Non-planar Films Research Articles

Conformal 3D printing of non-planar antennas on wrinkled and folded kapton films using point cloud data

We report a reverse engineering-driven method for conformal microextrusion three-dimensional (3D) printing of functional materials on complex 3D structures and thin films of near-arbitrary topography. A non-planar tool path programming algorithm for conformal microextrusion 3D printing based on point cloud data representations of object geometry is presented. We show that the optimal nozzle-substrate standoff distance for quality 3D printing depends on the substrate’s local geometric features (i.e. slope and curvature) and the tool trajectory. The impact and utility of the novel conformal microextrusion 3D printing process were demonstrated by fabrication of 3D spiral and Hilbert-curve loop antennas on various non-planar substrates, including wrinkled and folded Kapton films and origami. 3D-printed conformal antennas exhibited resonant frequencies ranging from 1.5 to 2.7 GHz with S 11 less than 10 db. This work provides a new method for conformal 3D printing on one-of-a-kind objects and non-planar films.

(Invited) The Morphological Stability of Passive Oxide Films

Understanding the kinetics of passive oxide formation and breakdown has been an ongoing problem for corrosion scientists for several decades. A model for the formation of a passive oxide film on a metal that is an extension of the Point Defect Model (PDM) will be presented. The potential description of the PDM is replaced with a boundary value problem that ensures Gauss' law is satisfied at the oxide-solution interface by considering the presence of the Helmholtz layer in solution. Our model predicts the observed linear variation of the steady-state film thickness with anode potential, and an increasing steady-state film thickness with increasing pH. A linear stability analysis of the moving metal-oxide and oxide-solution interfaces shows that, depending on the parameters, the passive film may be unstable to morphological perturbations of the film interfaces, leading to non-planar films and potentially the formation of a pit in the oxide. This also implies that one-dimensional models of oxide growth, that assume planar interfaces, can be inapplicable in broad classes of corrosion processes. The analysis shows that a morphological instability exists if the oxide dissolution mechanism is such that an increasing Helmholtz layer potential drop leads to an increasing dissolution rate. The instability behavior is consistent with the literature on breakdown of passivity in the presence of chloride ions. The theory provides insights on the initiation of passivity breakdown leading to pitting corrosion and the role that interfacial energy plays in determining stability.

Thermal fluctuations in capillary thinning of thin liquid films

Thermal fluctuations have been shown to influence the thinning dynamics of planar thin liquid films, bringing predicted rupture times closer to experiments. Most liquid films in nature and industry are, however, non-planar. Thinning of such films not just results from the interplay between stabilizing surface tension forces and destabilizing van der Waals forces, but also from drainage due to curvature differences. This work explores the influence of thermal fluctuations on the dynamics of thin non-planar films subjected to drainage, with their dynamics governed by two parameters: the strength of thermal fluctuations, $\unicode[STIX]{x1D703}$, and the strength of drainage, $\unicode[STIX]{x1D705}$. For strong drainage ($\unicode[STIX]{x1D705}\gg \unicode[STIX]{x1D705}_{tr}$), we find that the film ruptures due to the formation of a local depression called a dimple that appears at the connection between the curved and flat parts of the film. For this dimple-dominated regime, the rupture time, $t_{r}$, solely depends on $\unicode[STIX]{x1D705}$, according to the earlier reported scaling, $t_{r}\sim \unicode[STIX]{x1D705}^{-10/7}$. By contrast, for weak drainage ($\unicode[STIX]{x1D705}\ll \unicode[STIX]{x1D705}_{tr}$), the film ruptures at a random location due to the spontaneous growth of fluctuations originating from thermal fluctuations. In this fluctuations-dominated regime, the rupture time solely depends on $\unicode[STIX]{x1D703}$ as $t_{r}\sim -(1/\unicode[STIX]{x1D714}_{max})\ln (\sqrt{2\unicode[STIX]{x1D703}})^{\unicode[STIX]{x1D6FC}}$, with $\unicode[STIX]{x1D6FC}=1.15$. This scaling is rationalized using linear stability theory, which yields $\unicode[STIX]{x1D714}_{max}$ as the growth rate of the fastest-growing wave and $\unicode[STIX]{x1D6FC}=1$. These insights on if, when and how thermal fluctuations play a role are instrumental in predicting the dynamics and rupture time of non-flat draining thin films.

