Controlling Marangoni induced instabilities in spin-cast polymer films: how to prepare uniform films

In both research and industrial settings spin coating is extensively used to prepare highly uniform thin polymer films. However, under certain conditions, spin coating results in films with non-uniform surface morphologies. Although the spin coating process has been extensively studied, the origin of these morphologies is not fully understood and the formation of non-uniform spincast films remains a practical problem. Here we report on experiments demonstrating that the formation of surface instabilities during spin coating is dependent on temperature. Our results suggest that non-uniform spincast films form as a result of the Marangoni effect, which describes flow due to surface tension gradients. We find that both the wavelength and amplitude of the pattern increase with temperature. Finally, and most important from a practical viewpoint, the non-uniformities in the film thickness can be entirely avoided simply by lowering the spin coating temperature.Comment: 8 pages, 6 figures. electronic supplementary material: 3 pages, 4 figure

Development of Improved Rhenium Coatings for Fluorine Engine Thrust Chambers

Coating trials were undertaken to evaluate the application of rhenium to carbon-carbon composite sheet by plasma spraying. Optimum spray parameters and coating thickness were identified for production of coatings free from continuous defects and with adequate adherence to the substrate. A tungsten underlayer was not beneficial and possibly detracted from coating integrity. Stress calculations indicated that the proposed operating cycle of the rocket engine would not cause spalling of the rhenium coating. Calculations indicated that permeation of gases through the coating would not be significant during the expected life of the thrust chamber. The feasibility of applying rhenium coatings by laser melting was also studied. Poor wetting of the composite surface by the liquid rhenium precluded production of uniform coatings. Borate/carborate fluxes did not improve wetting characteristics

Strongly nonlinear wave dynamics in a chain of polymer coated beads

Strongly nonlinear phononic crystals were assembled from a chain of Parylene-C coated steel spheres in a polytetrafluoroethylene holder. This system exhibits strongly nonlinear properties and extends the range of materials supporting sonic-vacuum-type behavior. The combination of a high density core and a soft (low elastic modulus) coating ensures a relatively low velocity of wave propagation. The bead contact interaction caused by the deformation of the Parylene coating can be described by classical nonlinear elastic Hertz theory with an effective value of the elastic modulus equal to 15 GPa for the contact interaction. Strongly nonlinear solitary waves excited by impacts were investigated experimentally and compared to chains composed of uniform steel beads. Fracture of the polymer coating was detected under relatively large pulse amplitude

Spray coating apparatus having a rotatable workpiece holder

A spray coating apparatus is provided for rotating a workpiece relative to a spray station to obtain a uniform coating of the workpiece. In a typical example, the workpiece comprises a ceramic tile which is to be coated with a ceramic coating and the tile is to be used as a reusable component of the thermal protection system for a space shuttle. The apparatus for rotating the workpiece includes a base support having a first rotatable stage for rotation in the horizontal plane and a second rotatable stage for rotation in a second plane inclined at an angle, such as 45 degrees, to the horizontal plane and the workpiece is supported on this second stage. Thus the workpiece is rotatable in both of two planes of rotation

Effects of activated ceria and zirconia nanoparticles on the protective behaviour of silane coatings in chloride solutions

This work investigates the effect of CeO2 and ZrO2 nanoparticles on the corrosion protection performance of non-inhibited and cerium inhibited silane coatings in 3.5% and 5% NaCl solutions on electro-galvanized steel substrates. Atomic force microscopy (AFM) results show relatively uniform coating thickness and varying nanoparticle distribution depending on coating composition. The corrosion behaviour of the sol–gel coatings revealed that CeO2-ZrO2 nanoparticles reinforce the barrier properties of the silane films and seem to act as nano-reservoirs providing a prolonged release of cerium ions. This prolonged release of inhibitor from oxide nanoreservoirs confers longer protection to the metallic substrate

Spectral theory of electromagnetic scattering by a coated sphere

In this paper, we introduce an alternative representation of the electromagnetic field scattered from a homogeneous sphere coated with a homogeneous layer of uniform thickness. Specifically, we expand the scattered field using a set of modes that are independent of the permittivity of the coating, while the expansion coefficients are simple rational functions of the permittivity. The theory we develop represents both a framework for the analysis of plasmonic and photonic modes and a straightforward methodology to design the permittivity of the coating to pursue a prescribed tailoring of the scattered field. To illustrate the practical implications of this method, we design the permittivity of the coating to zero either the backscattering or a prescribed multipolar order of the scattered field, and to maximize an electric field component in a given point of space

