Insights of Evaporation Dynamics of an Interacting Linear Array of Droplet System

Abstract

Manipulation of an array of sessile droplets organized in an ordered structure turns out to be of immense consequence in a wide variety of applications ranging from photonics, near field imaging and inkjet printing on one hand to bio-molecular analysis and DNA sequencing on the other. While evaporation of a single isolated sessile droplet has been well studied, the collective evaporative dynamics of an ordered array of droplets on a solid substrate remains elusive. Physically, the closed region between the centre and side droplets in the ordered array reduces the mobility of the diffusing vapour, resulting in its accumulation along with enhanced local concentration and a consequent increment in the lifetime of the center droplet. Here, we present a theoretical model to account for evaporation lifetime scaling in closely placed ordered linear droplet arrays. In addition, the present theory predicts the limiting cases of droplet interaction; namely, critical droplet separation for which interfacial interaction cease to exist and minimum possible droplet separation (droplets on the verge of coalescence) for which droplet system achieves maximum lifetime scaling. Further experimental evidences demonstrate the applicability of the present scaling theory to extended dimensions of the droplet array, generalizing our physical conjecture. It is also worth noting that the theoretical timescale is applicable across a wide variety of drop-substrate combinations and initial droplet volumes. We also highlight that the scaling law proposed here can be extended seamlessly to other forms of confinement like an evaporating droplet inside a mini channel as encountered in countless applications ranging from biomedical engineering to surface patterning. Having established the framework of collective dynamics of droplet evaporation, we turn our attention to the case of self-agglomeration deposits, which are observed in evaporating sessile linear array of droplets. When a spilled drop of coffee dries on a solid surface, it leaves a dense, ring-like deposit along the perimeter, i.e., forming “coffee ring” on the surface. Ring-like stains are not particular to coffee and are commonly seen in the droplets containing dispersed solutes. Many of the industrial application requires uniform deposition like in inkjet printing, genotyping and complex assembly. In this section, we have presented the mixing of colloidal particles suppresses the coffee ring stains and forms uniform deposition. One of the promising areas where the self-aggregation of colloids is highly useful is photonic crystals. Photonic crystals have emerged as a potentially powerful platform that were previously impossible. Photonic crystals have emerged as a powerful tool to achieve light manipulation. Central concept for the photonic behavior is the formation of a photonic ‘band gap’ - a range of frequencies for which light is forbidden to exist within the bulk of the photonic crystal. The presence of a band gap depends on a particular periodic structure within the crystal. Self-agglomeration of colloidal particles can also form periodic arrays, which may serve as a template of photonic crystal. To find out the arrangement of the colloidal particles, packing fraction of colloidal particles have been calculated from centre to the edge of the agglomerate. Optical reflectance shows the variation in the reflectivity along the diametrical line which have been correlated to the peak reflectance of light beam incident on the colloidal crystal. We show that by controlling the evaporative behavior of the droplet in a linear array, it is possible to effect changes in the photonic behavior of the final precipitate.