Donut transformation. Toroidal transformation has been shown to

 

            Donut shaped
particles, referred to as toroidal particles, are particles that have
completely buckled or invaginated. Toroidal shapes have been shown to form from
the buckling on one side of the droplet followed by the buckling on the other
side of the droplet. The two cavities meet in the middle of the droplet
resulting in the final toroidal shapei
This was found in a single droplet experiment using an acoustic levitator, so
acoustic streaming forces could have been responsible for non-uniform shell
thicknesses and therefore the transformation from sphere to toroid.15

              

               Recently
there has been research into controlling toroidal transformation. Toroidal
transformation has been shown to occur at higher drying temperatures due to the
increased size and hollowness in particles formed at these temperatures.ii iii The
transformation can also be influenced by the polydispersity of the
nanoparticles within the droplet, as an increase in polydispersity gives a
decrease of rearrangement of the nanoparticles during the drying. This
transformation relieves strain on the shell.iv

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i Miglani,
A., & Basu, S. (2015). Sphere to
ring morphological transformation in drying nanofluid droplets in a contact-free
environment. Soft Matter, 11(11), 2268-2278.

 

ii Biswas,
P., Sen, D., Mazumder, S., Basak, C., & Doshi, P. (2016). Temperature Mediated Morphological
Transition during Drying of Spray Colloidal Droplets. Langmuir : The ACS
Journal of Surfaces and Colloids, 32(10), 2464-73.

 

iii Biswas,
P., Sen, Mazumder, & Doshi. (2017). Controlling
sphere to doughnut transformation during quick drying of colloidal micrometric
droplets. AIP Conference Proceedings, 1832(1), Dae Solid State Physics
Symposium 2016, Bhubaneswar, Odisha, India (26–30 December 2016)

 

iv Sen,
Bahadur, Mazumder, & Bhattacharya. (2012). Formation of hollow spherical and doughnut microcapsules by evaporation
induced self-assembly of nanoparticles: Effects of particle size and
polydispersity. Soft Matter, 8(39), 10036-10044