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We report morphological evolution and pattern formation during evaporative drying of a droplet of polymethylmethacrylate (PMMA) dissolved in tetrahydrofuran over a soft, swellable cross-linked Sylgard 184 substrate. In contrast to the well-known coffee ring formation due to the evaporation of a polymer solution droplet over a rigid substrate, we show that the situation becomes far more complicated over a Sylgard 184 substrate due to solvent penetration and associated swelling. The combined effect of evaporation and diffusive penetration leads to significantly faster solvent loss and results in the formation of an in situ thin polymer shell over the free surface of the evaporating droplet due to the attainment of local glass-transition concentration. The diffusive penetration of the solvent also leads to the spreading of the three-phase contact line (TPCL) of the droplet after dispensing. The vertical component of surface tension acting at the TPCL results in the formation of peripheral creases along the boundary of the droplet after the TPCL pins. With the progressive solvent loss, the shell eventually collapses, resulting in a buckled morphology with a central depression. We show that the evolution pathway and the final deposit morphology depend strongly on the initial PMMA concentration (Ci) in the droplet as it undergoes a transformation from a central depression surrounded by peripheral folds at lower Ci to a central depression along with radial wrinkles at higher Ci. During the late stage of the evolution process, the substrate undergoes de-swelling, which leads to flattening/rearrangement of the radial wrinkles, the extent of which again depends on Ci. We explored how the deposition pathway and patterns vary over a topographically patterned substrate and found out that the presence of topographic patterns leads to even faster solvent consumption due to enhanced diffusive penetration at the corrugated liquidâsubstrate interface, eventually resulting in deposition with a smaller footprint and partially aligned radial wrinkles. The results significantly enhance our understanding of droplet evaporation over a substrate into which the solvent can penetrate and unravel the complex physics, which is significantly dominated by swelling rather than evaporation only, which is common over a rigid, non-interacting substrate.
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Purpose: Economic development of India mainly depends on agricultural sectors. The Indian traditional agricultural system is mainly based on chemical fertilizer to get better yield. The main motto of this research work is to change the traditional faith of Indian farmers and rural Indian economy. Methods: Bioprocessing of feather prepared from an efficient newly isolated bacterial strain, identified as Bacillus wiedmanni SAB10 is used to produce a nitrogen rich liquid fertilizer. The cell-free hydrolysate was prepared from submerged fermentation of poultry litter (1.25%, w/v) as sole media with supplemented as chicken feather (1%, w/v) in 79.41 h with pH 10.6. Results: Fermented hydrolysate contains a significant quantity of total amino acid (503.02 mg/L) with diversity (Cystine, Phenylalanine, Tyrosine, lysine, Valine, Proline and Alanine), total oligopeptides (4.65 mg/ml) and thiol content (58.09 µg/ml) which influence growth and yield (1.02 fold) of moong beans (Vigna radiata) plant in pot trials and as well as successfully scale up in field trials by the farmers. This liquid fertilizer not only makes plant healthy and has drought tolerance (proline content- 0.023 mg/g) capacity but also increases the grain quality by spraying the fertilizer on foliage with a ratio of 2:1 (Water: Feather hydrolysate) for two times (before the 1st flash and 2nd flash of flowering). Conclusion: Fermented feather hydrolysate is used full as a foliage fertilizer for the cultivation of moong beans. Some commercial properties and its eco-friendly, cost-effectiveness will make it a smart liquid fertilizer in near future. Supplementary Information: The online version contains supplementary material available at 10.1007/s12649-022-01982-9.
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Strategic control of evaporation dynamics can help control oscillation modes and internal flow field in an oscillating sessile droplet. This article presents the study of an oscillating droplet on a bio-inspired "sticky" surface to better understand the nexus between the modes of evaporation and oscillation. Oscillation in droplets can be characterized by the number of nodes forming on the surface and is referred to as the mode of oscillation. An evaporating sessile droplet under constant periodic perturbation naturally self-tunes between different oscillation modes depending on its geometry. The droplet geometry evolves according to the mode of evaporation controlled by substrate topography. We use a bio-inspired, rose patterned, "sticky" hydrophobic substrate to perpetually pin the contact line of the droplet in order to hence achieve a single mode of evaporation for most of the droplet's lifetime. This allows the prediction of experimentally observed oscillation mode transitions at different excitation frequencies. We present simple scaling arguments to predict the velocity of the internal flow induced by the oscillation. The findings are beneficial to applications which seek to tailor energy and mass transfer rates across liquid droplets by using bio-inspired surfaces.
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Reversible morphology switching in a soft elastic film sandwiched between two parallel electrodes when subject to an externally applied electric field is reported herein. In contrast to electric field mediated instability of a thin liquid film, where the instability patterns remain permanent, in the present case the patterns debond completely or partially when the electric field is switched off, depending on whether the gap spacing (dG) between the film and the top electrode is >100 nm or not. The onset of instability is marked with the appearance of isotropic columns when the applied field strength (U) exceeds a critical value (Uc). The subsequent increase in U leads to the gradual transition of the instability patterns from pillars to bi-continuous labyrinths to an array of holes. Complete conformal contact is established between the film and the top electrode at U = UF. When U is reduced, the morphology changes in a reverse sequence. There is a significant level of hysteresis between the bonding and debonding stages, including persistence of the features at much lower voltages due to pinning of the patterns to the top electrode. Complete detachment occurs at a lower voltage UD when dG > 100 nm. The holes fluctuate before complete contact between the film and the top electrode due to competition between the destabilizing electric field and restoring forces due to stretching of the crosslinked polymer matrix.