Phase-sensitive optical coherence tomography reveals droplet penetration into a powder bed

Understanding the mixing behavior that unfolds when liquid droplets impact on powder beds requires a sub-surface dynamic imaging method that provides high spatial and temporal resolution. In this study, a high-resolution phase-sensitive optical coherence tomography (PhS-OCT) technique was introduced to provide in-depth insights into the interaction of droplets and powder beds. A theoretical model correlating changes in the optical path differences (OPD) of the OCT signal with physical properties of the powder bed was presented. Experiments were conducted on three powders with median particle diameters, i.e., the size corresponding to the 50^th percentile of the cumulative volume distribution, of 3 µm (mannitol), 4 µm (mannitol) and 109 µm (SV010). Water and ethanol droplets were used to illustrate the technique. The results demonstrated that OPD maps can qualitatively resolve the liquid-powder mixture region, both temporally and spatially, as the droplet diffuses into the powder bed, allowing for a quantitative description of diffusion behavior. A comparison of lactose and mannitol powder samples revealed varying diffusion rates, potentially attributed to differences in porosity, particle size, and agglomeration. Additionally, water and ethanol droplets exhibited significantly different (p = 0.005) diffusion profiles for the same powder properties. In general, the findings indicate the potential of the proposed PhS-OCT technique for evaluating the mixing behavior of multi-phase mixtures, holding promise in applications such as pharmaceutical drug formulations and additive manufacturing.