Rheology of white paints: How Van Gogh achieved his famous impasto

Multiscale organisation of lead carboxylates in artistic oil binders

The supramolecular and mesoscopic architectures of lead-saponified linseed oil, used by painters since the Renaissance, have been characterised and linked to their rheological properties. The multi-scale organization of saponified oils has been demonstrated by SAXS (Small Angle X-ray Scattering), FF-TEM (Freeze-Fracture Transmission Electron Microscopy) and DIC (Differential Interference Contrast): some of the lead soaps (formed when the oil is heated in the presence of PbO) are organized into microscopic lamellar domains, distributed in a continuous matrix made up of unorganized species (partially saponified triglycerides, glycerol, remaining soaps, etc.). The concentration of lead soaps in the oil controls the average size and interaction between the lamellar domains. Linseed oil + PbO 17 mol% is viscous and consists of aggregates of lamellar domains isolated within the continuous unorganized matrix. In contrast, in linseed oil + PbO 50 mol%, the domains are homogeneously dispersed and form what can be described as a three-dimensional network, giving the system viscoelastic properties.

Drying Drops of Paint Suspension: From "Fried Eggs" to Quasi-Homogeneous Patterns

Drying of multicomponent sessile drops is a complex phenomenon involving intricate mechanisms. Here, we study the evaporation of drops made of paint suspension and investigate the influence of the substrate temperature and suspension concentration on the resulting deposit patterns. At low concentrations and temperatures, the pigments appear highly concentrated in a narrow area at the center of the drop, a morphology we call "fried eggs". Increasing the temperature or concentration leads to more homogeneous patterns. From a top-view camera used for monitoring the whole evaporative process, we identify three mechanisms responsible for the final pattern: inward/outward flows that convect the pigments, gelation of the paint suspension where pigments accumulate, and final drying of the drop that freezes the location of the pigments onto the substrate. The relative kinetics of these three mechanisms upon concentration and temperature govern the deposit growth and the morphology of the final pattern. These observations are quantitatively supported by rheological measurements highlighting a strong increase of the viscosity with concentration, consistent with the gelation mechanism. Finally, we show that the kinetics of drop drying is controlled by the substrate temperature.

A holistic view on the role of egg yolk in Old Masters' oil paints

Old Masters like Botticelli used paints containing mixtures of oils and proteins, but "how" and "why" this was done is still not understood. Here, egg yolk is used in combination with two pigments to evaluate how different repartition of proteinaceous binder can be used to control the flow behavior as well as drying kinetics and chemistry of oil paints. Stiff paints enabling pronounced impasto can be achieved, but paint stiffening due to undesired uptake of humidity from the environment can also be suppressed, depending on proteinaceous binder distribution and colloidal paint microstructure. Brushability at high pigment loading is improved via reduction of high shear viscosity and wrinkling can be suppressed adjusting a high yield stress. Egg acts as antioxidant, slowing down the onset of curing, and promoting the formation of cross-linked networks less prone to oxidative degradation compared to oil alone, which might improve the preservation of invaluable artworks.

Rheo-SAXS characterization of lead-treated oils: Understanding the influence of lead driers on artistic oil paint's flow properties

From the 15^th century onwards, painters began to treat their oils with lead compounds before grinding them with pigments. Such a treatment induces the partial hydrolysis of the oil triglycerides and the formation of lead soaps, which significantly modify the rheological properties of the oil paint. Organization at the supramolecular scale is thus expected to explain these macroscopic changes. Synchrotron Rheo-SAXS (Small Angle X-ray Scattering) measurements were carried out on lead-treated oils, with different lead contents. We can now propose a full picture of the relationship between structure and rheological properties of historical saponified oils. At rest, lead soaps in oil are organized as lamellar phases with a characteristic period of 50 Å. Under shear, the loss of viscoelastic properties can be linked to the modification of this organization. Continuous shear resulted in a preferential and reversible orientation of the lamellar domains which increased with the concentration of lead soaps. The parallel orientation predominates over the entire shear range (0-1000 s^-1). Conversely, oscillatory shear coiled the lamellae into cylinders that oriented themselves vertically in the rheometer cell. This is the first report of such a vertical cylindrical structure obtained under shear from lamellae.

Drying colloidal systems: Laboratory models for a wide range of applications

The drying of complex fluids provides a powerful insight into phenomena that take place on time and length scales not normally accessible. An important feature of complex fluids, colloidal dispersions and polymer solutions is their high sensitivity to weak external actions. Thus, the drying of complex fluids involves a large number of physical and chemical processes. The scope of this review is the capacity to tune such systems to reproduce and explore specific properties in a physics laboratory. A wide variety of systems are presented, ranging from functional coatings, food science, cosmetology, medical diagnostics and forensics to geophysics and art.

Materials science challenges in paintings

Through the paintings of the old masters, we showcase how materials science today provides us with a vision of the processes involved in the creation of a work of art: the choice of materials, the painter's skill in handling these materials, and the perception of the finished work.