Soft lithography meets self-organization: some new developments in meso-patterning

This is a brief review of our recent and ongoing work on simple, rapid, room temperature, pressure-less and large area (˜ cm2) 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

Real-time inferencing of solid-liquid phase equilibria in solution polymerization of polyethylene

An algorithm for the real-time prediction of multi-component solid-liquid equilibrium (SLE) in a polyethylene (PE) process flowsheet has been attempted. While most of the available literature assumes the polymer to possess a single average molecular weight and lump the entire polymer as a single component, the present work proposes to consider a polydispersed multi-component fraction, characterized by the pseudo-component approach. The SLE model has been used to study the effects of monomer and polymer polydispersity in solution polymerization process. At first, it has been validated on the solubility data of n-alkanes, in n-hexane, polyethylene in M-xylene and on wax precipitation from a mixture of n-paraffins. The SLE model is based on perturbed chain SAFT (PC-SAFT) equation of state (EOS). An algorithm for the real-time prediction of SLE phase boundaries in solution polymerization has been subsequently presented. A simulation experiment on the polyethylene flowsheet highlighted the potential of real-time inferencing for industrial applications. (C) 2004 Elsevier B.V All rights reserved

Creating self-organized submicrometer contact instability patterns in soft elastic bilayers with a topographically patterned stamp

The surface of a thin elastic bilayer becomes spontaneously unstable when it is brought in proximity to another rigid contactor. The instability patterns, which are random and isotropic, exhibit a dominant lateral length scale of instability λ, which linearly scales with the bilayer thickness (h) as: λ = RFh. It is known that for an elastic bilayer, RF exhibits a nonlinear dependence on the ratios of individual film thicknesses (H) and shear moduli (M) of the two constituent layers, and can have values as low as 0.5 under specific conditions. This is in contrast to a near constant value of RF ≈ 3 for a single layer elastic film.(1) These isotropic contact instability patterns in a bilayer can be ordered, aligned and modulated using a topographically patterned stamp. The precise morphology of the aligned structures depends on commensuration between λ and the stamp periodicity (λP), and on the intersurface separation distance. A variety of patterns, like an array of circular holes, double periodic channels, etc., in addition to a positive and a negative replica of the stamp pattern, can be engineered with a simple stamp having 1D grating structure. A lower value of RF in a bilayer allows generating patterns with sub 500 nm lateral resolution, which is impossible to create by elastic contact lithography (ECL) of a single layer film due to strong surface tension effects in ultrathin films. Thus, control of elastic instability in a bilayer with a patterned stamp represents a flexible soft lithography tool allowing modulation of length scales, morphology, and order

Fabrication of Ordered 2D Colloidal Crystals on Flat and Patterned Substrates by Spin Coating

Spin coating is a simple and rapid method for fabricating ordered monolayer colloidal crystals on flat as well as patterned substrates. In this article, we show how a combination of factors, particularly concentration of the dispensed colloidal solution (Cn) and spin-coating speed, influences the ordering process. We have performed systematic experiments on different types of substrates with two types of colloidal particles (polystyrene and silica). We also show that even when perfect ordering is achieved at some locations, there might be a significant spatial variation in the deposit morphology over different areas of the sample. Our experiments reveal that higher Cn is required for obtaining perfect arrays, as the diameter of the colloids (dD) increases. Interestingly, a combination of higher Cn and rotational speed (expressed as revolutions per minute) is required to achieve perfect ordering on a topographically patterned substrate, as compared to that on a flat surface, because of loss of inertia of the particles during outward flow because of impact on the substrate features. Finally, we also identify the relation between the particle diameter and the height of the pattern features to achieve topography-mediated particle ordering

Ordered to Isotropic Morphology Transition in Pattern-Directed Dewetting of Polymer Thin Films on Substrates with Different Feature Heights

