Visualization of the Sol–Gel Transition in Porous Networks Using Fluorescent Viscosity-Sensitive Probes

The sol–gel transition involves the transformation of a colloidal suspension into a system-spanning, interconnected gel. This process is widely used to reinforce mechanically weakened porous artifacts, such as sculptures but the impact of the restricted geometry (porous network) on the gelation dynamics of the solution remains unclear. Here, using fluorescent viscosity-sensitive molecular rotors, confocal microscopy, and model pores, we visualize the local viscosity changes at the microscale that accompany the sol–gel transition of a methyltriethoxysilane solution into a gel network. We show that, with evaporation of the solvent, a viscosity gradient develops near the free surface, triggering the sol–gel transition inside small pores near the surface. In homogeneous porous media, this leads to skin formation, which reduces the evaporation rate. In heterogeneous porous media, a gradient in gel density develops toward the heart of the porous material, where the gel formation mainly occurs as capillary bridges within smaller pores.

1. Chemistry of sol-gel transition with methyltriethoxysilane Methyltriethoxysilane (C 2 H 5 O) 3 SiCH 3 (Sigma-Aldrich) is hydrolysed prior to its use by a 0.1 mol.L −1 solution of acetic acid (Sigma-Aldrich).The hydrolysis reaction is the following: where the resulting solution consists CH 3 Si(OH) 3 methylsilanetriol MTS in a solvent composed of water and ethanol (formed during the hydrolysis of the MTEOS).After (partial) hydrolysis, evaporation of the solvent triggers condensation reactions in the solution that lead to the formation of a highly branched gel network spanning through the whole sample. 1 Generally, condensation reactions between the MTS monomers formed start before the hydrolysis is complete.Two condensation reactions can describe the sol-gel process: 5 are methylsilanes at different condensation states.

DLS measurements
We used a Dynamic Light Scattering setup (ALV ALV/DLS/SLS-5000) to determine the particle size distribution with a wavelength of 633 nm at a 90°angle.Fig. S1 is the DLS measurement of three different stages of the sol-gel transition of MTEOS.After the hydrolysis

S2
of the MTEOS, the solution consists of monomers with the size of 2-3 nm (Fig. S1a) in suspension in the solvent.When the evaporation starts, the monomers start to aggregate and form oligomers with a broader size distribution with the radius of 2-12 nm.Bigger clusters are also formed, with a size around 1-2 µm.This shows the creation of a second population of bigger oligomers (Fig. S1b).With further evaporation, oligomers polymerized and interlinked to form a gel network (Fig. S1c).At that stage, due to the limitation of the DLS setup and multi scattering effect during gelation, exact particle sizes is not reliable anymore.τ 2 is the amplitude and lifetime of the second decay, respectively.All the values that are reported as a lifetime are the amplitude average lifetime ⟨τ ⟩ defined as: Which is in this case proportional to the steady-state intensity.tensity between both microscopes is also conducted (Fig. S2c).By using (Fig. S2c) and the present linear relationship between lifetime and intensity in these measurements, we are able to establish a correlation between all the intensity values obtained from the Zeiss microscope and a corresponding fluorescence lifetime value.

Evolution of the surface tension and contact on glass capillary wall of MTEOS during the sol-gel transition
The

Skin formation in round capillary
As described in the main text, the meniscus undergoes recession due to evaporation, leading to the accumulation of macromolecules at its surface.Consequently, a gradient of macromolecules is formed within the capillary.In Fig. S7, the Fluorescence decay curves of MTEOS at t=1080 min in a round capillary at various x positions during the Sol-Gel Transition are shown.
In Fig. S8, the fluorescence lifetime values are plotted against the distance from the surface of the meniscus.Notably, it is evident that the fluorescence lifetime gradient from the meniscus surface to the bulk region intensifies over time.

Protocole for the 2D porous media
We designed in this study quasi-2D micromodels of porous media following the protocol detailed in. 2 We recall here the main steps.Rectangular borosilicate microcapillaries (Vitrocom) are cut to the volume 13 × 3 × 0.3mm 3 .One side is melted with a torch and the capillaries are filled with soda lime glass beads of diameter 210-250 µm (Polyscience).To and heated near the glass transition temperature of the glass beads, so that they are sintered together by forming bridges but without deforming their overall shape.Quasi 2D porous media with two pore size distributions (heterogeneous porous media) can the also obtained by mixing the glass beads with NaCl crystals of sizes 150-300 µm prior to the filling of the capillaries.After the heating step, the NaCl crystals (still entrapped in the capillary as their melting point is much higher) are removed by washing several times the sintered porous media with water.

Figure 1 :Figure 2 :
Figure 1: Dynamic light scattering(DLS) measurements: The Three stage scheme of the solgel transition.a) Formation of the MTS monomers after the hydrolysis process b) Formation of the oligomers c) Growth of the clusters and percolation formation

AFigure 3 :Figure 4 :
Figure 3: Fluorescence lifetime measurement of MTEOS droplet+4-DASPI: a) fluorescence lifetime as a function of time at the center of the droplet b) fluorescence lifetime as a function of intensity

Figure 5 :
Figure 5: Surface tension evolution during the sol-gel transition of a 10 µL MTEOS droplet plotted as a function of time

2 Figure 7 :
Figure 7: Fluorescence decay curves of MTEOS at t=1080 min in a round capillary at various x positions during the Sol-Gel Transition.The blue curve represents the reference decay curve for the bulk solution at t=10 min.

Figure 8 :
Figure8: Skin Formation in a Round Capillary.The fluorescence lifetime as a function of time is presented for both the skin layer and bulk region, depicting the evolution of skin formation over time.Additionally, fluorescence lifetime profiles at four distinct time steps are plotted as a function of distance from the meniscus, allowing translation to viscosity as explained in the previous section.