Schematic showing the various stages during natural drying of acoustically levitated droplets in (i) 2% by weight of aqueous TM dispersion, (ii) SDS at a concentration <1% (by weight), (iii) SDS at a concentration >1% (by weight) added to 2% aqueous TM dispersion, and (iv) AHC added to equal concentration of SDS and TM (2% by weight each in aqueous dispersion).

From:
Binita Pathak and Saptarshi Basu (Department of Mechanical Engineering, Indian Institute of Science, Bangalore, Karnataka 560012, India),

“Phenomenology and control of buckling dynamics in multicomponent colloidal droplets”, J. Appl. Phys. 117, 244901 (2015); http://dx.doi.org/10.1063/1.4922980 

ABSTRACT: Self-assembly of nano sized particles during natural drying causes agglomeration and shell formation at the surface of micron sized droplets. The shell undergoes sol-gel transition leading to buckling at the weakest point on the surface and produces different types of structures. Manipulation of the buckling rate with inclusion of surfactant (sodium dodecyl sulphate, SDS) and salt (anilinium hydrochloride, AHC) to the nano-sized particle dispersion (nanosilica) is reported here in an acoustically levitated single droplet. Buckling in levitated droplets is a cumulative, complicated function of acoustic streaming, chemistry, agglomeration rate, porosity, radius of curvature, and elastic energy of shell. We put forward our hypothesis on how buckling occurs and can be suppressed during natural drying of the droplets. Global precipitation of aggregates due to slow drying of surfactant-added droplets (no added salts) enhances the rigidity of the shell formed and hence reduces the buckling probability of the shell. On the contrary, adsorption of SDS aggregates on salt ions facilitates the buckling phenomenon with an addition of minute concentration of the aniline salt to the dispersion. Variation in the concentration of the added particles (SDS/AHC) also leads to starkly different morphologies and transient behaviour of buckling (buckling modes like paraboloid, ellipsoid, and buckling rates). Tuning of the buckling rate causes a transition in the final morphology from ring and bowl shapes to cocoon type of structure.

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