Sreeram K Kalpathy

We critically understand the hydrogen bonding interactions and electronic transitions occurring in a thin film as well as in solution of a photo-responsive polymer, azo-polyurea (azo-PU). We synthesize azo-PU by covalent attachment of the azobenzene chromophore to the main chain of polyurea. Azo-PU shows reversible photoisomerization between trans and cis states upon light exposure, the occurrence of which is typically analysed using the π-π* and n-π* electronic transition peaks in the UV-visible absorption spectrum. We find that the π-π* and n-π* bands undergo a redshift and blueshift respectively on dissolving azo-PU in DMF solvent, resulting in a single overlapped peak in the spectrum. However, upon UV irradiation, these bands split into two independent transitions that are characteristic of azo-PU solid films. These observations are explained based on the changes in polymer-polymer and polymer-solvent interactions through hydrogen bonding and self-aggregation tendency. The experimental findings are corroborated using DFT simulations which provide useful insights into electronic orbital transitions, electron distribution, and hydrogen bonding interaction through IR vibrational modes.

Coating thin liquid films with complex rheological behaviour on permeable substrates is often an important requirement in several applications such as contact lenses, textiles, and paper-based electronics. Here, we extend the classical Landau-Levich problem of dip coating of Newtonian liquids on rigid substrates to liquids of power-law rheology on permeable substrates. Our results suggest distinct deviation from the classical Landau-Levich relation through exhibition of different regimes of varying dependence of coating film thickness on withdrawal speed. A process map is presented depicting these coating thickness regimes for a wide range of operating parameters such as the substrate permeability factor, power-law exponent of the liquid, and a rescaled capillary number.

The electrocapillary effect is the change in interfacial tension between two immiscible liquids when subjected to an electric field across the interface. The change in interfacial tension occurs due to variations in the surface charge density of adsorbed ions or polar species at the interface. In the present work, we provide fluid mechanical insights into electrocapillarity-based dynamics of an interface between a passive air layer and a thin liquid film that has small, but finite electrical conductivity. We formulate and solve mathematical equations that model the situation in which a liquid film bounded by an air layer of controllable thickness is sandwiched between two electrode plates, and an electric field is applied across the layers. As the liquid and air have different conductivities, free charges would accumulate at the liquid-air interface to ensure that current is conserved across the layers. In our model, the interfacial tension is assumed to vary as per the classical Lippmann's equation. The present mathematical model describes spatiotemporal variation of the film height and interfacial charge density as a function of the applied potential, air layer width, and the sensitivity of interfacial tension to the electric potential. Our results show that ultrathin liquid films (<100 nm) in presence of electrocapillary Marangoni effect are destabilized and break up faster than in its absence. Furthermore, by controlling the air gap width, different morphological patterns can be generated. Finally, we illustrate that the dynamics is profoundly affected if the electrode plate bounding the liquid film possesses patterned wettability.

The present work examines the effect of substrate surface porosity on the coating thickness and meniscus profile during dip coating under saturated porous media conditions. The classical Landau-Levich formulation is modified by encoding the influence of porosity in an effective Navier slip boundary condition at the porous substrate surface. It is shown that simplified Navier slip-based model works well for creeping flow through the porous medium. The film height profile equation is derived as a function of a rescaled capillary number (Ca‾) and a substrate permeability factor, with inertial effects neglected. Numerical solutions show that the classical 2/3rd power dependence of film thickness on capillary number is recovered only at sufficiently high Ca‾ values. As Ca‾ is decreased, a marked deviation is seen. The shrinking of the entrainment meniscus and the change in meniscus curvature are analyzed in detail. The theoretical results are also validated with a suitable experimental system.

Photoresponsive organic liquids and polymers may undergo reversible photochemical reactions with accompanying change in chemical structure upon exposure to a suitable wavelength of light. Spatial variations in chemical structure can cause surface tension gradients along thin films of such materials, resulting in Marangoni flows when the material is in its fluid state. Consequently the fluid film can self-organize into topographical patterns. Here, we provide a hydrodynamic model perspective of photochemically assisted patterning in thin fluid films using principles of momentum and mass transfer. Photochemical reaction occuring simultaneously along with hydrodynamic flow is modelled. Dynamical variation of the photoproduct concentration, accounting for first order chemical reaction kinetics, is considered. Our simulations highlight the counteracting effects of reaction kinetics and Marangoni flow as the mechanism responsible for pattern evolution. We capture various pillar and hole array morphologies obtained by controlling the direction of Marangoni flow and reaction-induced mass transfer. The resolution and timescales of pattern formation are computed as function of experimental control parameters such as the reaction rate coefficient (K0), Marangoni number (M), and distance between the film and light source. A process map comparing feature sizes on a photomask (w) with those of the patterns evolved in the material is developed. It reveals optimum w − M and w − K0 combinations required for faithful reproduction of photomask feature sizes and deviation from ideally templated patterns.

