusion in a homogeneous structure. (d) Heterogeneous diffusion in a heterogeneous structure. (e) Ensemble-averaged mean squared displacements (MSDs) of 30 bead trajectories classified as diffusing predominantly through the Laponite-rich phase (purple) or the Laponite-poor phase (orange), with variability indicated by 16–84% percentile bands ($n = 10^3 - 10^4$ for each curve) (see Section 2.6(iii) for further details). The black dashed line shows the fit to the poor-phase MSD using $\mathrm{MSD} = 4D\tau$, and the corresponding diffusion coefficient $D$ is also indicated with a fitting error. The numbers in parentheses represent the standard error ($1\sigma$) of the fitted diffusion coefficient, estimated from the fitting covariance assuming uncorrelated and normally distributed residuals.