In Situ X-Ray Imaging of Droplet Drying with Dense Particle Suspensions: Exploring the Mechanism of Coffee Ring Formation

Droplet deposition and drying is a key process in many manufacturing processes including: the spray deposition of paints and coatings, inkjet printing, and the manufacture of large area electronics. During the drying of droplets that contain small particles in suspension, it is well known that, under certain experimental conditions, the dried residue does not form a uniform deposit related to the shape of the initial liquid drop but instead a migration of particles to the pinned contact line of the drop leads to a characteristic ring deposit or <i>coffee stain</i>. In dilute particle suspensions this phenomenon is believed to be initiated by contact line pinning during evaporation, which leads to a radial outward fluid flow along the base of the drop increasing the local particle concentration at the contact line because evaporation removes the solvent. The precise conditions that trigger the formation of a coffee ring are still a topic of investigation but are believed to be associated with the mechanisms that lead to contact line pinning before the receding contact angle is reached during drying.<br/>There has been considerable experimental study of the drop drying process and in situ imaging of dilute particular suspensions using optical microscopy and individual particle tracking experiments has prove valuable for studying the dynamics of the coffee ring and fluid flow during droplet drying. However, many of the real-world applications of droplet-based manufacture use dense particulate suspensions as the fluid or ink and, although it is known that such dense suspensions can form a coffee ring, study of droplet drying in these cases has been limited to observing the change in drop shape.<br/>Here, we present an <i>in situ</i> study of the drying of dense suspensions of sub-micron ZrO₂ particles in aqueous suspension using a synchrotron X-Ray source to track changes in drop volume and shape during drying at 20, 45 and 60 °C on glass, silicon and Kapton substrates that show different contact angles of 24°, 36° and 73° respectively. We show that if the drop retains radial symmetry during the drying process, the signal intensity across the drop image can be transformed to determine the local density of the suspension and from this a semi-quantitative measure of the particle concentration in the liquid as a function of vertical and radial position in the drop. Hence, we can track the relative densification of the particle suspension as a function of position during the drying of the drop.<br/>Coffee rings are found to form more easily on the lower contact angle surfaces and at higher drying temperatures where evaporation is more rapid. Our results show that the density profile across drying drop is significantly different between drops that show uniform drying and those where a coffee ring forms. In the absence of a coffee ring, densification commences across the surface of the drop in a uniform manner and there is evidence for a possible surface crust during drying. However, under the conditions that lead to a coffee ring, initial densification is also more intense at the upper surface but is now much stronger towards the contact line. It is unclear whether the surface crust formation suppresses coffee ring formation or is a consequence of its absence.