Ordered non-close packed colloidal array with morphology control.

Non-close packed (NCP) colloidal arrays find wide applications in the fields of photonics, optical chip fabrication, nano sphere lithography and so on. However, unlike their close packed counterparts, such arrays cannot be obtained by direct self-organization of colloidal particles and require specialized techniques involving plasma/reactive ion etching, electric field driven assembly, substrate stretching or precise positioning of the particles. In this article, we present a facile template guided approach for fabricating ordered NCP arrays of colloidal particles. First, we employ soft lithography to replicate self-assembled hexagonal close packed (HCP) arrays of 'larger colloidal particles' (LPs) to obtain a topographically patterned positive and/or negative replica of the initial array. These replicas are then used as templates to spin coat 'smaller colloidal particles' (SPs), which may even have some degree of poly-dispersity, to obtain ordered NCP arrays. We further show that pattern morphology can be modulated based on whether a single or a double replicated template is used to confine the SPs, the concentration (Cn) of the SPs in the casting solution as well as the relative commensuration of the diameter of the SPs (ds) with that of the LPs (dL). Finally, we show that such NCP arrays can be transferred onto any flat surface by UVO mediated colloidal transfer printing.

Decisive Role of Polymer-Bovine Serum Albumin Interactions in Biofilm Substrates on "Philicity" and Extracellular Polymeric Substances Composition.

Formation of extracellular polymeric substances (EPS) is a crucial step for bacterial biofilm growth. The dependence of EPS composition on growth substrate and conditioning of the latter is thus of primary importance. We present results of studies on the growth of biofilms of two different strains each, of the Gram-negative bacteria Escherichia coli and Klebsiella pneumoniae, on four polymers used commonly in indwelling medical devices ─polyethene, polypropylene, polycarbonate, and polytetrafluoroethylene─immersed in bovine serum albumin (BSA) for 24 h. The polymer substrates are studied before and after immersing in BSA for 9 and 24 h, using contact angle measurement (CAM) and field emission scanning electron microscopy (FE-SEM) to extract, respectively, the "philicity" φ (defined as -cos θ, where θ is the contact angle of the liquid on the solid at a particular temperature and ambient pressure) and spatial Hirsch parameter H (defined from the relation F(r) ∼ r2H, where F(r) is the mean squared density fluctuation at the sample surface). H = 0.5, <0.5, or >0.5 signifies no correlation, anticorrelation, and correlation, respectively. The substrates are seen to transform from large hydrophobicity to near amphiphilicity with the formation of a BSA conditioning surface layer, and the H-values distinguish the length scales of 100, 500, and 2000 nm, with the anticorrelation increasing with length scale. Biofilms of E. coli did not grow on bare PTFE and HDPE substrates. Biofilms grown on BSA-covered surfaces are studied with CAM, FE-SEM, Fourier transform infrared (FTIR), and surface-enhanced Raman spectroscopy (SERS). Both spectra and φ-values were independent of bacterial species but dependent on the polymer, while H-values show some bacterial variation. Thus, EPS composition and wetting properties of the corresponding bacterial biofilms seem to be decided by the interaction of the conditioning BSA layer with the specific polymer substrate.

Time-structuring in the evolution of 2D nanopatterns through interactions with substrate.

Hydrophobic dodecanethiol capped gold nanoparticles (AuNPs) are found to self-assemble into two-dimensional patterns in monolayers of amphiphiles spread at the air-water interface of a Langmuir trough. In this communication we investigate the role of the nanoparticle-monolayer (FNMA) and monolayer-monolayer (FMMA) lipophilic attraction in influencing morphology and dynamics of AuNP cluster patterns in fatty acid monolayers. FNMA and FMMA are progressively varied by changing n, where n is the number of -CH2 groups in the alkyl tails of the amphiphilic fatty acid (CH3(CH2)nCOOH) molecules forming the monolayer. Compressibility measurements on the pristine and nanoparticle-laden monolayers show that, while the compressibility of the pristine monolayer decreases with increasing n, pointing to a progressive increase in FMMA, the effect of nanoparticles (increase in compressibility or lowering of FMMA) is discernible only for 14 < n < 22. The corresponding pattern morphology, observed with a Brewster Angle Microscope (BAM) at an in-plane resolution of 450 nm for 6 hours, reveals that there are essentially three stages in pattern evolution, lamellae of Au nanoclusters spread over the fatty-acid monolayer background (the λ state) followed by a network of nanoclusters with high node density (the ν state) and finally rings (circular/elongated) of random sizes with very low node density (the ρ state), evolving from an initial unsegregated state, without appreciable change in the average nanoparticle number density over the field of view. Increasing FNMA alongwith FMMA is found to shift a certain state to later times, thus playing the role of a viscous drag and introducing a delay in the timeline. The mean square fluctuation of BAM intensity remains flat and then decays as f(ξ) = ξ(2H) over smaller length scales, where ξ is the spatial separation and H the Hurst exponent. The study of f(ξ) over time reveals the growth of a sub-diffusive regime (H < 0.5) at the intermediate length scale, in almost all the films coinciding with the emergence of the ρ state. The growth of this sub-diffusive regime is slower for stronger FNMA and FMMA, the interactions thus acting as control parameters in dictating the time structure of the spatio-temporal patterns.

