We show that the morphology of a thin polymer film spin coated directly on to a topographically patterned substrate is strongly influenced by the wettability of the substrate, in addition to other well-known parameters such as concentration of the polymer solution (c(n)), spin speed (RPM), and spin duration. Similar to spin coating on a flat surface, (1, 2) on a topographically patterned substrate as well, a continuous film forms only above a critical polymer solution concentration (c(t)*), for a specific RPM and dispensed drop volume. It is believed that for c(n) > c(t)*, the resulting continuous film on a topographically patterned substrate has an undulating top surface, where the surface undulations are in phase with the underlying substrate patterns. (3) On the basis of experiments involving spin coating of polymer thin films on topographically patterned grating substrates, we show that the surface undulations on the film are in phase with the substrate patterns only when the substrate is completely wetted (CW) by the solvent. In contrast, when the substrate is partially wetted (PW) by the solvent, then the undulations are 180° out of phase with respect to the substrate patterns. We further show that for c(n)< c(t)*, a variety of ordered and disordered structures, like array of aligned droplets, isolated strips of polymers, etc., result on both CW and PW substrates, depending on c(n).

It is known that dewetting of a polystyrene (PS) thin film on a silicon substrate gets completely suppressed upon addition of small amount of C 60 nanoparticles (NP). 1 The NPs migrate to the film−substrate interface and forms an enriched surface layer of the particles that eventually stabilizes the film by apparent pinning. In this article we quantitatively highlight the unexplored effect of substrate surface energy (γ S ) on the migration of the NPs to the film−substrate interface and their contribution on dewetting suppression. Depending on the relative magnitudes of NP concentration (C NP ) and γ S , we identify three distinct stability regimes. In regime 1 (C NP < 0.2%) there is no suppression of dewetting and the final polygonal arrangement of droplets closely resemble dewetted structures in particle free films. However, the size of the polygons becomes smaller in NP containing films when γ S < γ C60 (NP surface energy) and larger as γ S exceeds γ C60 . In regime 2 (0.3% < C NP < 0.75%) the films dewet partially, and the extent of dewetting is seen to strongly dependent on the relative magnitudes of γ C60 and γ S . While dewetting proceeds up to the stage of partial hole growth and coalescence when γ S < γ C60 , some random isolated holes are seen to form when γ S > γ C60 . On the basis of direct AFM imaging, we show that in both regimes 1 and 2 the NPs migrate to the substrate−film interface only when γ S > γ C60 . We show complete suppression of dewetting in regime 3 (C NP > 1.0%), where the particles are seen to migrate to the substrate for all values of γ S . The work highlights that entropy driven migration of particles takes place on substrates with any γ S only above a critical NP concentration (C NPC ) and only on substrates with γ S > γ C60 when C NP < C NPC . The findings, apart from dewetting suppressing, can guide potential design criteria for applications such as electron extracting layer in organic photovoltaic.

Zinc oxide (ZnO) is a versatile material for different commercial applications such as transparent electrodes, piezoelectric devices, varistors, SAW devices etc because of its high piezoelectric coupling, greater stability of its hexagonal phase and its pyroelectric property. In fact, ZnO is a potential material for gas sensor applications. Good quality ZnO films were deposited on glass and quartz substrates by a novel CVD technique using zinc acetate as the starting solution. X-ray diffraction confirmed the crystallinity of the zinc oxide film and SEM study revealed uniform deposition of fine grains. Undoped ZnO films were used for detection of dimethylamine (DMA) and H 2 at different temperatures by recording the change in resistivity of the film in presence of the test gases. The response was faster and the sensitivity was higher compared to the earlier reported ZnO based sensors developed in our laboratory. The main objective of this work was to study the selectivity of the ZnO film for a particular gas in presence of the others. The operating temperature was found to play a key role in the selectivity of such sensors.

We study the rheology of dry and wet granular materials in the steady quasistatic regime using the discrete element method in a split-bottom ring shear cell with focus on the macroscopic friction. The aim of our study is to understand the local rheology of bulk flow at various positions in the shear band, where the system is in critical state. We develop a general(ized) rheology, in which the macroscopic friction is factorized into a product of four functions, on top of the classical ( ) m I rheology, each of which depends on exactly one dimensionless control parameter, quantifying the relative importance of different micro-mechanical machanisms. These four control parameters relate the time scales of shear rateġ t , particle stiffness t k , gravity t g and cohesion t c , respectively, with the governing time scale of confining pressure t p . Whileġ t is large and thus of little importance for most of the slow flow data studied, it increases the friction in critical state, where the shear rate is high and decreases friction by relaxation (creep) where the shear rate is low. t g and t k are comparable to t p in the bulk, but become more or less dominant relative to t p at the extremes of low pressure at the free surface and high pressure deep inside the bulk, respectively. The effect of wet cohesion on the flow rheology is quantified by t c decreasing with increasing cohesion. Furthermore, the proposed rheological model predicts well the shear thinning behavior both in the bulk and near the free surface; shear thinning rate becomes less near the free surface with increasing cohesion.

Controlled dewetting of a thin polymer film on a topographically patterned substrate is an interesting approach for aligning isotropic dewetted structures. In this article, we investigate the influence of substrate feature height (H(S)) on the dewetting pathway and final pattern morphology by studying the dewetting of polystyrene (PS) thin films on grating substrates with identical periodicity (λ(P) = 1.5 μm), but H(S) varying between 10 nm and 120 nm. We identify four distinct categories of final dewetted morphology, with different extent of ordering: (1) array of aligned droplets (H(S) ≈ 120 nm); (2) aligned undulating ribbons (H(S) ≈ 70-100 nm); (3) multilength scale structures with coexisting large droplets uncorrelated to the substrate and smaller droplets/ribbons aligned along the stripes (H(S) ≈ 40-60 nm); and (4) large droplets completely uncorrelated to the substrate (H(S) < 25 nm). The distinct morphologies across the categories are attributed to two major factors: (a) whether the as-cast film is continuous (H(S)≤ 80 nm) or discontinuous (H(S)≥ 100 nm) and (b) in case of a continuous film, whether the film ruptures along each substrate stripe (H(S)≥ 70 nm) or with nucleation of random holes that are not correlated to the substrate features (H(S)≤ 60 nm). While the ranges of H(S) values indicated in the parentheses are valid for PS films with an equivalent thickness (h(E)) ≈ 50.3 nm on a flat substrate, a change in h(E) merely alters the cut-off values of H(S), as the final dewetted morphologies and transition across categories remain generically unaltered. We finally show that the structures obtained by dewetting on different H(S) substrates exhibits different levels of hydrophobicity because of combined spatial variation of chemical and topographic contrast along the surface. Thus, the work reported in this article can find potential application in fabricating surfaces with controlled wettability.