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IITH creates a unique holistic ecosystem for education that offers interactive learning, a very flexible academic structure, cutting-edge research, strong industry collaboration, and entrepreneurship. This is an environment which enables students and faculty to translate their dreams into realities.

IIT Hyderabad

Drying complex fluids is a common phenomenon where a liquid phase transforms into a dense or porous solid. This transformation involves several physical processes, such as the diffusion of liquid molecules into the surrounding atmosphere and the movement of dispersed phases through evaporation-driven flow. As a result, the solute forming a dried deposit exhibits unique patterns and often displays structural defects like desiccation cracks, buckling, or wrinkling. Various drying configurations have been utilized to study the drying of colloids, the process of their consolidation, and fluid-flow dynamics. This review focuses on the drying of colloids and the related phenomena, specifically the drying-induced effects observed during sessile drop drying. We first present a theoretical overview of the physics of drying pure and binary liquid droplets, followed by drying colloidal droplets. Then, we explain the phenomena of pattern formation and desiccation cracks. Additionally, the article briefly describes the impact of evaporation-driven flows on the accumulation of particles and various physical parameters that influence deposit patterns and cracks. © 2023 Author(s).

Department of Chemical Engineering

World University root node

Physics of Fluids

Physics of drying complex fluid drop: Flow field, pattern formation, and desiccation cracks

The packing of particles is ubiquitous, and it is of fundamental importance, particularly in materials science in the nanometric length scale. It becomes more intriguing when constituent particles deviate from spherical symmetry owing to the inherent complexity in quantifying their positional and rotational correlation. For quantitative estimation of packing fraction, it requires a thorough analysis of the positional correlation of jammed particles. This article adopts a novel approach for determination of the packing fraction of strongly correlated nano-ellipsoids in a microsphere using small-Angle scattering. The method has been elucidated through a quantitative analysis of structural correlation of nano-hematite ellipsoids in 3D micrometric granules, which are realized using rapid evaporative assembly. Owing to the deviation from spherical symmetry, the conventional analysis of scattering data fails to interpret the actual packing fraction of the anisotropic particles. The structural correlation gets smeared out because of orientation distribution among the packed anisotropic particles, which leads to an anomaly in the estimation of packing fraction using the conventional analysis approach. It is illustrated that consideration of an interparticle distance distribution function of the correlated nano-ellipsoids becomes indispensable in determining their packing fraction. © 2022 American Chemical Society

Langmuir

Jamming of Nano-Ellipsoids in a Microsphere: A Quantitative Analysis of Packing Fraction by Small-Angle Scattering

Crystallization demands structural regularity. Therefore, when a solvent is frozen to form a crystalline solid, it forces the dispersed solute to partition out of the crystalline phase. This forms the basis of ice-templating, wherein a solvent (often water) containing dispersed particles and/or macromolecules is frozen so that the dispersed solute phase is expelled by the ice crystals and is consolidated at their boundaries, when the ice crystals impinge. Removal of the ice crystals results in the formation of a macroporous material, whose pores are bounded by walls formed by the freezing-induced self-assembly of the solute. This versatile technique can be applied to a wide variety of solutes since their expulsion and aggregation due to solvent freezing is largely independent of solute chemistry. Ice-templating has therefore become an increasingly popular synthetic tool in the materials science community. In this review, we summarize the process of freezing and the interaction of the solute with the freezing solvent. We specifically review the literature on how solute-solute interactions influence the self-assembled structure that results from ice templating, and identify lacunae in our current understanding of ice-templated materials synthesis. © 2022 RSC

Materials Advances

Materials prepared by Freezing-Induced Self-Assembly of Dispersed Solutes: A Review

Drying of complex fluids is a fascinating subject of interest to several growing fields, for example, forensic science, lithography, printing and coating technologies. In this article, we report that the drying of charge stabilized colloidal dispersions between two parallel plates is a route to intriguing self-assembly patterns. We show that when the dispersions are dried in parallel plate confinement, particles deposit as spiral patterns after the complete evaporation of the solvent irrespective of the confinement spacing. The formation of such patterns is understood by analyzing the underlying three phase contact line dynamics during the drying process. Compared to the usual discrete stick-slip motion of the contact line, typically observed in several drying configurations, in the parallel plate drying configuration, the contact line is found to exhibit continuous stick-slip motion. The de-pinning of the contact line is found to occur only locally and is observed to propagate in both clockwise and anticlockwise directions, leading to the patterning of colloids as spirals. Furthermore, we show that while the number of turns in the spiral deposit is influenced by the dispersion volume and particle concentration, the spiral patterns form irrespective of the shape of the particles in the dispersion. This journal is © The Royal Society of Chemistry.

Soft Matter

Patterning of colloids into spirals: Via confined drying

Hodges and Tangparitkul (Hodges, C. S.; Tangparitkul, S. M. Langmuir 2019, 35, doi: 10.1021/acs.langmuir.9b01442) in their Comment on "Patterns in Drying Drops Dictated by Curvature Driven Particle Transport" argue that the coffee-eye deposits in dried pendant drops can also be formed if the particles or particle clusters in the drying drop are large enough to sediment during the course of evaporation. In our reply to this comment, we compare these two different mechanisms, namely, gravity settling and curvature-driven interfacial migration of particles in the drying particle-laden drops, with an aim towards placing them in a correct perspective. © 2019 American Chemical Society.

