A control over the nature of deposit pattern obtained after the evaporation of solvent from a sessile drop containing dispersed materials has been demonstrated to have applications in materials engineering, separation technology, printing technology, manufacture of printed circuit boards, biology, and agriculture. In this article, we report an experimental investigation of the effect of particle shape and DLVO (Derjaguin-Landau-Verwey-Overbeek) interactions on evaporation-driven pattern formation in sessile drops. The use of a model system containing monodisperse particles where particle aspect ratio and surface charge can be adjusted reveals that a control over the nature of deposit pattern can be achieved by tuning the particle-particle and particle-substrate interactions. A clear coffee-ring formation is observed when the strength of particle-particle repulsion is higher than the particle-substrate attraction. However, complete suppression of ringlike deposits leading to a uniform film is achieved when particle-substrate and particle-particle interactions are attractive. Results illustrate that for the system of submicron ellipsoids that are hydrophilic, the nature of deposit patterns obtained after evaporation depends on the nature of interactions and not on particle shape.

The recent surge to explore and exploit the effect of particle shape has led to several interesting and unique observations in various diverse fields, ranging from biology to material science. To this end, the development of novel synthesis methods has contributed immensely, and has directed this exciting research on the role of particle shape in colloidal systems and other diverse fields. Furthermore, such investigations have resulted in the development of advanced materials, which are novel and multifunctional. In this article, we review recent advances in the study of suspensions containing shape anisotropic particles. In particular, we investigate those pertaining to rod-like or ellipsoidal particles and highlight recent results in three areasevaporation driven assembly, packing, interfacial behavior and the use of these particles in emulsion stabilization.

The adsorption of particles to the fluid-fluid interface is a key factor for the stabilization of fluid-fluid interfaces such as those found in emulsions, foams and bijels. However, for the formation of stable particle-laden interfaces, the particles must migrate to the interface from the bulk. Recent studies show that the adsorption of particles to the interface formed during emulsification is influenced by the surface charge of the particles. To further investigate this phenomenon, we study the effect of the surface charge of the particle on the adsorption kinetics of particles to the oil-water interface. By suspending a drop of aqueous dispersion of charge stabilized nanoparticles in decane, the adsorption dynamics of particles to the decane-water interface is studied using the dynamic surface tension measurements. When the particles are highly charged (low salt), a negligible change in the interface tension is observed indicating that almost no particles are adsorbed. These results show that the charged particles experience an energy barrier when they approach the interface. But when the particle surface charge is screened by the addition of monovalent salt, a significant reduction in surface tension is observed indicating the migration and adsorption of particles to the decane-water interface. We estimate the effective diffusivity of particles to the interface by analyzing the initial decay in the measured surface tension by considering particle laden drops containing different amounts of salt using the modified Ward and Tordai theory. This effective diffusivity is used to calculate the energy barrier for the adsorption of particles to the interface. The energy barrier from the analysis of dynamic surface tension data agrees well with the concept of image charge repulsion which inhibits the adsorption of highly charged particles to the interface. By considering various types of relevant interactions, we derive an analytical expression that qualitatively captures the effect of the surface charge on the equilibrium surface coverage of particles at the drop surface.

Liquid drops containing insoluble solutes when dried on solid substrates leave distinct ring-like deposits at the periphery or along the three-phase contact line-a phenomena popularly known as the coffee-ring or the coffee stain effect. The formation of such rings as well as their suppression is shown to have applications in particle separation and disease diagnostics. We present an experimental study of the evaporation of sessile drops containing silica rods to elucidate the structural arrangement of particles in the ring, an effect of the addition of surfactant and salt. To this end, the evaporation of aqueous sessile drops containing model rod-like silica particles of aspect ratio ranging from ∼4 to 15 on a glass slide is studied. We first show that when the conditions such as (1) solvent evaporation, (2) nonzero contact angle, (3) contact line pinning, (4) no surface tension gradient driven flow, and (5) repulsive particle-particle/particle-substrate interactions, that are necessary for the formation of the coffee-ring are met, the suspension drops containing silica rods upon evaporation leave a ring-like deposit. A closer examination of the ring deposits reveals that several layers of silica rods close to the edge of the drop are ordered such that the major axis of the rods are oriented parallel to the contact line. After the first few layers of ordered arrangement of particles, a random arrangement of particles in the drop interior is observed indicating an order-disorder transition in the ring. We monitor the evolution of the ring width and particle velocity during evaporation to elucidate the mechanism of the order-disorder transition. Moreover, when the evaporation rate is lowered, the ordering of silica rods is observed to extend over large areas. We demonstrate that the nature of the deposit can be tuned by the addition of a small quantity of surfactant or salt.

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.