Studying Soft Interfaces with Shear Waves: Principles and Applications of the Quartz Crystal Microbalance (QCM)

The response of the quartz crystal microbalance (QCM, also: QCM-D for “QCM with Dissipation monitoring”) to loading with a diverse set of samples is reviewed in a consistent frame. After a brief introduction to the advanced QCMs, the governing equation (the small-load approximation) is derived. Planar films and adsorbates are modeled based on the acoustic multilayer formalism. In liquid environments, viscoelastic spectroscopy and high-frequency rheology are possible, even on layers with a thickness in the monolayer range. For particulate samples, the contact stiffness can be derived. Because the stress at the contact is large, the force is not always proportional to the displacement. Nonlinear effects are observed, leading to a dependence of the resonance frequency and the resonance bandwidth on the amplitude of oscillation. Partial slip, in particular, can be studied in detail. Advanced topics include structured samples and the extension of the small-load approximation to its tensorial version.

Characterization and Design of Programmable Self-Folding Polymer Films

This paper presents the characterization and design aspects of a novel fabrication method that integrates photolithography and self-folding to create polymer polyhedral structures. A two-step UV exposure process is used to produce patterned polymer films with flexible folds of low cross-linking density and stiff faces of high cross-linking density. Solvent is diffused into the folds during the development step of the photolithography process due to their low cross-linking density. The solvent concentration is non-uniform across the thickness of the folds and causes a strain gradient at these regions when the solvent is removed by heating the films, which enables self-folding. Experiments are performed to calibrate an equation that relates the dimensions of the folds and their achieved fold angle. An analytical model is introduced to elucidate the form of the equation and provide physical meaning to the calibration parameter. The formula is incorporated into a computational implementation of the unfolding polyhedra method that considers smoothly bent folds. This method, enhanced with the experimentally calibrated formula, enables the design of planar films programmed to self-fold into target three-dimensional shapes when heated. Polyhedral shapes are fabricated to demonstrate the developed method for origami-based fabrication. A parametric study quantifying the accuracy of the designed polyhedral forms with smooth folds as compared against those with idealized creased folds is performed.

Stimulus responsive 3D structures by self-morphing polymeric materials

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI-COLUMBIA AT REQUEST OF AUTHOR.] Inspired by ubiquitous phenomena of shape transformation in nature, extensive research effort has been devoted to self-morphing materials in recent decades. The self-morphing materials transfer their shapes from 2D planar films into 3D structures under environmental triggers such as humidity, pH, temperature, and light. Due to the extreme values of shape transformation, this kind of materials are invaluable for fabrication of various devices and systems, including flexible electronics, displays, artificial muscles, microfluidic valves and gates, actuation components in soft robotics and so on. The quintessence of fabricating such materials lies in programming structural anisotropies in them. Although various strategies and techniques have been developed to realize such a goal, this research area is still in its infant stages. New strategies and new techniques are still strongly desired. This dissertation focuses on exploring new possibilities of generating and programming anisotropies to develop novel self-morphing materials. New types of anisotropies are realized by controlling the distribution of polymeric crystal phase (Chapter 2), swellable guest medium (Chapter 3), laser induced graphene (Chapter 4), phase change microstructures (Chapter 5), and soft-stiff hybridized structures (Chapter 6) in self-morphing materials. Programmable shape changing behaviors, such as bending, folding, helical curling and buckling, were demonstrated on these materials by pattering the anisotropic structures. Moreover, for the first time, we demonstrate that the CO2 laser direct writing, which is normally used as a cutting tool in industry, has shown great potential in programming anisotropies in these newly developed self-morphing materials. These demonstrated strategies and techniques offer unique capabilities in fabricating functional self-morphing devices such as soft gripper, locomotive robot, rewritable paper, reconfigurable pneumatic actuator, and acoustic metamaterials.

Numerical Study of Multilayer Planar Film Structures for Ideal Absorption in the Entire Solar Spectrum

Here, we have theoretically proposed an ideal structure of selective solar absorber with multilayer planar films, which can absorb the incident light throughout the entire solar spectrum (300–2500 nm) and over a wide angular range, whatever the polarization angle of 0°~90°. The efficiency of the proposed absorber is proven by the Finite-Difference Time Domain (FDTD) simulation. The average absorption rate over the solar spectrum is up to 96.6%. The planar design is extremely easy to fabricate and modify, and this structure does not require lithographic processes to finish the absorbers. Improvements of the solar absorber on the basis of planar multilayer-film structures is attributed to multiple asymmetric highly lossy Fabry–Perot resonators. Because of having many virtues, such as using different refractory and non-noble metals, having angle and polarization independence, and having ideal absorption for entire solar spectrum, our proposed absorbers are promising candidates for practical industrial production of the solar-energy harvesting.

Reactive Inorganic Vapor Deposition of Perovskite Oxynitride Films for Solar Energy Conversion

The synthesis of perovskite oxynitrides, which are promising photoanode candidates for solar energy conversion, is normally accomplished by high-temperature ammonolysis of oxides and carbonate precursors, thus making the deposition of their planar films onto conductive substrates challenging. Here, we proposed a facile strategy to prepare a series of perovskite oxynitride films. Taking SrTaO2N as a prototype, we prepared SrTaO2N films on Ta foils under NH3 flow by utilizing the vaporized SrCl2/SrCO3 eutectic salt. The SrTaO2N films exhibit solar water-splitting photocurrents of 3.0 mA cm-2 at 1.23 V vs. RHE (reversible hydrogen electrode), which increases by 270% compared to the highest photocurrent (1.1 mA cm-2 at 1.23 V vs. RHE) of SrTaO2N reported in the literature. This strategy may also be applied to directly prepare a series of perovskite oxynitride films on conductive substrates such as ATaO2N (A=Ca,Ba) and ANbO2N (A=Sr,Ba).

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.