Simpler, faster, and softer: Towards broad application of Laser Speckle Imaging in art conservation and soft matter

Laser Speckle Imaging (LSI) is a technique that employs interference of laser light to visualize motion inside materials. Based on the interference of visible light, it can detect motion as small as a few to hundreds of nanometers. LSI is also unique because it is an optical technique that requires an opaque material. Most light-based methods require a transparent material to be able to visualize the inside structure. However, most daily-life materials are not transparent, so their inner structure is not easily studied with optical methods. LSI was originally developed as a medical imaging tool to visualize the motion of red blood cells in veins. It has also been applied in materials science, but only in a few research groups over the world.

While most optical methods visualize structure in semi-transparent materials, LSI visualizes dynamics in opaque materials, and does so with unparalleled sensitivity. The hardware required to perform LSI is relatively simple, just a laser and camera pointed at the same location on the sample surface. With this combination of yielding unique information, while being an experimentally accessible technique, it is surprising that LSI is not more widely applied.

In this thesis we explore a few adaptations to LSI that will help to make it more accessible. We also explore new applications in the fields of art conservation and soft matter science to show the techniques potential and inspire an even broader application.

The interference pattern captured by the camera has to be processed to obtain the spatially resolved dynamics. In Chapter 2 we develop a new processing algorithm based on the fast Fourier transform, which is both fast and quantitative. We harness the potential of this new approach by constructing a portable laser speckle imaging setup that performs quantitative data processing in real-time on a tablet.

In Chapter 3 we put this portable and real-time LSI setup to work, and we explore potential applications in the conservation of oil paintings. We use LSI to measure pigment motion as a proxy for solvent presence in the paint layer. The exposure of oil paintings to organic solvents for varnish removal or to water for the removal of surface dirt can affect the chemical and physical properties of oil paint in an undesired way. Solvents can temporarily plasticise and swell the polymerised oil paint binding medium, enhancing both the thermal mobility and mechanical displacement of pigments embedded in this film. The enhancement of these microscopic motions can affect both the chemical and physical stability of the object as a whole. By quantifying the amount of solvent in the paint layer, LSI can help gaining insight in the solvent uptake in order to ultimately minimize it during cleaning treatments.

We more closely study one of those solvent application methods in Chapter 4. Evolon® CR is increasingly used in paintings conservation for varnish removal from oil paintings. Its key benefits over traditional cotton swabs are limiting solvent exposure and reducing mechanical action on the paint surface. However, this non-woven microfilament textile was not originally engineered for conservation use and little is known about its chemical stability towards organic solvents. Our results show that the tissue is generally chemically and physically stable to organic solvents when exposed on timescales that are typical in conservation practice. We developed a method to quantify solvent-retention in real-time and revealed that the presence of varnish on paintings results in lower dynamics of solvents within the paint in comparison to unvarnished paint. Comparing various solvents, it was found that cleaning with acetone resulted in a roughly six-fold increase in dynamics compared to ethanol and isopropanol. We also combine LSI with UV-imaging to compare the degree of varnish removal with the amount of solvent in the paint. By combining those two measures, a conservator could find the optimal treatment by maximizing the cleaning result with minimal solvent penetration.

In Chapter 5 we move our attention from art conservation to soft matter. Syneresis, the compaction of a material accompanied by fluid expulsion, is a typical mechanical instability which exists among colloidal gel based materials and that negatively affects the quality of relevant applications such as yoghurt and mayonnaise. We shed light onto the internal dynamics of model colloidal gels undergoing syneresis with LSI. The resulting dynamical maps capture the distinct differences in spatial and temporal relaxation patterns between colloidal gels comprising solid and liquid particles. This indicates different mechanisms of syneresis between the two systems and highlights the importance of the constituent particles and their mobile or restrictive interfaces in the mechanical relaxation of the colloidal gels during syneresis.

In Chapter 6 we combine LSI with rheology to simultaneously measure the global and local flow properties during one measurement. Rheological properties of complex fluids are often governed by localized events such as cooperative motions, shear banding or wall slip. Current methods to perform rheological imaging either lack the resolution to resolve individual events or rely on optical imaging which requires transparent samples thereby precluding the study of many industrially-relevant materials. We show how LSI can be used to measure the local flow behavior with a high sensitivity over the full field of view. We quantitatively visualize localized areas with jamming in a concentrated corn starch suspension under oscillatory deformation in a double plate geometry. The approach is not limited to the rheological constraints of this test type and geometry. With a constant shear stress tests we capture intermittent jamming and un-jamming events in the same concentrated corn starch suspension.

In the General Discussion we reflect on the progress we made to making LSI more accessible, and discuss potential applications, limitations and how to proceed. A major part of this chapter is an approach to extend LSI to 3D-imaging. We have made technical progress and we have shown a few examples of potential applications for LSI in this thesis. Although our work may help in making LSI more accessible, we mainly hope to inspire application in other fields by showing what is possible.