Bacterial spores respond to humidity similarly to hydrogels.

Bacterial spores have outstanding properties from the materials science perspective, which allow them to survive extreme environmental conditions. Recent work by [S. G. Harrellson et al., Nature 619, 500-505 (2023)] studied the mechanical properties of Bacillus subtilis spores and the evolution of these properties with the change of humidity. The experimental measurements were interpreted assuming that the spores behave as water-filled porous solids, subjected to hydration forces. Here, we revisit their experimental data using literature data on vapor sorption on spores and ideas from polymer physics. We demonstrate that upon the change of humidity, the spores behave like rubber with respect to their swelling, elasticity, and relaxation times. This picture is consistent with the knowledge of the materials comprising the bacterial cell walls-cross-linked peptidoglycan. Our results provide an interpretation of the mechanics of bacterial spores and can help in developing synthetic materials mimicking the mechanical properties of the spores.

Durable Self-Cleaning Coatings for Architectural Surfaces by Incorporation of TiO2 Nano-Particles into Hydroxyapatite Films.

To prevent soiling of marble exposed outdoors, the use of TiO2 nano-particles has been proposed in the literature by two main routes, both raising durability issues: (i) direct application to marble surface, with the risk of particle leaching by rainfall; (ii) particle incorporation into inorganic or organic coatings, with the risk of organic coating degradation catalyzed by TiO2 photoactivity. Here, we investigated the combination of nano-TiO2 and hydroxyapatite (HAP), previously developed for marble protection against dissolution in rain and mechanical consolidation. HAP-TiO2 combination was investigated by two routes: (i) sequential application of HAP followed by nano-TiO2 ("H+T"); (ii) simultaneous application by introducing nano-TiO2 into the phosphate solution used to form HAP ("HT"). The self-cleaning ability was evaluated before and after prolonged exposure to simulated rain. "H+T" and "HT" coatings exhibited much better resistance to nano-TiO2 leaching by rain, compared to TiO2 alone. In "H+T" samples, TiO2 nano-particles adhere better to HAP (having flower-like morphology and high specific surface area) than to marble. In "HT" samples, thanks to chemical bonds between nano-TiO2 and HAP, the particles are firmly incorporated in the HAP coating, which protects them from leaching by rain, without diminishing their photoactivity and without being degraded by them.

An Ideal Solid Solution Model for C-S-H.

A model for an ideal solid solution, developed by Nourtier-Mazauric et al. [Oil & Gas Sci. Tech. Rev. IFP, 60 [2] (2005) 401], is applied to calcium-silicate-hydrate (C-S-H). Fitting the model to solubility data reported in the literature for C-S-H yields reasonable values for the compositions of the end-members of the solid solution and for their equilibrium constants. This model will be useful in models of hydration kinetics of tricalcium silicate because it is easier to implement than other solid solution models, it clearly identifies the driving force for growth of the most favorable C-S-H composition, and it still allows the model to accurately capture variations in C-S-H composition as the aqueous solution changes significantly at early hydration times.

Thermo-Mechanical Compatibility of Viscoelastic Mortars for Stone Repair.

The magnitude of the thermal stresses that originate in an acrylic-based repair material used for the reprofiling of natural sandstone is analyzed. This kind of artificial stone was developed in the late 1970s for its peculiar property of reversibility in an organic solvent. However, it displays a high thermal expansion coefficient, which can be a matter of concern for the durability either of the repair or of the underlying original stone. To evaluate this risk we propose an analytical solution that considers the viscoelasticity of the repair layer. The temperature profile used in the numerical evaluation has been measured in a church where artificial stone has been used in a recent restoration campaign. The viscoelasticity of the artificial stone has been characterized by stress relaxation experiments. The numerical analysis shows that the relaxation time of the repair mortar, originating from a low T g , allows relief of most of the thermal stresses. It explains the good durability of this particular repair material, as observed by the practitioners, and provides a solid scientific basis for considering that the problem of thermal expansion mismatch is not an issue for this type of stone under any possible conditions of natural exposure.

Nucleation, growth and evolution of calcium phosphate films on calcite.

Marble, a stone composed of the mineral calcite, is subject to chemically induced weathering in nature due to its relatively high dissolution rate in acid rain. To protect monuments and sculpture from corrosion, we are investigating the application of thin layers of hydroxyapatite (HAP) onto marble. The motivation for using HAP is its low dissolution rate and crystal and lattice compatibility with calcite. A mild, wet chemical synthesis route, in which diammonium hydrogen phosphate salt was reacted with marble, alone and with cationic and anionic precursors under different reaction conditions, was used to produce inorganic HAP layers on marble. Nucleation and growth on the calcite substrate was studied, as well as metastable phase evolution, using scanning electron microscopy, grazing incidence X-ray diffraction, and atomic force microscopy. Film nucleation was enhanced by surface roughness. The rate of nucleation and the growth rate of the film increased with cationic (calcium) and anionic (carbonate) precursor additions. Calcium additions also influenced phase formation, introducing a metastable phase (octacalcium phosphate) and a different phase evolution sequence.

Chemo-mechanics of salt damage in stone.

Many porous materials are damaged by pressure exerted by salt crystals growing in their pores. This is a serious issue in conservation science, geomorphology, geotechnical engineering and concrete materials science. In all cases, a central question is whether crystallization pressure will cause damage. Here we present an experiment in which the crystallization pressure and the pore saturation are varied in a controlled way. We demonstrate that a strain energy failure criterion can be used to predict when damage will occur. The experiment considered is the most widely used means to study the susceptibility to salt crystallization, so quantification of this test has far-reaching implications.

