Role of motility and nutrient availability in drying patterns of algal droplets

Sessile drying droplets in various bio-related systems attracted attention due to the complex interactions between convective flows, droplet pinning, mechanical stress, wettability, and the emergence of unique patterns. This study focuses on the drying dynamics of Chlamydomonas reinhardtii (chlamys), a versatile model algae used in molecular biology and biotechnology. The experimental findings shed light on how motility and nutrient availability influence morphological patterns- a fusion of macroscopic fluid dynamics and microbiology. This paper further discusses the interplay of two competing stressors during drying- nutrient scarcity (quantitative analysis) and mechanical stress (qualitative analysis), where the global mechanical stress does not induce cracks. Interestingly, motile chlamys form clusters under nutrient scarcity due to metabolic stress, indicating the onset of flocculation, a common feature observed in microbial systems. Moreover, non-motile chlamys exhibit an "anomalous coffee-ring effect" in the presence of nutrients, with an inward movement observed near the droplet edge despite sufficient water in the droplet. The quantitative image processing techniques provide fundamental insights into these behaviors in classifying the patterns into four categories (motile+with nutrients, motile+without nutrients, non-motile+with nutrients, and non-motile+without nutrients) across five distinct drying stages- Droplet Deposition, Capillary Flow, Dynamic Droplet Phase, Aggregation Phase, and Dried Morphology.