We show that the electric field driven surface instability of viscoelastic films has two distinct regimes: (1) The viscoelastic films behaving like a liquid display long wavelengths governed by applied voltage and surface tension, independent of its elastic storage and viscous loss moduli, and (2) the films behaving like a solid require a threshold voltage for the instability whose wavelength always scales as approximately 4xfilm thickness, independent of its surface tension, applied voltage, loss and storage moduli. Wavelength in a narrow transition zone between these regimes depends on the storage modulus.

In recent times, deep eutectic solvents (DES) have received attention as an extractive media for separations. In this work, the water stability of eight menthol-based DESs and two tetrabutylammonium chloride (N4444Cl) based DESs with organic acid-based hydrogen bond donors (HBD) at a temperature of 298.15 K and atmospheric pressure were studied. dl-Menthol and N4444Cl were considered as the hydrogen bond acceptors (HBA). Molecular dynamics simulation (MD) was used as a tool to examine the distribution of molecules of DES and water in either phase. The intermolecular nonbonded interaction among the species of the systems was analyzed with radial distribution function, interaction energy, and hydrogen-bonding analysis to understand the stability of DESs in an aqueous medium. The results showed that the strong hydrogen bond plays a crucial role in the water stability of the DES. The degree of hydrogen bonding in HBD–water in terms of HBDs obtained by MD simulation can be presented in the order of acetic acid > levulinic acid > butanoic acid > pyruvic acid > hexanoic acid > octanoic acid > decanoic acid > dodecanoic acid. The strength of the hydrogen bond was attributed to the structure of solvents and the alkyl chain length of the HBD group. Overall, the order of stability of DES in water based on a “relative stability factor” was found as dl-menthol:acetic acid (1:1) < dl-menthol: levulinic acid (1:1) < dl-menthol:butanoic acid (1:1) < dl-menthol:pyruvic acid (1:2) < dl-menthol:hexanoic acid (1:1) < dl-menthol:octanoic acid (1:1) < dl-menthol: decanoic acid (1:1) < dl-menthol:dodecanoic acid (2:1). The transfer of molecules in the system from the aqueous phase to the DES rich phase was analyzed with the help of mean-square displacement and diffusion-coefficients. dl-Menthol and organic acids starting from octanoic acid and higher ones can be used in aqueous systems as solvents. Finally, dl-menthol:octanoic acid (1:1) -based DES was used to benchmark and predict the extraction efficiency of a pesticide (nitenpyram) from an aqueous feed. Hydrogen bond analysis demonstrated higher interactions of nitenpyram with dl-menthol and octanoic acid as compared to water. The MD simulation of the ternary system consisting of DES, water, and nitenpyram showed encouraging results, and gave an excellent agreement with experimental literature data in terms of extraction efficiency (∼42 to 46.7%) and distribution ratio (0.72).

Electric-fi eld-induced hierarchical, multiscale patterning of incompletely cross-linked viscoelastic polydimethylsiloxane (PDMS) fi lms is achieved by spatiotemporal variation of the fi eld, which produces a multiplicity of complex mesopatterns from the same electrode. Experiments and simulations are employed to uncover pathways of hierarchical pattern formation. Spatial modulation of the fi eld is introduced by employing different types of simply patterned electrodes: stripes, elevated concentric circular rings, and boxpatterned ridges. Multiscale complex structures consisting of increasingly fi ner primary, secondary, and tertiary hierarchical structures are fabricated by progressively ramping up the electric fi eld while maintaining the integrity of the already formed structures. The latter is achieved by partially cross-linking the fi lms before patterning, which engenders optimal viscosity to prevent a rapid ripening and coalescence of earlier formed patterns. These multiscale structures can be controlled by the geometry and periodicity of patterned electrodes, the strength of the electric fi eld, and its programmable temporal variation. The PDMS patterns are made permanent by complete cross-linking after a desired multiscale structure is obtained. FULL PAPERwww.afm-journal.de www.MaterialsViews.com

This is a brief review of our recent and ongoing work on simple, rapid, room temperature, pressure-less and large area (~ cm 2 ) imprinting techniques for high fidelity meso-patterning of different types of polymer films. Examples include soft solid polymer films and surfaces like cross-linked polydimethylsiloxane (PDMS) and polyacrylamide (PAA) based hydrogels, thermoplastics like polystyrene (PS), polymethylmethacrylate (PMMA) etc both on planar and curved surfaces. These techniques address two key issues in imprinting: (i) attainment of large area conformal contact with the stamp, especially on curved surfaces, and (ii) ease of stamp detachment without damage to the imprinted structures. The key element of the method is the use of thin and flexible patterned foils that readily and rapidly come into complete conformal contact with soft polymer surfaces because of adhesive interfacial interactions. The conformal contact is established at all length scales by bending of the foil at scales larger than the feature size, in conjunction with the spontaneous deformations of the film surface on the scale of the features. Complex two-dimensional patterns could also be formed even by using a simple one-dimensional master by multiple imprinting. The technique can be particularly useful for the bulk nano applications requiring routine fabrication of templates, for example, in the study of confined chemistry phenomena, nanofluidics, bio-MEMS, micro-imprinting, optical coatings and controlled dewetting.

Chemical pattern directed spin-dewetting of a macroscopic droplet composed of a dilute organic solution of liquid crystal (LC) formed an ordered array of micro- and nanoscale LC droplets. Controlled evaporation of the spin-dewetted droplets through vacuum drying could further miniaturize the size to the level of ∼90 nm. The size, periodicity, and spacing of these mesoscale droplets could be tuned with the variations in the initial loading of LC in the organic solution, the strength of the centripetal force on the droplet, and the duration of the evaporation. A simple theoretical model was developed to predict the spacing between the spin-dewetted droplets. The patterned LC droplets showed a reversible phase transition from nematic to isotropic and vice versa with the periodic exposure of a solvent vapor and its removal. A similar phase transition behavior was also observed with the periodic increase or reduction of temperature, suggesting their usefulness as vapor or temperature sensors. Interestingly, when the spin-dewetted droplets were confined between a pair of electrodes and an external electric field was applied, the droplets situated at the hydrophobic patches showed light-reflecting properties under the polarization microscopy highlighting their importance in the development of micro- or nanoscale LC displays. The digitized LC droplets, which were stationary otherwise, showed dielectrophoretic locomotion under the guidance of the external electric field beyond a threshold intensity of the field. Remarkably, the motion of these droplets could be restricted to the hydrophilic zones, which were confined between the hydrophobic patches of the chemically patterned surface. The findings could significantly contribute in the development of futuristic vapor or temperature sensors, light reflectors, and self-propellers using the micro- or nanoscale digitized LC droplets.