(489j) Buckling of a Drying Drop of Colloidal Dispersion: Experiments

It is well known that drying drops of colloidal dispersions undergo complex morphological transitions involving buckling of a particle-packed outer shell during drying[1,2,3]. Recent theoretical investigations [4] have shown that there exists a critical capillary pressure for buckling of drying droplets of colloidal dispersions, which is related to the mechanical properties of the particle network formed during drying. The theory attributes the buckling to the capillary stresses generated during drying and presents exact buckling conditions related to particle size, rigidity, and nature of packing.

We performed experiments where drops of dispersions, containing colloidal particles of varying size and shear modulus were dried at elevated temperatures (80-160°C) and under vacuum. The drops were placed on superhydrophobic surfaces so as to maintain a spherical geometry throughout the drying process. The high drying rates led to the formation of a particle shell at the periphery of the drop. With continued evaporation, the drops shrink causing compressive stress in the particle shell. We observed that large drops buckled on drying while smaller drops retained their spherical shape. Further, for a fixed drop size, shells made of smaller particles buckled while those containing large particles remained spherical. The experimental results compared favorably with model predictions for different particle sizes and drop volumes.

The attached figure shows scanning electron microscopy images of (a) a buckled shell formed from a colloidal dispersion containing 1μm polystyrene particles, and (b) an unbuckled shell formed from a colloidal dispersion containing 15μm polystyrene particles

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