Changyong Cao

Harnessing Localized Ridges for High-Aspect-Ratio Hierarchical Patterns with Dynamic Tunability and Multifunctionality

A simple method for fabricating high-aspect-ratio, hierarchical, and dynamically tunable surface patterns is invented by harnessing localized-ridge instabilities in gold nanofilms coated on elastomer substrates (a); a theoretical model to calculate the critical parameters (e.g., wavelength and amplitude) for designing the new patterns is developed (b); and novel applications of the patterns as super-hydrophobic coatings (c) and biomimetic cell-culture substrates (d) capable of on-demand tunability are demonstrated.

Improving Contact Interfaces in Fully Printed Carbon Nanotube Thin-Film Transistors

Single-walled carbon nanotubes (CNTs) printed into thin films have been shown to yield high mobility, thermal conductivity, mechanical flexibility, and chemical stability as semiconducting channels in field-effect, thin-film transistors (TFTs). Printed CNT-TFTs of many varieties have been studied; however, there has been limited effort toward improving overall CNT-TFT performance. In particular, contact resistance plays a dominant role in determining the performance and degree of variability in the TFTs, especially in fully printed devices where the contacts and channel are both printed. In this work, we have systematically investigated the contact resistance and overall performance of fully printed CNT-TFTs employing three different printed contact materials-Ag nanoparticles, Au nanoparticles, and metallic CNTs-each in the following distinct contact geometries: top, bottom, and double. The active channel for each device was printed from the dispersion of high-purity (>99%) semiconducting CNTs, and all printing was carried out using an aerosol jet printer. Hundreds of devices with different channel lengths (from 20 to 500 μm) were fabricated for extracting contact resistance and determining related contact effects. Printed bottom contacts are shown to be advantageous compared to the more common top contacts, regardless of contact material. Further, compared to single (top or bottom) contacts, double contacts offer a significant decrease (>35%) in contact resistance for all types of contact materials, with the metallic CNTs yielding the best overall performance. These findings underscore the impact of printed contact materials and structures when interfacing with CNT thin films, providing key guidance for the further development of printed nanomaterial electronics.

Additive engineering for high-performance room-temperature-processed perovskite absorbers with micron-size grains and microsecond-range carrier lifetimes

Perovskite photovoltaics have attracted remarkable attention recently due to their exceptional power conversion efficiencies (PCE). State-of-the-art perovskite absorbers typically require thermal annealing steps for high film quality. However, the annealing process adds cost and reduces yield for device fabrication and may also hinder application in tandem photovoltaics and flexible/ultra-low-cost optoelectronics. Herein, we report an additive-based room-temperature process for realizing high-quality methylammonium lead iodide films with micron-sized grains (>2 μm) and microsecond-range carrier lifetimes (τ1 = 931.94 ± 89.43 ns; τ2 = 320.41 ± 43.69 ns). Solar cells employing such films demonstrate 18.22% PCE with improved current–voltage hysteresis and stability without encapsulation. Further, we reveal that room-temperature-processed perovskite film grain size strongly depends on the precursor aggregate size in the film-deposition solution and that additive-based tuning of aggregate properties enables enlarging grains to the micron scale. These results offer a new pathway for more versatile, cost-effective perovskite processing.

Urinary catheter capable of repeated on-demand removal of infectious biofilms via active deformation.

Biofilm removal from biomaterials is of fundamental importance, and is especially relevant when considering the problematic and deleterious impact of biofilm infections on the inner surfaces of urinary catheters. Catheter-associated urinary tract infections are the most common cause of hospital-acquired infections and there are over 30 million Foley urinary catheters used annually in the USA. In this paper, we present the design and optimization of urinary catheter prototypes capable of on-demand removal of biofilms from the inner luminal surface of catheters. The urinary catheters utilize 4 intra-wall inflation lumens that are pressure-actuated to generate region-selective strains in the elastomeric urine lumen, and thereby remove overlying biofilms. A combination of finite-element modeling and prototype fabrication was used to optimize the catheter design to generate greater than 30% strain in the majority of the luminal surface when subjected to pressure. The catheter prototypes are able to remove greater than 80% of a mixed community biofilm of Proteus mirabilis and Escherichia coli on-demand, and furthermore are able to remove the biofilm repeatedly. Additionally, experiments with the prototypes demonstrate that biofilm debonding can be achieved upon application of both tensile and compressive strains in the inner surface of the catheter. The fouling-release catheter offers the potential for a non-biologic, non-antibiotic method to remove biofilms and thereby for impacting the thus far intractable problem of catheter-associated infections.

Tunable stiffness of electrorheological elastomers by designing mesostructures

Electrorheological elastomers have broad and important applications. While existing studies mostly focus on microstructures of electrorheological elastomers, their mesoscale structures have been rarely investigated. We present a theory on the design of mesostructures of electrorheological elastomers that consist of two phases with different permittivity. We show that the deformation of elastomers can reorient their mesostructures, which consequently results in variations of their effective permittivity, leading to stiffening, softening, or instability of the elastomer. Optimal design of the mesostructures can give giant tunable stiffness. Our theoretical model is further validated by results from numerical simulations.

