Browsing by Author "Holmes, Douglas P."

Now showing 1 - 12 of 12
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    Bending, Creasing, and Snapping of Soft, Slender Structures
    Pandey, Anupam (Virginia Tech, 2014-07-28)
    Crosslinked polymers or elastomers are examples of soft, synthetic material that can bend, crease, snap, wrinkle in response to external stimulus like pH, humidity, electric field or swelling. If a droplet of favorable solvent is placed on top of a thin, elastomer beam, it bends drastically to accommodate the excessive swelling stress. Keeping the solvent and its volume constant if we just increase the thickness of the beam, microscopic surface creases appear on the top surface. In this thesis, we experimentally characterize this transition between global bending to surface creasing. Closing of Venus flytrap leaves is a classic example of well known snap-through instability. A knowledge of the timescale of snapping is crucial in designing advanced functional materials. We perform the simplest experiment of poking an soft, elastomer arch at its apex till it snaps. Combining our experiments with analytical model we are able to predict the purely geometric nature of the snapping timescale. We also develop a simple scaling law that captures the dynamics of jumping toy poppers.
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    The Biomechanics of Tracheal Compression in the Darkling Beetle, Zophobas morio
    Adjerid, Khaled (Virginia Tech, 2019-11-05)
    In this dissertation, we examine mechanics of rhythmic tracheal compression (RTC) in the darkling beetle, Zophobas morio. In Chapter 2, we studied the relationship between hemolymph pressure and tracheal collapse to test the hypothesis that pressure is a driving mechanism for RTC. We found that tracheae collapse as pressure increases, but other physiological factors in the body may be affecting tracheal compression in live beetles. Additionally, as the tracheae compress, they do so in varying spatial patterns across the insect body. In chapter 3, we examined spatial variations in the taenidial spacing, stiffness, and tracheal thickness along the length of the tracheae. We related variations in Young's modulus and taenidial spacing with measurements of collapse dimples and found that spatial patterns of Young's modulus correlate with dimensions of collapse dimples. This correlation suggests an intuitive link between tracheal stiffness variations and the unique patterns observed in compressing tracheae. Lastly, in chapter 4, we studied the non-uniform collapse patterns in 3-D. By manually pressurizing the hemocoel and imaging using synchrotron microcomputed tomography (SR-µCT), we reconstructed the tracheal system in its compressed state. While previous studies used 2-D x-ray images to examine collapse morphology, ours is the first to quantify collapse patterns in 3-D and compare with previous 2-D quantification methods. Our method is also the first to make a direct measure of tracheal volume as the tracheal system compresses, similar to the phenomenon that occurs during rhythmic tracheal compression.
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    Buckling at the Fluid - Soft Solid Interface; A Means for Advanced Functionality within Soft Materials
    Tavakol, Behrouz (Virginia Tech, 2015-09-02)
    Soft materials and compliant structures often undergo significant deformation without failure, a unique feature making them distinct from classical rigid materials. These substantial deformations provide a means for faster or more energy efficient deformations, which can be achieved by taking advantage of elastic instabilities. We intend to utilize structural instabilities to generate advanced functionality within soft materials. In particular, we use the buckling of thin, flexible plates to control or enhance the flow of fluid in a micro channel. The buckling deformation is created or altered via two different stimuli, first a mechanical strain and then an electrical signal. We investigate the behavior of each system under different conditions experimentally, numerically, or theoretically. We also show that the coupled interaction between fluid and the soft film plays a critical role in the shape of deformation and consequently in the functionality of the mechanism. We first embed a buckled thin film in a fluid channel within a soft device. By applying a mechanical strain to the device, we show both experimentally and numerically that the height of the buckled film changes accordingly as does the flow rate. We then offer an analytical solution by extending the classical lubrication theory to higher-order terms as a means to more accurately describe the flow in a channel with a buckled thin film, and in general, the flow in channels with any constrictions provided the Reynolds number is low. Next, we use an electrical signal to make a confined dielectric film undergo out-of-plane buckling deformation. The thin film is sandwiched between two flexible electrodes and the mechanism is implemented in a microfluidic device to pump the fluid into a micro channel. We show that the critical buckling voltage at which the thin film buckles out of the plane is mainly a function of voltage while the shape of deformation and so the functionality of this mechanism depend considerably on the applied boundary conditions. Finally, we enhance the fluid-soft structure response of the actuating mechanism by substituting flexible electrodes with fluid electrodes, resulting in a significant increase in the actuation frequency as well as a reduction in the critical buckling voltage.
