Evaporating solute-containing droplets tend to temporarily become sticky as they transition from wet droplets to dry particles. Stickiness plays a crucial role in promoting beneficial powder properties through agglomeration but also in potential fouling of drying equipment. Spray drying, a processing technology turning a swarm of droplets into powder, is particularly susceptible to stickiness. Therefore, it is desirable to predict when a droplet becomes sticky. However, predicting the drying and collision behavior of droplets in spray drying is challenging due to multiple factors: transient drying dynamics, intra-droplet solute redistribution, morphological changes, and varying impact velocities. This dissertation unravels the intricate behavior of partially dried droplets in spray drying scenarios. The primary goal is to understand how their collision outcomes vary with varying degrees of drying. A key aspect is developing predictive strategies for these collision outcomes, considering the distinct drying patterns and internal concentration gradients within solute-containing droplets...@en

Properties of powders produced from drying solute-containing droplets arise from the dynamic redistribution of solute during drying. While insights on the dynamic redistribution are instrumental for the rational design of powders and for the optimized operation of equipment such as spray dryers, experimental techniques that allow measuring the spatio-temporal concentration of solute in drying droplets are scarce. In this work, we explore and demonstrate the use of optical coherence tomography (OCT) to measure the spatio-temporal concentration of solute in drying droplets and the development of a solidifying shell at the liquid-air interface, using aqueous droplets of maltodextrin as a model system. This work provides a solid foundation for the use of OCT to quantify the dynamic redistribution of solute and link it to the development of the morphology of the produced particles and agglomerates.

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Sticking of particles has a tremendous impact on powder-processing industries, especially for hygroscopic amorphous powders. A wide variety of experimental methods has been developed to measure at what combinations of temperature and moisture content material becomes sticky. This review describes, for each method, how so-called stickiness curves are determined. As particle velocity also plays a key role, we classify the methods into static and dynamic stickiness tests. Static stickiness tests have limited particle motion during the conditioning step prior to the measurement. Thus, the obtained information is particularly useful in predicting the long-term behavior of powder during storage or in packaging. Dynamic stickiness tests involve significant particle motion during conditioning and measurement. Stickiness curves strongly depend on particle velocity, and the obtained information is highly relevant to the design and operation of powder production and processing equipment. Virtually all methods determine the onset of stickiness using powder as a starting point. Given the many industrial processes like spray drying that start from a liquid that may become sticky upon drying, future effort should focus on developing test methods that determine the onset of stickiness using a liquid droplet as a starting point.

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