A Novel Boundary-Integral Based Finite Element Method for 2D and 3D Thermo-Elasticity Problems

In this article a new hybrid boundary integral-based (HBI) finite element method (FEM) is presented for analyzing two-dimensional (2D) and three-dimensional (3D) thermoelastic problems with arbitrary distribution of body force and temperature changes. The method of particular solution is used to decompose the displacement field into homogeneous part and particular part. The homogeneous solution is obtained by using the HBI-FEM with fundamental solutions, yet the particular solution related to the body force and temperature change is approximated by radial basis function (RBF). The detailed formulation for both 2D and 3D HBI-FEM for thermoelastic problems are given, and two different approaches for treating the inhomogenous terms are presented and compared. Five numerical examples are presented to demonstrate the accuracy and performance of the proposed method. When compared with the existing analytical solutions or ABAQUS results, it is found that the proposed method works well for thermoelastic problems and also when using a very coarse mesh, results with satisfactory accuracy can be obtained.

Evaluation of Effective Thermal Conductivity of Fiber-Reinforced Composites

In this paper, effective thermal conductivity of fiber-reinforced composites are estimated by the newly developed hybrid finite element method (FEM). In the hybrid FEM, foundational solutions are employed to approximate the intra-element displacement field in any given element, while the polynomial shape functions used in traditional FEM are utilized to interpolate the frame field. The homogenization procedures using the representative volume element are integrated with the hybrid fundamental solution based finite element method (HFS-FEM) to estimate the effective thermal conductivity of the composites and to investigate the effect of fiber volume fraction and fiber arrangement pattern on the effective thermal conductivity. A special element with an inclusion is constructed by means of related special fundamental solutions. Due to the fact that the proposed special element exactly satisfy its boundary conditions along the fibre-matrix interface, only element boundary integrals are involved and significant mesh reduction can be achieved. Mesh regeneration may be avoided as well when the fiber volume fraction is slightly changed. The accuracy of the numerical results obtained by the proposed method is verified against with that obtained from commercial software package ABAQUS. The results indicate that the proposed method is efficient and accurate in analyzing the micromechanical thermal behavior of fiber-composites and has the potential to be scaled up to macro-scale modeling of practical problems of interest.

Hybrid Fundamental Solution Based Finite Element Method: Theory and Applications

An overview on the development of hybrid fundamental solution based finite element method (HFS-FEM) and its application in engineering problems is presented in this paper. The framework and formulations of HFS-FEM for potential problem, plane elasticity, three-dimensional elasticity, thermoelasticity, anisotropic elasticity, and plane piezoelectricity are presented. In this method, two independent assumed fields (intraelement filed and auxiliary frame field) are employed. The formulations for all cases are derived from the modified variational functionals and the fundamental solutions to a given problem. Generation of elemental stiffness equations from the modified variational principle is also described. Typical numerical examples are given to demonstrate the validity and performance of the HFS-FEM. Finally, a brief summary of the approach is provided and future trends in this field are identified.

Mesh reduction strategy: special element for modelling anisotropic materials with defects

In this paper we present a hybrid finite element method (HFS-FEM) to model efficiently and accurately anisotropic materials with defects by developing special elements for elliptic hole/crack based on their associated Green’s functions. The hybrid method is formulated based on two independent assumptions: intra-element field in terms of the combination of fundamental solutions and inter-element frame fields along the element boundary. A modified functional, which is satisfying the governing equation, boundary and continuity conditions between elements, is proposed to derive the element stiffness. In this work, the foundational solutions of the anisotropic materials following Stroh formalism are employed to approximate the intra-element displacement field of general elements, while the special fundamental solutions satisfying the required boundary conditions for hole or crack are used for the special elements containing defects. Two examples are presented to assess the performance of the proposed method. Numerical results obtained for the stress concentration factor (SCF) and stress intensity factor (SIF) are extremely accurate for the investigated cases.

Incorporation of silicone oil into elastomers enhances barnacle detachment by active surface strain

Abstract Silicone-oil additives are often used in fouling-release silicone coatings to reduce the adhesion strength of barnacles and other biofouling organisms. This study follows on from a recently reported active approach to detach barnacles, which was based on the surface strain of elastomeric materials, by investigating a new, dual-action approach to barnacle detachment using Ecoflex®-based elastomers incorporated with poly(dimethylsiloxane)-based oil additives. The experimental results support the hypothesis that silicone-oil additives reduce the amount of substratum strain required to detach barnacles. The study also de-coupled the two effects of silicone oils (ie surface-activity and alteration of the bulk modulus) and examined their contributions in reducing barnacle adhesion strength. Further, a finite element model based on fracture mechanics was employed to qualitatively understand the effects of surface strain and substratum modulus on barnacle adhesion strength. The study demonstrates that dynamic substratum deformation of elastomers with silicone-oil additives provides a bifunctional approach towards management of biofouling by barnacles.