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    Buckling of Dielectric Elastomeric Plates for Soft, Electrically Active Microfluidic Pumps
    Tavakol, Behrouz; Bozlar, Michael; Punckt, Christian; Froehlicher, Guillaume; Stone, Howard A.; Aksay, Ilhan A.; Holmes, Douglas P. (The Royal Society of Chemistry, 2014-05-15)
    Elastic instabilities, when properly implemented within soft, mechanical structures, can generate advanced functionality. In this work, we use the voltage-induced buckling of thin, flexible plates to pump fluids within a microfluidic channel. The soft electrodes that enable electrical actuation are compatible with fluids, and undergo large, reversible deformations. We quantified the onset of voltage-induced buckling, and measured the flow rate within the microchannel. This embeddable, flexible microfluidic pump will aid in the generation of new stand-alone microfluidic devices that require a tunable flow rate.
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    Control and Manipulation of Microfluidic Flow via Elastic Deformations
    Holmes, Douglas P.; Tavakol, Behrouz; Froehlicher, Guillaume; Stone, Howard A. (The Royal Society of Chemistry, 2013-05-08)
    We utilize elastic deformations via mechanical actuation to control and direct fluid flow within a flexible microfluidic device. The device consists of a microchannel with a flexible arch prepared by the buckling of a thin elastic film. The deflection of the arch can be predicted and controlled using the classical theory of Euler buckling. The fluid flow rate is then controlled by coupling the elastic deformation of the arch to the gap within the microchannel, and the results compared well with analytical predictions from a perturbation calculation and numerical simulations. We demonstrate that placement of these flexible valves in series enables directed flow towards regions of externally applied mechanical stress. The simplicity of the experimental approach provides a general design for advanced functionality in portable microfluidics, self-healing devices, and in situ diagnostics.
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    Experimental and Theoretical Studies of Friction and Adhesion of Elastomeric Materials
    Rezaei Mojdehi, Ahmad (Virginia Tech, 2017-10-26)
    In this dissertation, four distinct but in some ways related topics, mostly related to experimental and theoretical investigations of friction and adhesion of elastomeric materials, are presented. First, an experimental and theoretical study of the interaction between elastic beams and granular media under compressive loading is performed. Buckling loads of beams with different dimensions and boundary conditions within granular media of different depths and grain sizes are measured, and theoretically approximated using the Ritz energy approach, based on the concept of beam on an elastic foundation. Several nondimensional parameters and a scaling law are derived to characterize different interaction regimes between the beams and granular support. The findings from this work is believed to be helpful for improved understanding of interactions between elastic beams and surrounding elastic foundation with applications to piles, oil pipelines, and robotic needle insertion into soft tissues. Second, the role of axial compliance on the friction of extensible strips is investigated. Significant changes were observed in the static and kinetic friction of strips, when the effective axial compliance was changed. The underlying causes of the changes in the frictional response are explained and quantitatively predicted using an extended shear lag model. We believe that this study provides insights into the effect of axial compliance on the frictional response of materials, paving the way for design and optimization of systems where the static and kinetic friction forces play an important role. Third, the effect of normal force and rate on the kinetic friction of two different elastomers, namely acrylic and silicone-based elastomers is evaluated. A custom-built pendulum test setup was used to perform the friction test in dynamic conditions. Two substantially different responses with respect to the change in normal force were observed and the role of different contributions to the frictional response of viscoelastic materials, i.e. bulk hysteresis friction, adhesion friction, and cohesion friction, are discussed. Different scenarios such as modifying the surface by using graphite powder, reducing test velocity, and also performing drop tests to characterize the surface hysteresis of the elastomers, were considered to further explore the origin of frictional responses of the elastomers. This study could improve insights gained from Dynamic Mechanical Analysis (DMA) data when obtaining and interpreting the effect of normal force on kinetic COF of elastomers with potential applications to tires, shoes, etc. where friction plays an important role. Last, a generalized scaling law, based on the classical fracture mechanics approach, is developed to predict the bond strength of adhesive systems. The proposed scaling law, which depends on the rate of change of bond area with compliance, is in apparent discrepancy with the previously reported scaling relationship that depends on the ratio of area to compliance. This distinction can have a profound impact on the expected bond strength of systems, particularly when failure mechanism changes or the compliance of the load train is increased. Furthermore, the shear lag model is implemented to derive a closed-form relation for the system compliance and the conditions where the two models deviate from each other are discussed and demonstrated. The results obtained from this approach could lead to a better understanding of the relationship between the bond strength and the geometry and mechanical properties of adhesive systems, with applications to different types of adhesive joints such as bio-inspired adhesive, biomedical adhesive tapes, and structural adhesive joints.