A two-dimensional network of Au nanoclusters on water surface: example of facile control of nanospace.

We report isotherm and Brewster Angle Microscopy (BAM) studies on mixed monolayers of Stearic Acid (StA) and Dodecanethiol-capped gold nanoparticles (AuNPs) (10% by weight) at the air-water interface. The surface pressure-specific molecular area isotherm for the mixed monolayer shows small hysteresis as well as a lowering of the slope change in going from L2-L2' and L2'-S phases, indicating lesser number of monolayer phases for the mixed film relative to a pristine StA monolayer. The BAM study shows formation of a two-dimensional, stable, reproducible, and extensive network of nanoparticle clusters with near-circular pores, in the 5-20 mN/m surface pressure range. This assembly of nanospaces has a pore area peak at approximately 75 microm2, width approximately 120 microm2 and can be disassembled and reassembled simply by changing the surface pressure.

Chain-length dependence of lipophilic force: comparison with the two-body van der Waals' force.

Hydrophobic dodecanethiol capped Au nanoparticles (AuNPs) form two-dimensional patterns in monolayers of amphiphilic fatty acids ([Formula: see text]) at the air water interface. An interplay between the various lipophilic interactions, in turn, decides the NP cluster size, where stronger NP-monolayer and monolayer-monolayer attraction in fatty acid monolayers with longer tail length oppose nanoparticle aggregation resulting in a decrease in cluster size in both in-plane and out-of-plane directions. The decrease in the in-plane cluster size is steepest for 14 < n < 18, n being the total number of C atoms in the fatty acid, and levels off for higher fatty acids and cannot be explained on the basis of the two body van der Waals pair potential atleast at initial phases of pattern formation. The potential can be used only at later times, closer to stability.

Critical behavior of a two-dimensional complex fluid: Macroscopic and mesoscopic views.

Liquid disordered (L_{d}) to liquid ordered (L_{o}) phase transition in myristic acid [MyA, CH_{3}(CH_{2})_{12}COOH] Langmuir monolayers was studied macroscopically as well as mesoscopically to locate the critical point. Macroscopically, isotherms of the monolayer were obtained across the 20^{∘}C-38^{∘}C temperature (T) range and the critical point was estimated, primarily from the vanishing of the order parameter, at ≈38^{∘}C. Mesoscopically, domain morphology in the L_{d}-L_{o} coexistence regime was imaged using the technique of Brewster angle microscopy (BAM) as a function of T and the corresponding power spectral density function (PSDF) obtained. Monolayer morphology passed from stable circular domains and a sharp peak in PSDF to stable dendritic domains and a divergence of the correlation length as the critical point was approached from below. The critical point was found to be consistent at ≈38^{∘}C from both isotherm and BAM results. In the critical regime the scaling behavior of the transition followed the two-dimensional Ising model. Additionally, we obtained a precritical regime, over a temperature range of ≈8^{∘}C below T_{c}, characterized by fluctuations in the order parameter at the macroscopic scale and at the mesoscopic scale characterized by unstable domains of fingering or dendritic morphology as well as proliferation of a large number of small sized domains, multiple peaks in the power spectra, and a corresponding fluctuation in the peak q values with T. Further, while comparing temperature studies on an ensemble of MyA monolayers with those on a single monolayer, the system was found to be not strictly ergodic in that the ensemble development did not strictly match with the time development in the system. In particular, the critical temperature was found to be lowered in the latter. These results clearly show that the critical behavior in fatty acid monolayer phase transitions have features of both complex and nonequilibrium systems.

Evolution of self-organized two-dimensional patterns of nanoclusters through demixing.

A mixture of dodecanethiol-capped Au nanoparticles (AuNPs) and the amphiphilic fatty acid, stearic acid, spread as a monomolecular layer on water surface, is observed with Brewster angle microscopy (BAM) to form a two-dimensional network of AuNP clusters through demixing, at concentration of AuNPs by weight (ρ[over ¯])>10% and the surface pressure (π)≥10mNm^{-1}. For π=15mNm^{-1}, the number of nodes (n) remains unchanged till ∼2 hours and then changes over to a lower n state, where the pattern consists of almost perfect circles with greater in-plane thickness of the AuNP lamellae. For the higher n state the mean-square fluctuation of BAM intensity remains flat and then decays as f(ξ)=ξ^{2α} with α∼0.6 (correlated fluctuations) over the length scales of 400µm-6µm and below 6µm, respectively. For the lower n state the fluctuation decays almost over the entire length scale with α=0.3, indicating emergence of aperiodicity from quasiperiodicity and a changeover to anticorrelated fluctuations. These patterns can be looked at as two distinct chaotic trajectories in the I-I^{'} phase space of the system (I being the scattered light intensity at any position of the pattern and I^{'} its gradient) with characteristic Lyapunov exponents.