Mechanical instabilities in drying protein droplets under substrate-free conditions

Understanding how proteins of different origins behave during drying is essential for controlling the mechanical stability and final structure of protein-based materials. Here, we examine the drying dynamics of acoustically levitated droplets containing either napin, a plant-derived seed storage protein, or native phosphocaseinate, a dairy protein complex, to uncover how their intrinsic physicochemical and mechanical properties govern evaporation-driven instabilities. Compared to sessile droplets, levitated droplets dry under symmetric evaporation conditions, minimizing substrate effects and contact-line pinning. During drying, both systems develop a solid skin at the air-liquid interface, which undergoes mechanical buckling once compressive stresses exceed the skin's rigidity. Despite similar overall drying kinetics, differences emerge between the two protein systems: native phosphocaseinate droplets form ductile, crack-free shells, whereas napin droplets display brittle fracture and surface cracking. These contrasting behaviours reflect fundamental differences between plant and animal proteins in terms of interfacial activity and network formation. The results are further supported by comparison with model colloid-polymer films, establishing a direct link between interfacial mechanics, crack formation, and film ductility. By revealing how protein origin governs drying-induced instabilities, this work provides mechanistic insight into the design of protein-based materials and supports the development of sustainable, plant-derived protein alternatives for food, pharmaceutical, and soft-material applications.