The displacement of a more viscous fluid by a less viscous fluid renders the fluid-fluid interface unstable and leads to intricate patterns called viscous fingers. Since the fluids experience shear during displacement, it should be possible to influence the emergence of patterns and instability dynamics through control of rheological parameters, such as elasticity or relaxation time in case of a viscoelastic fluid. In this article, we record and analyze the interfacial fingering patterns that emerge when a Newtonian fluid (glycerol-water mixtures of different viscosities) displaces a shear-thinning viscoelastic fluid (aqueous cornstarch suspensions of varying concentrations) in a radial Hele-Shaw cell geometry. While Newtonian-non-Newtonian fluid pair displacements have drawn attention of researchers in the past, the current work showcases the various regimes of emergent patterns over a wide range of viscosity ratios of the two fluids, and the effect of fluid elasticity on the rate of its displacement. As the ratio of viscosities of the inner and outer fluids is increased, radial branched patterns are replaced by more stable interfaces that display finger coalescence. Increasing the viscosity of the displacing fluid and the concentration-dependent elasticity of the outer viscoelastic fluid both lead to significant suppression of interfacial instabilities. A linear stability analysis of the interface, using viscosity ratio as the only control parameter, is employed to predict the dominant wavelength of interfacial perturbation. The perturbation wavelength computed numerically is found to match closely with the spacing between fingers measured experimentally at the onset of interfacial instability. It is suggested that control of instabilities during miscible displacement of a viscoelastic fluid (mud slurries, for example) by a Newtonian fluid has implications in material processing, such as in ensuring minimal mixing of phases while maximizing sweep efficiency during material recovery.

Self-cleaning superhydrophobic cement based surfaces are predominantly fabricated by functionalization of either superhydrophilic micro/nano powders or their structural components with toxic chemicals (alkylsilanes, perfluoropolyethers). In this article, a non-toxic, scalable and cost-effective fabrication colloidal lithography approach is reported to prepare superhydrophobic cement surfaces. This study will firstly report the discovery of superhydrophobicity in Periwinkle flowers, followed by biomimicking their surface micro-conical textures on the polydimethylsiloxane (PDMS) and cement surfaces. The protruding micro-cones possess average height of 15 ± 2 µm, bottom diameter of 12 ± 2 µm and pitch of 20 ± 4 µm. The Wenzel roughness of the petals measured by a 3D non-contact optical profilometer is 2.3 ± 0.12. The water static contact angle (SCA) on the petals is 148 ± 2°, roll-off angle (RA) is 13 ± 1° and contact angle hysteresis (CAH) is 12 ± 3° demonstrating quasi-superhydrophobicity. A key highlight is the development of textured cement and PDMS surfaces of up to 152 cm2 areas starting from 2.25 cm2 areas of petals, i.e. achieving ∼ 68 times bigger areas as compared to that of the master templates. The textured PDMS surfaces exhibited superhydrophobicity displaying SCA of 156 ± 2°, CAH of 9 + 1° and RA of 10 ± 1°, while the textured cement surfaces exhibited a transition from superhydrophilicity to quasi- superhydrophobicity with SCA of 147 ± 2°, CAH of 16 + 3° and RA of 17 ± 3°. The rolling water droplets took away the mud particles demonstrating self-cleaning ability. This study showcases an environmentally sustainable solution to prevent water leakages in the cement-based constructions and can be easily adapted by the non-specialist users.

Polymorph (spinel/ilmenite) composite nanofibers of nickel titanate (NTO) were prepared by a sol-gel assisted electrospinning process followed by pyrolysis using the styrene-acrylonitrile copolymer as a precursor at three different pyrolysis soaking temperatures (i.e. T= 773, 973, and 1173 K). The magnetic behavior of these composite NTO nanofibers was studied under isothermal and non-isothermal conditions in the temperature range of 20-300 K. The magnetic parameters such as coercivity (Hc), remanence (Mr), and saturation magnetisation (Ms) were found to be strongly dependent onT. The highestHcandMrwere observed for NTO nanofibers developed at 973 K, which have a mosaic structured morphology with spinel and ilmenite NTO crystallite sizes of ~39 nm and ~24 nm, respectively. On the other hand, the highestMsand switching field distribution were observed for mosaic structured NTO nanofibers having smaller crystallites (~13 nm and 24 nm for spinel and ilmenite NTO, respectively, with high inter-particle distance and high porosity) developed at 773 K, which are also rich in spinel NTO content. The correlation between the variation in magnetic behavior and structural/morphological features of NTO nanofibers is useful for NTO-based soft magnetic and multiferroic applications.

In this work, ordered patterns having nanoscale features and resolutions in the range of 5–75 μm are produced from thin films of azo-polyurea (azo-PU) using photoinduced dewetting. Azo-PU is synthesized through polyaddition reactions, and reversible isomerization between cis – trans states is demonstrated by exposure to light of wavelengths 365 and 450 nm. This photochemical response is exploited to produce patterns by selective exposure using photomasks followed by subsequent annealing to induce polymer flow and dewetting. Marangoni flow is found to be a dominant mechanism that controls morphological evolution of patterns. The patterns can be switched between pillar-like and hole-like morphologies by stimulating the forward or backward isomerization reaction, and pattern resolution can be controlled using photomasks of different feature sizes. Preferential adsorption of proteins on these patterns is proposed as a potential application.

Solar distillation is an immensely promising, renewable energy-based solution route for potable water production from grey water. Noble plasmonic nanoparticles (e.g., Au, Ag) are typically employed in solar distillation systems for efficiency enhancement. However, their high cost renders such technologies economically nonviable. In this study, we evaluate the prospect of using Cu-coated Al as non-noble plasmonic nanoparticles for solar distillation based on the energy balance across various parts of a solar still (basin, glass cover and fluid). We compute the absorption and scattering cross-sections of these nanoparticles, and hence their optical activity. We show that particles of sizes <60 nm have greater absorption cross-section as compared to scattering cross-section in the ultraviolent (200 to 360 nm) and visible light spectrum (360 to 700 nm). The effects of nanoparticle concentration (0.004 % to 0.024 % of sizes 10–22 nm) in the nanofluid on the yield of distillate, energy efficiency and temperature of the fluid are analyzed. We report that Al-Cu nanoparticles enhance energy efficiency of a conventional solar still by as much as 64 % at a concentration of 0.012 %. The efficiency increases with increasing nanoparticle concentration and begins to saturate after 0.012 % concentration.