Reply to "comment on patterns in drying drops dictated by curvature-driven particle transport'"

Evaporation of colloidal dispersions leading to patterning of particles is a simple and elegant route for controlling the self-assembly of particles on a solid surface. In this article, we demonstrate that the configuration in which a colloidal dispersion is dried greatly influences the patterning of particles on a solid surface after complete evaporation of the solvent. Evaporation experiments are carried out using well-characterized stable aqueous dispersions of hematite ellipsoids and polystyrene spheres. The drying of particle laden sessile drops always give a "coffee-ring" deposit irrespective of the particle concentration. At a particle concentration ≥0.3 wt% circular cracks appear in the annular region of the coffee-ring deposit owing to the ordered arrangement of ellipsoids. In stark contrast, the deposits formed by drying the dispersion of ellipsoids in the sphere-on-plate configuration show a transition from "concentric rings" to "concentric cracks" in the micro-structure of the particulate film with an increase in the concentration of particles. Further, our experimental findings reveal that long-range circular cracks and long-range assemblies of particles can be achieved by drying of the dispersion in the sphere-on-plate configuration. While the nature of patterns-that is-coffee-rings and concentric rings-is independent of the shape of the particles, a strikingly different crack morphology is shown to be dictated by the shape of the particles in the dispersion. The results presented show that the drying of colloidal dispersions in the sphere-on-plate configuration enables the fabrication of a long range ordered assembly of particles over an area as large as few square millimeters. © 2019 the Owner Societies.

Physical Chemistry Chemical Physics

Influence of the drying configuration on the patterning of ellipsoids-concentric rings and concentric cracks

Hypothesis: Spray drying is a facile technique to transform colloidal dispersion into micro-granules of controlled size, shape, and morphology. There is significant interest to understand the structural integrity, different morphology of the granules obtained post spray drying which find potential application in many technological fields. The shape of the constituent particles in the colloidal dispersion that is spray dried is expected to influence the micro-structural features of the micro-granules. Experiments: We investigate the formation of micro-granules consisting of nano-ellipsoids through controlled spray drying. The morphological features and the packing of ellipsoids in the granules are quantitatively analyzed by using small angle neutron scattering, small angle X-ray scattering and high-resolution field emission scanning electron microscopy. The time evolution of the micro-structure and the structural integrity of the granules are investigated by re-dispersing the powder granules in water. Findings: The morphology of the granules are found to be strongly correlated with the aspect ratio of the ellipsoid. While the drying of droplets containing lower aspect ratio ellipsoids give rise to mostly spherical granules, in stark contrast, for higher aspect ratio ellipsoids, micro-granules of different morphologies are formed including doughnut shaped granules. A plausible mechanism explaining such an aspect ratio dependent shape transformation is proposed. © 2019 Elsevier Inc.

Journal of Colloid and Interface Science

Spray drying of colloidal dispersions containing ellipsoids

Patterns generated by controlled evaporation of droplets containing colloids are dictated by internally generated flows. This advective particle transport is crucial to the efficacy of printing and coating processes and is also an elegant route to the self-assembly of particles. We propose a novel particle transport route, which involves adsorption of particles to the interface and subsequent curvature-driven migration of the particles along the interface. This interface-mediated transport can be exploited to control the distribution of particles in the dried patterns, which we experimentally elucidate by achieving gravity-induced drop shape changes. Our experiments demonstrate that the interplay between the bulk and the interfacial transport leads to strikingly different patterns: while dried aqueous sessile drops of colloidal dispersions produce well known "coffee-rings", dried pendant drops lead to "coffee-eyes". We support our experimental findings using scaling arguments. In previous studies, the effect of gravity-induced change in drop shape on the patterns formed in drying drops has been neglected. However, we show that the structure of the patterns formed by the colloidal particles after solvent evaporation is markedly different when the drops are deformed by gravity. © 2018 American Chemical Society.

Patterns in Drying Drops Dictated by Curvature-Driven Particle Transport

The orientation and morphology of cracks in coffee-ring like particulate deposit obtained by drying sessile drops containing anisotropic magnetic particles strongly depends on the magnitude of the applied magnetic field and its direction. This opens up the possibility of tuning the micro-structure of cracks via suitable manipulation of magnetic and hydrodynamic torques on the particles which has potential applications in nano-fabrication and field driven self-assembly. We report a systematic study of magnetic field driven self-assembly of hematite ellipsoids in sessile drops dried on solid substrates and resulting crack patterns. The experiments are carried out over a wide range of applied magnetic field strength (|B→|) varying from 0 to 400 G and ellipsoids of two different aspect ratios. Dried coffee-ring deposits of ellipsoids in absence of the external applied magnetic field and at low field strength, |B→|≤20G exhibit circular cracks. However, at |B→|⩾30G, the cracks are observed to be linear and perpendicular to the direction of the applied magnetic field. Random cracks are observed in the intermediate field range of 20G<|B→|<30G. Thus our experiments reveal that there exists a critical magnetic field at which the orientation of cracks change from circular to linear. The knowledge of the critical field is exploited to measure the hydrodynamic torque experienced by nano-ellipsoids and fluid velocities during evaporation, which are challenging to measure experimentally. © 2017 Elsevier Inc.