Advances in understanding damage by salt crystallization.

The single most important cause of the deterioration of monuments in the Mediterranean basin, and elsewhere around the world, is the crystallization of salt within the pores of the stone. Considerable advances have been made in recent years in elucidating the fundamental mechanisms responsible for salt damage. As a result, new methods of treatment are being proposed that offer the possibility of attacking the cause of the problem, rather than simply treating the symptoms. In this Account, we review the thermodynamics and kinetics of crystallization, then examine how a range of technological innovations have been applied experimentally to further the current understanding of in-pore crystallization. We close with a discussion of how computer modeling now provides particularly valuable insight, including quantitative estimates of both the interaction forces between the mineral and the crystal and the stresses induced in the material. Analyzing the kinetics and thermodynamics of crystal growth within the pores of a stone requires sensitive tools used in combination. For example, calorimetry quantifies the amount of salt that precipitates in the pores of a stone during cooling, and dilatometric measurements on a companion sample reveal the stress exerted by the salt. Synchrotron X-rays can penetrate the stone and identify the metastable phases that often appear in the first stages of crystallization. Atomic force microscopy and environmental scanning electron microscopy permit study of the nanometric liquid film that typically lies between salt and stone; this film controls the magnitude of the pressure exerted and the kinetics of relaxation of the stress. These experimental advances provide validation for increasingly advanced simulations, using continuum models of reactive transport on a macroscopic scale and molecular dynamics on the atomic scale. Because of the fundamental understanding of the damage mechanisms that is beginning to emerge, it is possible to devise methods for protecting monuments and sculptures. For example, chemical modification of the stone can alter the repulsive forces that stabilize the liquid film between the salt and mineral surfaces, thereby reducing the stress that the salt can generate. Alternatively, molecules can be introduced into the pores of the stone that inhibit the nucleation or growth of salt crystals. Many challenges remain, however, particularly in understanding the complex interactions between salts, the role of metastable phases, the mechanism of crack initiation and growth, and the role of biofilms.

Transport of water in small pores.

Experimental measurements of the thermal expansion coefficient (alpha), permeability (k), and diffusivity (D) of water and 1 M solutions of NaCl and CaCl(2) are interpreted with the aid of molecular dynamics (MD) simulations of water in a 3 nm gap between glass plates. MD shows that there is a layer approximately 6 A thick near the glass surface that has alpha approximately 2.3 times higher and D about an order of magnitude lower than bulk water. The measured D is approximately 5 times lower than that for bulk water. However, when the MD results are averaged over the thickness of the 3 nm gap, D is only reduced by approximately 30% relative to the bulk, so the measured reduction is attributed primarily to tortuosity of the pore space, not to the reduced mobility near the pore wall. The measured alpha can be quantitatively explained by a volume-weighted average of the properties of the high-expansion layer and the "normal" water in the middle of the pore. The permeability of the porous glass can be quantitatively predicted by the Carman-Kozeny equation, if 6 A of water near the pore wall is assumed to be immobile, which is consistent with the MD results. The properties and thickness of the surface-affected layer are not affected significantly by the presence of the dissolved salts.

Thermal expansion of confined water.

Dilatometric measurement of the thermal expansion of water in porous silica shows that the expansion coefficient increases systematically as the pore size decreases below about 15 nm. This behavior is quantitatively reproduced by molecular dynamics (MD) simulations based on a new dissociative potential. According to MD, the structure of the water is modified within approximately 6 A of the pore wall, so that it resembles bulk water at a higher pressure. On the basis of this observation, it is possible to account for the measured expansion, as the thermal expansion coefficient of bulk water increases with temperature over the range considered in this study.

Molecular mechanisms causing anomalously high thermal expansion of nanoconfined water.

Anomalously high thermal expansion is measured in water confined in nanoscale pores in amorphous silica and the molecular mechanisms are identified by molecular dynamics (MD) simulations using an accurate dissociative water potential. The experimentally measured coefficient of thermal expansion (CTE) of nanoconfined water increases as pore dimension decreases. The simulations match this behavior for water confined in 30 A and 70 A pores in silica. The cause of the high expansion is associated with the structure and increased CTE of a region of water approximately 6 A thick adjacent to the silica. The structure of water in the first 3 A of this interface is templated by the atomically rough silica surface, while the water in the second 3 A just beyond the atomically rough silica surface sits in an asymmetric potential well and displays a high density, with a structure comparable to bulk water at higher pressure.

Dangling bond deflection model: growth of gel network with loop structure.

It has been shown that the closed-loop structure in the model gel networks is responsible for their stiffness. However, the creation of loops has been underestimated in most of the existing kinetic aggregation models [e.g., DLCA (diffusion-limited cluster-cluster aggregation) and derivatives]. A dangling bond deflection (DEF) mechanism is proposed to model the fluctuation of dangling branches or dead ends under thermal excitation. The random deflections of the dangling branches can create loops in the network by forming intracluster bonds, and proceed during both the gelling and aging processes. The resulting DLCADEF networks have extensive loop structure with a negligible number of dangling branches. Its growth kinetics and fractal behavior resemble those of real gels, including volume-invariant gel time and fractal dimension of about 2. The DLCADEF model is the first attempt to model the gel growth with loop formation by the physically realistic fluctuation mechanism. The mechanical properties of the resulting networks will be studied and verified by comparison with real gels.