A starch edible surface coating delays banana fruit ripening

A rice starch edible coating blended with sucrose esters was developed for controlling the postharvest physiological activity of Cavendish banana to extend postharvest quality during ripening at 20 ± 2 °C. Coating effectiveness was assessed against changes in fruit physiochemical parameters such as weight loss, titratable acidity, total soluble solids, flesh fruit firmness, ion leakage, colour change, respiration, ethylene production, chlorophyll degradation and starch conversion were determined. The topography of coating material on the fruit surface was evaluated by scanning electron microscope (SEM). Surface morphology studies highlighted the binding compatibility of the coating matrix with the fruit peel character and formed a continuous uniform layer over the fruit surface. The results showed that the coating was effective in delaying ethylene biosynthesis and reducing respiration rate. Other factors impacting included delayed chlorophyll degradation, reduced weight loss and retention of fruit firmness for the first six days, all of which improved the commercial value of the fruit. The shelf life of coated fruit was prolonged for 12 days in comparison with the untreated control which ripened within seven days and lost marketability after Day 6. The pilot study demonstrates the effectiveness of a starch-based edible coating formulation for improving the ambient storage capacity of banana fruit

Relationship of optical coating on thermal radiation characteristics of nonisothermal cylindrical enclosures

A numerical ray tracing technique was applied to simulate radiation propagating from various non-isothermal cylindrical cavities to determine the effect of optical coating (surface emissivity). In general, the analysis showed that the optical coating and temperature within a cavity have a significant effect on emitted radiation based on cavity dimension. Temperature thresholds were found to exist where the same optical coating may either reduce or increase cavity performance (apparent emissivity). Parametric values of apparent emissivity results are presented over a wide range of variables to correlate cylindrical cavity radiation for non-uniform cavity emissivity values. A universal curve was developed to aid in selecting wall emissivity values for design considerations

Avery Final Report: Identification and Cross-Directional Control of Coating Processes

Coating refers to the covering of a solid with a uniform layer of liquid. Of special industrial interest is the cross-directional control of coating processes, where the cross-direction refers to the direction perpendicular to the substrate movement. The objective of the controller is to maintain a uniform coating under unmeasured process disturbances. Assumptions that are relevant to coating processes found in industry are used to develop a model for control design. We show how to identify the model from input-output data. This model is used to derive a model predictive controller to maintain flat profiles of coating across the substrate by varying the liquid flows along the cross direction. The model predictive controller computes the control action which minimizes the predicted deviation in cross-directional uniformity. The predictor combines the estimate obtained from the model with the measurement of the cross-directional uniformity to obtain a prediction for the next time step. A filter is used to obtain robustness to model error and insensitivity to measurement noise. The tuning of the noise filter and different methods for handling actuator constraints are studied in detail. The three different constraint-handling methods studied are: the weighting of actuator movements in the objective function, explicitly adding constraints to the control algorithm, i.e. constrained model predictive control, and scaling infeasible control actions calculated from an unconstrained control law to be feasible. Actuator constraints, measurement noise, model uncertainty, and the plant condition number are investigated to determine which of these limit the achievable closed loop performance. From knowledge of how these limitations affect the performance we find how the plant could be modified to improve the process uniformity. Also, because identification of model parameters is time-consuming and costly, we study how accurate the identification must be to achieve a given level of performance. The theory developed throughout the paper is rigorously verified though simulations and experiments on a pilot plant. The effect of interactions on the closed loop performance is shown to be negligible for this pilot plant. The measurement noise and the actuator constraints are shown to have the largest effect on closed loop performance

Nanometer-Scale Precision Tuning of 3D Photonic Crystals Made Possible Using Polyelectrolytes with Controlled Short Chain Length and Narrow Polydispersity

Nanometer‐scale tuning of the optical properties of prefabricated photonic crystals is achieved via layer‐by‐layer assembly of polyelectrolytes in the interstitial spaces of the photonic lattice. The key to the approach is using polyelectrolytes with controlled short chain lengths. This ensures they do not block the air voids, thereby maintaining uniform coating and thus precise and reproducible optical tuning.Ministerio de Economía y Competitividad MAT2011-23593, CSD2007-00007Junta de Andalucía FQM3579, FQM5247European Union 30708