Luca Chiari

The self-propulsion mechanisms of liquid marbles containing aqueous alcohol solutions are investigated, with particular focus on the transition from the initial acceleration to the steady-state motion. By combining surface tension measurements using quasi-elastic laser scattering (QELS) and tracer particle visualization, we quantitatively analyze the relationship between interfacial forces and flow fields. In steady-state constant-speed motion, both measurements demonstrate that the propulsion is primarily driven by the surface tension difference between the front and rear sides of the liquid marbles. In contrast, during the initial acceleration phase, the forward Marangoni flow was found to exceed the propulsion velocity, indicating a significantly increased contribution of the forward flow to self-propulsion. The comparison between the propulsion velocity and the surrounding flow velocity reveals that the dominant driving mechanism switches from flow-driven propulsion in the acceleration regime to surface tension-driven propulsion in the steady and deceleration regimes. This work provides experimental evidence of a temporal transition in propulsion mechanisms and offers new insights into how self-propelled systems reach steady-state motion.

This study investigates the influence of polysaccharides with varying dextrose equivalent on the free volume and pharmaceutical stability of lyophilized formulations. Free volume, estimated from the ortho-positronium pick-off annihilation lifetime using positron annihilation lifetime spectroscopy, was used as a proxy for the molecular mobility of the formulation matrices and their stability. Formulations containing polysaccharides with higher dextrose equivalent, such as maltodextrin, exhibited reduced free volume and enhanced stability. Furthermore, the addition of a sugar or a polyol to these formulations was found to be effective, producing even smaller free volume. These findings underscore the potential for rational excipient design based on the dextrose equivalent to improve formulation stability.

The influence of surface polishing on hydrogen-related defects in austenitic stainless steel is investigated, focusing particularly on interactions between hydrogen, dislocations, and vacancies. Using positron annihilation lifetime spectroscopy, the defects formed in hydrogen-charged and tensile-strained samples with different surface coarseness are quantitatively analysed. Mechanical polishing introduces dislocations in the near-surface region, elevating the local hydrogen concentration and promoting the formation of hydrogen-induced vacancies. A positive correlation is observed between surface roughness and vacancy cluster size, while the overall vacancy concentration decreases with increasing surface coarseness. The results reveal that polishing-induced surface deformation enhances the vacancy agglomeration process. As large vacancy clusters may adversely affect the mechanical properties, these findings underscore the importance of surface finishing processes in austenitic stainless steel exposed to hydrogen.

When a 1-hexanol droplet is laid down on a hexanol aqueous solution surface, it exhibits a self-propelled motion. The Marangoni flow in front of the droplet plays an important role in the self-propulsion, but it was so far unclear how the flow is converted into the driving force. In this study, the time-resolved surface tension, surface flow speed, water surface height, and droplet position of a unidirectionally moving hexanol droplet were simultaneously measured and converted to position distributions. The flow in the aqueous solution was also visualized by using particles. The results of the surface tension difference between the front and the rear of the droplet were not dependent on the droplet speed, whereas the surface flow speed and the water level difference between the front and the rear of the droplet were clearly dependent on the droplet speed. Because 90% of the relative flow below the droplet was directed backward, the viscous force at the bottom of the droplet mainly decelerated the droplet. These results suggest that the self-propelled motion originated by the Marangoni convection at the front of the droplet was converted into the pressure difference caused by the change in water level between the front and the rear of the droplet.

The interaction of hydrogen with lattice defects plays a crucial role in the hydrogen embrittlement mechanism, but the origin of hydrogen-related defects remains unclear. In this study we investigate the formation process of hydrogen-induced vacancies in austenitic stainless steel SUS 304 by positron annihilation lifetime spectroscopy. Positron lifetime measurements of hydrogen-charged samples subjected to tensile testing by different strains show that the formation of hydrogen-induced vacancies first appears when a strain of about 5% is applied. Electropolishing of the hydrogen-charged layer reveals the generation of vacancy-hydrogen complexes in the bulk underneath the hydrogen-charged layer, which develop into vacancy clusters by further application of stress. From the PALS results and complementary X-ray diffraction analysis, the dislocation density required for the formation of hydrogen-induced vacancies is quantitatively determined. Clarification of the conditions for the formation of hydrogen-induced vacancies provides important input and reference data for models of the hydrogen embrittlement in stainless steels.