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 (<b>H</b>_<b>S</b>) on the dewetting pathway and final pattern morphology by studying the dewetting of polystyrene (PS) thin films on grating substrates with identical periodicity (<b>λ</b>_<b>P</b> = 1.5 μm), but <b>H</b>_<b>S</b> 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 (<b>H</b>_<b>S</b> ≈ 120 nm); (2) aligned undulating ribbons (<b>H</b>_<b>S</b> ≈ 70–100 nm); (3) multilength scale structures with coexisting large droplets uncorrelated to the substrate and smaller droplets/ribbons aligned along the stripes (<b>H</b>_<b>S</b> ≈ 40–60 nm); and (4) large droplets completely uncorrelated to the substrate (<b>H</b>_<b>S</b> < 25 nm). The distinct morphologies across the categories are attributed to two major factors: (a) whether the as-cast film is continuous (<b>H</b>_<b>S</b> <b>≤</b> 80 nm) or discontinuous (<b>H</b>_<b>S</b> <b>≥</b> 100 nm) and (b) in case of a continuous film, whether the film ruptures along each substrate stripe (<b>H</b>_<b>S</b> <b>≥</b> 70 nm) or with nucleation of random holes that are not correlated to the substrate features (<b>H</b>_<b>S</b> <b>≤</b> 60 nm). While the ranges of <b>H</b>_<b>S</b> values indicated in the parentheses are valid for PS films with an equivalent thickness (<b>h</b>_<b>E</b>) ≈ 50.3 nm on a flat substrate, a change in <b>h</b>_<b>E</b> merely alters the cut-off values of <b>H</b>_<b>S</b>, as the final dewetted morphologies and transition across categories remain generically unaltered. We finally show that the structures obtained by dewetting on different <b>H</b>_<b>S</b> 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

Creating Self-Organized Submicrometer Contact Instability Patterns in Soft Elastic Bilayers with a Topographically Patterned Stamp

The surface of a thin elastic bilayer becomes spontaneously unstable when it is brought in proximity to another rigid contactor. The instability patterns, which are random and isotropic, exhibit a dominant lateral length scale of instability λ, which linearly scales with the bilayer thickness (h) as: λ = <i>R</i>_F<i>h</i>. It is known that for an elastic bilayer, <i>R</i>_F exhibits a nonlinear dependence on the ratios of individual film thicknesses (<i>H</i>) and shear moduli (<i>M</i>) of the two constituent layers, and can have values as low as 0.5 under specific conditions. This is in contrast to a near constant value of <i>R</i>_F ≈ 3 for a single layer elastic film. These isotropic contact instability patterns in a bilayer can be ordered, aligned and modulated using a topographically patterned stamp. The precise morphology of the aligned structures depends on commensuration between λ and the stamp periodicity (λ_P), and on the intersurface separation distance. A variety of patterns, like an array of circular holes, double periodic channels, etc., in addition to a positive and a negative replica of the stamp pattern, can be engineered with a simple stamp having 1D grating structure. A lower value of <i>R</i>_F in a bilayer allows generating patterns with sub 500 nm lateral resolution, which is impossible to create by elastic contact lithography (ECL) of a single layer film due to strong surface tension effects in ultrathin films. Thus, control of elastic instability in a bilayer with a patterned stamp represents a flexible soft lithography tool allowing modulation of length scales, morphology, and order

Control of morphology in pattern directed dewetting of thin polymer films

We investigate the creation of large area mesoscale structures by controlling the dewetting pathways of thin polymer films on physically heterogeneous substrates comprising a two-dimensional (2-D) array of square pillars. Depending on the initial configuration and thickness of the film, dewetting produces a variety of both ordered and disordered structures. The substrate pattern strongly influences the dewetting pathways as well as the organization and size of the polymer structures. The key findings are: (i) the lateral confinement imposed by the substrate pattern can reduce the length-scale of the dewetted structure by one to two orders of magnitude as compared to dewetting on the same homogeneous substrate. (ii) When the polymer film is thin (40 nm), dewetting progresses by nucleation of large holes, uncorrelated to the substrate pattern. The dewetted pattern in this case forms on multiple length-scales consisting of similar10 small mu m droplets resulting from the coalescence of the holes and small (<1 small mu m) droplets at the interstitial locations due to stick-slip of the retracting contact line