Suppression of the coffee-ring effect by tailoring the viscosity of pharmaceutical sessile drops

Significance 

When a drop of liquid evaporates on a solid surface, its suspended particulate matter is deposited in ring-like fashion. This phenomenon, known as the coffee-ring effect (CRE). Droplet-based processing techniques involving drying processes are highly susceptible to the CRE. The ring-like pattern results in non-uniform solute distribution leading to low-quality products. Typically, CRE results from the capillary flow of the liquid from the droplet center to the contact line to replenish the lost liquid due to the evaporation. It carries along a solute the gets deposited at the edge of the droplet. CRE is detrimental where uniform distribution of the dried deposits is desired, like in the case of the printed electronics. Besides causing heterogeneity of the dried deposits, CRE also causes undesirable changes in the physical nature of the deposits. Thus, a thorough understanding of the microstructure of the CRE deposits is instrumental in developing effective strategies to suppress its effects.

CRE has been extensively studied in the literature. In particular, three main theoretical strategies for preventing or at least reducing its effects have been proposed: avoiding the pinning of the contact line, preventing the capillary flow of liquid to the contact line, and lastly, preventing the transportation of the solid particles to the contact line. On the other hand, practical strategies include manipulating the shape of the solute and substrate surface and adding surfactants to the solution. Nevertheless, despite the remarkable effects, the practical strategies for eliminating or reducing CRE are still limited. In addition, previous findings established that adjusting the solution viscosity could effectively control the solute deposition that is key in preventing CRE. However, these assertions are yet to be practically validated, especially for the pharmaceutical context that uses multiple solute types.

To this note, Janine Wilkinson, Chak Tam, Dr. Alexandros Askounis and Professor Sheng Qi from the University of East Anglia developed a simple practical method for suppressing the CRE during the manufacturing of pharmaceutical products involving drying of droplets. The approach involved modifying the viscosity of the pharmaceutical sessile droplets. The authors started their work by investigating the effects of viscosity on multicomponent sessile droplets comprising a drug model (paracetamol) and polymeric pharmaceutical excipient (chitosan). The paracetamol was combined with different concentrations of chitosan to create a viscosity-based approach for CRE suppression. The original research article is now published in the journal, Colloids and Surfaces A: Physicochemical and Engineering Aspects.

The research team showed that when aqueous paracetamol solution was dried on glass, it recrystallized to form a coffee-ring structure due to CRE. Upon addition of chitosan, the deposition of crystalline solutes on the periphery and around the edge was suppressed because chitosan increases the viscosity that suppresses CRE by avoiding radial flow towards the edge of the droplet. This could be further attributed to the fact that the formulation with an optimal viscosity resulted in homogenous solute distribution during the drying process, and resulted a uniformly dispersed pharmaceutical film after drying.

In a nutshell, the use of viscosity as a practical tool for controlling the deposition of solutes to suppressed CRE in multicomponent pharmaceutical solutions was demonstrated. Overall, two main contributions for understanding CRE suppression were provided. The first involved obtaining a uniform film after drying the droplet by increasing the solution viscosity. And the second, the authors, for the first time, reported that after attaining optimal viscosity, a further increase in viscosity was not beneficial because it did not only prolong the drying process but also increased the risk of developing non-uniform thickness. The study provided more insights into the mechanistic understanding of CRE suppression. In a statement to Advances in Engineering, the authors said that the simple and practical solution to eliminate the coffee-ring effect will improve the efficacy of droplet-based processing techniques used in various industries, including pharmaceutical and additive manufacturing.

Advances in Engineering-coffee-ring effect

Reference

Wilkinson, J., Tam, C., Askounis, A., & Qi, S. (2021). Suppression of the coffee-ring effect by tailoring the viscosity of pharmaceutical sessile dropsColloids And Surfaces A: Physicochemical and Engineering Aspects, 614, 126144.

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