Methodology of studying non-planar films and membranes of complex structure

Among the thin-walled structural elements combining lightness and high strength, film and membrane elements are used most widely. Smart coatings with a complex structure are being actively developed nowadays. Proceeding from their functional duty, it is advisable to produce non-planar forms — shell films, membranes and coatings — which can have a complex structure specified by the designer or acquired in the process of manufacture and operation. Study of the mechanical properties of the shell films and membranes with a complex structure using standard uniaxial method of testing appeared ineffective. Complex structures with macro-heterogeneity should not be studied by indentation methods, capable of determining the material properties in the vicinity of the point in question. We developed an experimental-theoretical method for determination of the mechanical characteristics of non-planar films or membrane compositions of complex structure. At the stage of experiment, the stiffness of a non-planar sample (e.g., spherical, cylindrical or toroidal shape) fixed along the contour and loaded with by a unilateral surface pressure is estimated. Then, using the ratios derived from the nonlinear theory of shells, the integral mechanical characteristics of the shell sample material are determined: the reduced modulus of elasticity (elastic strain) or reduced conditional modulus of elasticity (plastic strain), deformation curves, etc. The relations for thin spherical membranes for the case of large displacements and deformations, as well as relations for thin cylindrical membranes of variable radius are considered. Results of the case study of rubber spherical membrane with holes and defect-free catenoidal shell are presented to illustrate the developed methodology.

Morphological stability of steady-state passive oxide films

Understanding the kinetics of passive oxide formation and breakdown has been an ongoing problem for corrosion scientists for several decades. Here, we present a model for the formation of a passive oxide film on a metal that is an extension of the Point Defect Model (PDM) by Macdonald and coworkers. We replace the potential description of the PDM with a boundary value problem and ensure that Gauss's law is satisfied at the oxide-solution interface by considering the presence of the Helmholtz layer in solution. Our model predicts the observed linear variation of the steady-state film thickness with anode potential, and an increasing film thickness with increasing pH. We perform a linear stability analysis of the model and show that, depending on the parameters, the passive film may be unstable to morphological perturbations of the film interfaces, leading to nonplanar films and potentially the formation of a pit in the oxide. This also implies that one-dimensional models, which assume planar interfaces, can be inapplicable in broad classes of corrosion processes. The analysis shows that a morphological instability exists if the oxide dissolution mechanism is such that an increasing Helmholtz layer potential drop leads to an increasing dissolution rate. The instability behavior is consistent with the literature on breakdown of passivity in the presence of chloride ions. The theory provides insights on the initiation of passivity breakdown leading to pitting corrosion and the role that interfacial energy plays in determining stability.

13th International Conference on Films and Coatings

13th International Conference on Films and Coatings

Predicting and Understanding Order of Heteroepitaxial Quantum Dots

Heteroepitaxial self-assembled quantum dots (SAQDs) will allow breakthroughs in electronics and optoelectronics. SAQDs are a result of Stranski-Krastanow growth whereby a growing planar film becomes unstable after an initial wetting layer is formed. Common systems are Ge$_{x}$Si$_{1-x}$/Si and In$_{x}$Ga$_{1-x}$As/GaAs. For applications, SAQD arrays need to be ordered. The role of crystal anisotropy, random initial conditions and thermal fluctuations in influencing SAQD order during early stages of SAQD formation is studied through a simple stochastic model of surface diffusion. Surface diffusion is analyzed through a linear and perturbatively nonlinear analysis. The role of crystal anisotropy in enhancing SAQD order is elucidated. It is also found that SAQD order is enhanced when the deposited film is allowed to evolve at heights near the critical wetting surface height that marks the onset of non-planar film growth.

Microinterferometry: three-dimensional reconstruction of surface microtopography for thin-film and wetting studies by reflection interference contrast microscopy (RICM)

We present an improved theory of image formation by reflection interference contrast microscopy (RICM) for structural studies of stratified films on planar substrates and propose a new theoretical approach to analyzing the surface profile of nonplanar films. We demonstrate the validity of the new approach by analyzing the fringe patterns of RICM images from wedge-shaped liquid films and spherical probes. By simulation of various scenarios, we study the effect of finite-aperture illumination and the shape of the nonplanar interface on the interference fringe pattern of RICM images. We show how the reconstruction of the microscopic topography of the sample from the fringe spacing is corrected by angular and curvature correction terms. We discuss the variation of the mean intensity of the fringe patterns and the decay in the fringe amplitude with increasing fringe order that is caused by nonplanar interfaces of different slope.

Effect of the conditions during photostimulated epitaxy on the structure of the resulting films

A study has been made of how electromagnetic radiation influences the growth of epitaxial structures. Experiments on the growth of IV–VI films used a high-power xenon flashlamp and pulsed and cw UV lasers. The conditions which result in the growth of nonplanar film structures were determined. An experimental study was made of the properties of indium-doped films, including the transition layer between the film and the substrate and film peeling processes. A model for processes at the crystallization surface is examined. The kinetics of defects must be taken into consideration in an analysis of photostimulated epitaxy.