Dillip K Satapathy

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.

We show that loosely packed homogeneous monolayers of soft colloidal particles can be fabricated via simple evaporative drying of aqueous dispersion containing colloidal particles. Owing to the inherent amphiphilicity of the soft microgels, they are preferentially adsorbed onto the water-vapor interface of the drying droplet, resulting in a uniform deposition of the particles throughout the fluid-substrate contact area. Distinct order-to-disorder transition is also identified within the dried deposits on changing spatial locations from edge to center of the circular stain. Such monolayer deposits exhibit vibrant colors on irradiating with white light due to interference.

Cracks in a colloidal film formed by evaporation induced drying can be controlled by changing drying conditions. We show, for the first time that the crack morphologies in colloidal films are dependent on shape of constituting particles apart from the microstructure and particle assembly. In order to investigate the particle shape effect on crack patterns, monodispered spherical and ellipsoidal particles are used in sessile drop experiments. On observing the dried sessile drop we found cracks along the radial direction for spherical particle dispersions and circular crack patterns for ellipsoidal particle dispersions. The change in crack pattern is a result of self assembly of shape anisotropic particles and their ordering. The ordering of particles dictate the crack direction and the cracks follow the path of least resistance to release the excess stress stored in the particle film. Ellipsoids having different aspect ratio (∼3 to 7) are used and circular crack patterns are repeatedly observed in all experiments.

In this article, we report the influence of substrate temperature (Tsub) on the evaporation driven patterning of colloids on solid substrates. When the drops are dried in an environment maintained at temperature, Tenv, lower than Tsub, the temperature difference between the drop apex and the three-phase contact line leads to thermal Marangoni flow. We show that the interplay between the radial capillary flow, the thermal Marangoni flow, and the descending rate of the drop surface can be tuned to modulate the spatial distribution of colloids in the dried deposits. At ΔT (=Tsub - Tenv) ≥ 45 °C, the distribution of particles in the interior region of the pattern is nearly uniform with a significant decrease in concentration of particles in the ring-like deposit at the edge. The deposits formed at 15 °C ≤ ΔT ≤ 40 °C are accompanied by a particle depleted zone in the center, which has not been reported to date. The formation of the central depletion zone arises from the suppression of the thermal Marangoni flow at the penultimate stage of drying and the interplay between the radial capillary flow and the descending rate of the drop surface. At ΔT < 15 °C, the dried deposits are found to exhibit coffee-ring-like stains.

We report a complete suppression of the coffee-ring effect resulting in the formation of loosely packed two-dimensional particle monolayers by controlled evaporation of sessile drops containing soft microgel particles. These particulate deposits show gradual order-to-disorder transitions that are not abrupt like deposits of hard colloids. The areal coverage of the monolayer deposits can also be precisely controlled by tuning the particle concentration. The preferential adsorption of soft-microgel particles to the water-vapor interface, and their radial flow along the interface towards the drop edge facilitates the creation of the novel monolayer deposits.

We investigate the sessile drop evaporation aided self-assembly of microgel particles by varying their softness. Evaporation of sessile drops containing amphiphilic microgel particles at suitable concentrations results in uniform monolayer deposits that span the entire drop area. At lower concentrations, the deposits are in the form of monolayer coffee rings whose width scales with particle concentration. Using softer microgels synthesised with a lower quantity of crosslinker, we show that the monolayer coffee rings do not form at low particle concentrations. The microgels adsorbed at the interface deform, and the extent of deformation depends on the softness of the microgels as well as their concentration at the interface. Upon complete evaporation of the solvent, the microgel-laden interface is transferred to the substrate. The final deposit shows hexagonal particle arrays where the interparticle separation increases with increasing microgel softness and decreases with particle concentration in the drop. Further insight into the role of microgel softness in the microstructure of the particulate deposits is obtained by measuring the viscoelasticity of the particle-laden interface. Interestingly, the interface loaded with lesser crosslinked microgels exhibits viscoelastic nature even at lower particle concentrations, whereas the higher crosslinked microgels show viscous behaviour.

