1,721,020 research outputs found
Scaling theory of adsorption-induced stresses in polymer brushes grafted onto compliant structures
No full textThe lateral forces exerted on a substrate by a layer of end-grafted polymer molecules are calculated on the basis of simple scaling arguments. The results are cast in terms of an equilibrium surface stress and an elastic constant, which describes the rate of change of the surface stress upon deformation of the substrate. This allows for straightforward integration of the present results into a continuum framework describing the response of a compliant structure, which facilitates device design and analysis. The results are illustrated with calculations for end-grafted poly(styrene) and poly(ethylene oxide), and the implications for building micromechanical devices based on adsorption-induced deformation are discussed
Multiscale modeling of adsorbed molecules on freestanding microfabricated structures
No full textThis paper outlines a multiscale model to quantitatively describe the chemomechanical coupling between adsorbed molecules and thin elastic films. The goal is to provide clear, quantitative connections between molecular interactions, adsorption distribution, and surface stress, which can be integrated with conventional thin film mechanics to quantify device performance in terms of molecular inputs. The decoupling of molecular and continuum frameworks enables a straightforward analysis of arbitrary structures and deformation modes, e.g., buckling and plate/membrane behavior. Moreover, it enables one to simultaneously identify both chemical properties (e.g., binding energy and grafting density) and mechanical properties (e.g., modulus and film geometry) that result in chemically responsive devices. We present the governing equations for scenarios where interactions between adsorbed molecules can be described in terms of pair interactions. These are used to quantify the mechanical driving forces that can be generated from adsorption of double-stranded DNA and C60 (fullerenes). The utility of the framework is illustrated by quantifying the performance of adsorption-driven cantilevers and clamped structures that experience buckling. We demonstrate that the use of surface-grafted polyelectrolytes (such as DNA) and ultracompliant elastomer structures is particularly attractive since deformation can be tuned over a very wide range by varying grafting density and chemical environment. The predictions illustrate that it is possible to construct (1) adsorption-based tools to quantify molecular properties such as polymer chain flexibility and (2) chemically activated structures to control flow in microfluidic devices
Chemo-mechanical interactions between adsorbed molecules and thin elastic films
No full textA general mechanics framework is presented to describe the interaction of molecular groups adsorbed on deformable thin films. Equations describing film deformation are expressed in terms of the pair potential that governs the interaction between adsorbed groups and large-deflection deformation descriptions. We illustrate that the pair potential can be used to define two-dimensional constitutive parameters for the adsorbed groups that describe their energetic contribution during arbitrary deformations. The framework is applicable to a wide range of interactions, including electrochemical interactions between a solution and film surface, interactions between neighboring adsorbed biopolymers and interactions between living cells. Key dimensionless parameters involving molecular interaction, surface coverage, film dimensions, and elastic properties are highlighted, and simplified governing equations for small deflections are identified. Solutions are presented that describe the influence of adsorbed groups on bending of cantilevers, pinned films and built-in (clamped) films. For clamped films, the critical molecular interaction parameters required to induce buckling are identified, and post-buckling behavior discussed in terms of the implications for making biological sensors
High-resolution NMR spectroscopy on a chip by structural compensation of magnetic susceptibility mismatch
No full textMicrofluidic compensation structures are introduced as a tool to homogenize magnetic field gradients present in microfluidic NMR spectroscopy. Magnetic field distortions are eliminated by compensating the magnetic susceptibility mismatch between the sample fluid and chip material through additional, lithographically defined compensation structures. This strategy will allow high resolution NMR spectroscopy on a chip, enabling a range of metabolomic applications
Microfluidic waves
No full textThe propagation of pressure waves in fluidic channels with elastic covers is discussed in view of applications to flow control in microfluidic devices. A theory is presented which describes pressure waves in the fluid that are coupled to bending waves in the elastic cover. At low frequencies, the lateral bending of the cover dominates over longitudinal bending, leading to propagating, non-dispersive longitudinal pressure waves in the channel. The theory addresses effects due to both the finite viscosity and compressibility of the fluid. The coupled waves propagate without dispersion, as long as the wave length is larger than the channel width. It is shown that in channels of typical microfluidic dimensions, wave velocities in the range of a few 10 m s?1 result if the channels are covered by films of a compliant material such as PDMS. The application of this principle to design microfluidic band pass filters based on standing waves is discussed. Characteristic frequencies in the range of a few kHz are readily achieved with quality factors above 30
GPU-based simulations of fracture in idealized brick and mortar composites
Full text linkStiff ceramic platelets (or bricks) that are aligned and bonded to a second ductile phase with low volume fraction (mortar) are a promising pathway to produce stiff, high-toughness composites. For certain ranges of constituent properties, including those of some synthetic analogs to nacre, one can demonstrate that the deformation is dominated by relative brick motions. This paper describes simulations of fracture that explicitly track the motions of individual rigid bricks in an idealized microstructure; cohesive tractions acting between the bricks introduce elastic, plastic and rupture behaviors. Results are presented for the stresses and damage near macroscopic cracks with different brick orientations relative to the loading orientation. The anisotropic macroscopic initiation toughness is computed for small-scale yielding conditions and is shown to be independent of specimen geometry and loading configuration. The results are shown to be in agreement with previously published experiments on synthetic nacre
Flexible and conductive bilayer membranes of nanoporous gold and silicone: synthesis and characterization
No full textThis work describes a simple fabrication process to produce a highly flexible bilayer membrane, consisting of a nanoporous gold layer embedded into the surface of a thin elastomer film. The nanoporous gold film shows excellent adhesion due to mechanical interlocking with the elastomer substrate, which penetrates its nanoscale pores. As the bilayer is stretched, the nanoporous gold layer cracks and the resulting bilayer has an effective elastic modulus that is only slightly higher than the elastomer (E?1.35 MPa). The film also exhibits low resistivity, which smoothly varies from ?1×10-6 to ?3×10-5 ? m as elongated to ?25% strain. The advantages and limitations of the bilayer with respect to sensing and actuation are briefly outlined
Kinetics of capillary wetting in nanoporous films in the presence of surface evaporation
No full textA liquid in contact with a nanoporous Au film forms a halo of constant width around the droplet. Due to the large surface area-to-volume ratio, one would expect either complete wetting or rejection of the liquid. Instead, a stable halo width is observed, depending on pore size, void fraction, film thickness, and the liquid. This is due to competition between capillary flow and evaporation through the surface of the nanoporous film. A theory is presented that predicts the halo width from the film geometry and liquid properties. Without adjustable parameters, the theory is in good agreement with experimental results
Quantitative end-grafting of DNA onto flat and nanoporous gold surfaces
No full textThe adsorption of end-thiolated single- and double-stranded DNA on gold surfaces has been studied with particular attention to the dependence of the adsorption density on the DNA base pair length. Quantitative understanding of probe immobilization and target binding is crucial for improving the performance of hybridization-based sensors and microarray
Nonlinear pressure-flow relationships for passive microfluidic valves
No full textAn analytical solution is presented for the nonlinear pressure-flow relationship of deformable passive valves, which are formed by bonding a deformable film over etched channels separated by a weir. A fluidic pathway connecting the channels is opened when the upstream pressure creates a tunnel along a predefined narrow strip where the film is not bonded to the weir. When the width of the strip is comparable to the inlet channel width, the predicted closed-form pressure-flow rate relationship is in excellent agreement with experiments, which determine pressures by measuring film deflections for prescribed flow rates. The validated closed-form models involve no fitting parameters, and provide the foundation to design passive diodes with specific nonlinear pressure-flow characteristics
- …
