Effect of shell formation on Shock-induced atomization of an evaporating nanofluid droplet

Understanding the breakup behaviour of evaporating nanofluid droplets under extreme aerodynamic loads is critical for applications ranging from drug delivery and material deposition to defence and propulsion systems. This study investigates the shock-induced atomisation dynamics of an acoustically levitated, evaporating TM-10 nanofluid droplet subjected to a coaxially propagating blast wave and compressible vortex ring. The blast waves are generated using a compact wire-explosion-based shock generator. Upon interaction with the droplet, the initial blast wave introduces a sharp velocity discontinuity, followed by a monotonically decaying flow field and a compressible vortex ring that ultimately results in the droplet disintegration. The nanofluid droplet is externally heated by a laser to induce evaporation, which leads to a temporal increase in nanoparticle concentration, viscosity, and agglomeration. As the droplet surface recedes, nanoparticles accumulate near the interface, initiating a sol–gel transition once the volume fraction exceeds the gelation threshold. Continued evaporation causes the formation of a solid outer shell as the local particle volume fraction approaches the maximum packing limit.

Shock interactions are examined at three key stages of evaporation: (i) steady liquid phase, (ii) gel-shell phase, and (iii) solid-shell phase. Each regime exhibits distinct atomisation responses across these stages due to evolving shell morphology. The presence of a viscous, gelatinous shell resists deformation, leading to a bag-on-sheet mode of atomisation occurring at the equatorial sheet. The resulting puncture of the shell enables inner liquid to escape, forming secondary jets and initiating further atomisation. The puncture results in the weakening of the shell, leading to delamination and rupture. In the solid-shell stage, these interactions become more violent, resulting in brittle fragmentation and catastrophic shell fracture.

This work demonstrates how evaporation-induced interfacial transitions in nanofluids fundamentally alter atomisation mechanisms under shock loading, offering insights into the evolving droplet composition, shell formation, and its interplay with extreme flow conditions.