Hypothesis: Evaporating sessile drops containing surface active colloids is a promising route to self-assemble two-dimensional nanostructures. The standard protocol is to first self-assemble surface active nanoscale particles at the water-vapour interface and subsequently transfer it on to a solid surface. Colloidal monolayers with very few morphologies have been fabricated, exploiting this bottom-up self-assembly technique. However, the evaporation kinetics under controlled humidity conditions may dramatically alter the microstructure of self-assembled colloidal monolayers at the liquid–vapor interface and that on the solid surfaces, an aspect that has not been fully addressed in the prior studies. Experiments: To this end, we present an experimental study of evaporation driven self-assembly of soft poly(N-isopropylacrylamide) (pNIPAM) microgel particles loaded in a sessile drop. The surface-active microgel particles spontaneously populate the water-vapour interface facilitating the suppression of the coffee-ring effect and the formation of monolayer stains. The role of evaporation kinetics under controlled humidity conditions on the colloid's microstructure adsorbed to the solvent-air interface and on the morphology of the colloidal monolayer transferred onto the solid surface are studied in detail. Findings: The formation of particle-free and particle-rich regions at the water–vapor interface is observed for sessile drops evaporated under saturated humidity conditions. We show that the evaporation induced shrinkage of the interface area and the enhancement of the areal density of microgel particles adsorbed onto the interface leads to a restructuring of the particle-laden interface. The rearrangement of microgel particles along the water–vapor interface resembling the de-wetting assisted patterns is transferred to the solid substrate upon complete evaporation of the solvent. The microgel particles in the deposit assemble into domains with enhanced crystalline order. The evolution of Voronoi entropy across the monolayer deposit patterns obtained by the standard and slow evaporation routes are presented.

The drying of a sessile drop consisting of colloidal particles and the formation of particulate deposits with spatially periodic cracks were ubiquitous. The drying induced stress, which is generated during the evaporation of a colloidal drop, is released by the formation of cracks. We find that the morphology of cracks formed in particulate films dried at substrate temperature, Tsub = 25 °C is markedly different from that of cracks formed at Tsub > 45 °C. The cracks are disordered in the former case, but ordered and periodic in the latter. The disorderedness of cracks observed at Tsub = 25 °C is mainly due to the formation of a coffee-ring like particle deposit that exhibits a larger height gradient. The ultimate deposit pattern after complete drying is observed to be different for colloidal dispersion drops evaporated at different substrate temperatures. This is attributed to temperature-dependent solvent flow mechanisms and capillary-driven flow, which occur inside the colloidal drop during the course of drying. In addition, for the coffee-ring-like particulate deposit obtained at Tsub ≤ 45 °C, the ratio between the width of the deposit w and the radius of the ring R scales with the volume fraction of the colloids φ, w/R ∼ φ0.5, in the range of volume fractions studied in this work. The deposited patterns obtained at temperature Tsub > 45 °C are largely dominated by the capture of particles by the receding liquid-vapor interface. This is due to the faster rate of decrease of the liquid-vapor interface position with an increase in substrate temperature.

Primary and secondary cracks often coexist in a dried colloidal deposit. While, primary cracks have been the focus of investigation in most of the theoretical and experimental studies, understanding the formation and time evolution of secondary cracks is scarce. In addition, secondary cracks at times are undesirable especially when used as a template in the nano-lithography. Therefore, a control over their formation and suppression is of high importance. We consider the drying of sessile drops containing colloidal ellipsoids on a solid substrate. The resulting coffee-ring-like deposit of ellipsoids is found to accompany primary cracks in the circumferential direction and secondary cracks in the radial direction. A systematic study is performed to understand the nucleation and propagation of cracks. Our experiments explicitly showed that the primary cracks always nucleate at the top surface of the deposit and then penetrate the interior of the deposit. However, in contrast, the secondary cracks always form at the bottom surface of the deposit, which is in contact with the solid substrate. The secondary cracks are found to form only when the deposit thickness exceeds a critical value. In addition, we discuss methods to completely suppress the formation of secondary cracks by (i) tuning the aspect ratio of the ellipsoidal particles, and (ii) incorporating a small fraction of spherical particles in the drying drop of ellipsoids prior to the evaporation experiments.

The control of the morphology of desiccation cracks is fascinating not only from the application point of view but also from the rich physics behind it. Here, we present a seemingly simple method to tailor the morphology of desiccation cracks by exploitation of the combined effect of particle shape anisotropy and the shape of the confining boundary. This allows us to make circular, square, and triangular-shaped desiccation cracks in the vicinity of the confining boundaries. As the colloidal dispersion dries in confined wells, a drying front appears at the center of the well. With further evaporation, the drying front recedes toward the boundary from the center of the well. We show that the temporal evolution of the drying front is strongly influenced by the shape of the well. Subsequently, desiccation cracks appear in the penultimate stage of drying, and the morphology of the cracks is governed by the shape of the drying front and hence by the shape of the wells. The spatial evolution of the crack pattern is quantified by estimation of the curvature of the cracks, which suggests that the influence of the confining boundary on crack formation is long-ranged. However, the cracks in the dried deposit consisting of spherical particles remain unaffected by the shape of the well, and the cracks are always radial. We establish a one-to-one correspondence between the shape of the drying front and the morphology of the crack pattern in the final dried deposit of ellipsoids.