Dispensing a water drop on the thin film of a solution composed of cellulose acetate (CA) in dimethyl formamide (DMF) forms a thin and porous CA layer at the water–DMF interface. While a denser water drop on a rarer CA–DMF film manifests a Rayleigh–Taylor instability—RTI, the dynamically forming porous layer at the water–DMF interface triggers a Saffman–Taylor instability—STI. The combined effects of RTI and STI enable the formation, growth, coalescence, and branching of an array of periodic finger patterns to finally develop into a flower-like morphology. A general linear stability analysis (GLSA) of a thin bilayer composed of a Newtonian and incompressible water layer resting on a Darcy–Brinkman porous medium could predict the length and the time scales of such a finger formation phenomenon. The GLSA uncovers the crucial roles of pressure gradients originating from the gravitational effects, osmotic forces, the Marangoni effect, and capillary forces on the dynamics of the finger formation. While the density difference between water and CA–DMF layer plays a crucial role in deciding the initial finger spacing, the osmotic pressure dictates the formation, growth, branching, and coalescence of fingers. The length-FL and number-Navg of fingers are found to scale as FL∼We0.33Re−0.25 and Navg∼We0.33Re0.25. Further, an inverse relationship of the concentration of CA (C) with ∼We−0.3 and ∼Re−0.7 highlights its role in the formation and growth of fingers. The loading of CA in DMF, the viscosity and density of the CA–DMF film, and the curvature of the fingers are found to be other parameters that decide morphologies.

A combined theoretical and experimental study is presented to investigate the interplay of forces in the spin-dewetting process in order to achieve enhanced control over droplet generation. In this regard, toluene–polystyrene (PS) film is spin dewetted on a solid substrate to generate an array of droplets. The underlying mechanisms of the spin dewetting of the films into the droplets are explained with the help of a theoretical model followed by a long-wave linear stability analysis (LWLSA). Stabilizing forces like solution viscosity and surface tension play essential roles. The study uncovers that the centripetal force stretches the film radially outward, before it becomes ultrathin and undergoes dewetting under the influence of van der Waals forces, while the surface tension force acts as a stabilizing influence. On the other hand, the viscous force kinetically stabilizes the system to expedite or delay drop formation on the substrate. An imbalance of these factors ultimately decides the droplet spacing, which leads to interesting morphologies such as singlet, doublet, triplet, and clusters of droplets at specific PS concentrations in the range 0.0001–0.0005%, with a ~10–14 nm average droplet height. The experimental data revealed that, at ~3000 rpm, PS (0.01–0.1%) results in critical droplet spacings of λ_max~98–172 μm, leading to immediate dewetting and uniform droplet formation. Our theoretical predictions are in close agreement with the experimental results, validating the present model. The insights gained in this work provide a foundation by presenting a robust framework for controlled droplet generation by optimizing process parameters to achieve the desired droplet size, distribution, and uniformity. The findings have broad applications in material science, biomedical engineering, and related disciplines.

Biosynthesis of gold-amylase nanocomposites (Au-Am-NC) has been achieved wherein the cysteine residues with thiol groups of α-amylase enzyme reduce the gold ions to engender metal-enzyme link. The Au-Am-NC, thus synthesized (~30 nm) could easily degrade the starch iodine (SI) complex generated, which eventually is translated into a prototype for the determination of the glycemic index (GI) of a starch source. In a way, in-situ hydrolysis of SI complex is achieved inside the droplet with the help of the Au-Am-NC, which breaks the α-1, 4 glycosidic bonds of the starch molecules to liberate iodine from helical SI complexes. Experiments uncover that the color intensity of the microdroplets decreases as the enzymatic activity of the nanocomposite increase. The variation in color intensity of droplet is correlated to the GI of various starch sources. Interestingly, an SI droplet of soluble starch shows more fading than wheat, corn, and waxy corn starch. The high branch density of waxy corn starch reduces the hydrolysis rate, showing less SI droplet color fading. The methodology has shown the potential to be translated into a biomedical device to determine the GI of various edible starch sources.

Using high-resolution computational fluid dynamics (Navier–Stokes equations and the level-set method), we quantify the impact of stepwise, linear, and quadratic contact-angle profiles on the Laplace pressure, interface morphology, and flow velocity.