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    Friction of Extensible Strips: an Extended Shear Lag Model with Experimental Evaluation
    Mojdehi, Ahmad R.; Holmes, Douglas P.; Williams, Christopher B.; Long, Timothy E.; Dillard, David A. (2016-02-22)
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    Interfacial debonding from a sandwiched elastomer layer
    Mukherjee, Bikramjit (Virginia Tech, 2016-06-25)
    The problem of a thin elastomeric layer confined between two stiff adherends arises in numerous applications such as microelectronics, bio-inspired adhesion and the manufacture of soft biomedical products. A common requirement is that the debonding of the elastomeric layer from the adherends be controlled to avoid undesirable failure modes. This level of control may necessitate understanding the collective role of the interfacial adhesion, material properties, part geometries, and loading conditions on the debonding. Analytical and numerical approaches using the finite element method and a cohesive zone model (CZM) for the interfacial debonding are used in this dissertation to delineate the role of the afore-mentioned parameters on the initiation and propagation of debonding for both rigid and non-rigid adherends. Extensively studied in the dissertation is the debonding of a semi-infinite relatively stiffer adherend from an elastomer layer with its other surface firmly bonded to a rigid base. The adherend is pulled upwards by applying normal displacements either on its entire unbonded surface or on the edge of its part overhanging from the elastomer layer. The adherend and the elastomeric layer materials are assumed to be linear elastic, homogeneous and isotropic and the elastomer is assumed to be incompressible. Viscoelasticity of the elastomer is considered in the first part of the work. Plane strain deformations of the system with a bilinear traction-separation (TS) relation in the CZM are analyzed. Two non-dimensional numbers, one related to the layer confinement and the other to the interfacial TS parameters, are found to determine if debonding initiates at interior points in addition to at corner points on the adherend/elastomer interface, and if adhesion-induced instability is exhibited. This work is extended to axisymmetric problems in which debonding can take place at both interfaces. Motivated by an industrial demolding problem, numerical experiments are conducted to derive insights into preferential debonding at one of the two interfaces, including for curved adherends. Results reported herein should help engineers design an elastomer layer sandwiched between two adherends for achieving desired failure characteristics.
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    Non-equilibrium Dynamics of Nanoscale Soft Matter Deformation
    Fergusson, Austin D. (Virginia Tech, 2014-09-12)
    Life is soft. From the fluid-like structure of lipid bilayers to the flexible folding of proteins, the realm of nanoscale soft matter is a complex and vibrant area of research. The lure of personalized medicine, advanced sensing technology, and understanding life at a fundamental level pushes research forward. This work considers to areas: (1) lipid bilayer dynamics in the presence of substrate defects and (2) the inverse temperature transition of elastic proteins. Molecular dynamics simulations as well as umbrella sampling were employed. The behavior of the bilayers discussed in the work provides evidence that small defects on confining surfaces can promote nucleation of lipid tethers. Results the second part of this work indicate elastin-like peptides experiencing inverse temperature transitions may be capable of performing amounts of work similar to RNA polymerase; additionally, resilin's inverse temperature transition may be closely linked to the molecule's ability to efficiently transmit energy through the similar coil-β secondary structure transition seen in both cases. These insights into the inverse transition temperature are relevant for the design of bio-inspired sensors and energy storage devices.
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    Novel methods for microfluidic mixing and control
    Chawan, Aschvin Bhagirath (Virginia Tech, 2014-01-11)
    Microfluidics is a constantly evolving area of research. The implementation of new technologies and fabrication processes offers novel methodologies to solve existing problems. There are currently a large number of established techniques to address issues associated with microscale mixing and valving. We present mixing and valving techniques that utilize simplified and inexpensive techniques. The first technique addresses issues associated with microscale mixing. Exercising control over animal locomotion is well known in the macro world but in the micro-scale world, control requires more sophistication. We present a method to artificially magnetize microorganisms and use external permanent magnets to control their motion in a microfluidic device. This effectively tethers the microorganisms to a location in the channel and controls where mixing occurs. We use the bulk and ciliary motion of the microswimmers to generate shear flows, thus enhancing cross-stream mixing by supplementing diffusion. The device is similar to an active mixer but requires no external power sources or artificial actuators. The second technique examines a methodology involving the integration of electroactive polymers into microfluidic devices. Under the influence of high applied voltages, electroactive polymers with fixed boundary conditions undergo out-of-plane deformation. We use this finding to create a valve capable blocking flow in microchannels. Electrolytic fluid solutions are used as electrodes to carry the voltage signal to the polymer surface. Currently we have demonstrated this methodology as a proof of concept, but aim to optimize our system to develop a robust microvalve technology.
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    Structural Modeling and Optimization of Aircraft Wings having Curvilinear Spars and Ribs (SpaRibs)
    De, Shuvodeep (Virginia Tech, 2017-09-22)
    The aviation industry is growing at a steady rate but presently, the industry is highly dependent on fossil fuel. As the world is running out of fossil fuels and the wide-spread acceptance of climate change due to carbon emissions, both the governments and industry are spending a significant amount of resources on research to reduce the weight and hence the fuel consumption of commercial aircraft. A commercial fixed-wing aircraft wing consists of spars which are beams running in span-wise direction, carrying the flight loads and ribs which are panels with holes attached to the spars to preserve the outer airfoil shape of the wing. Kapania et al. at Virginia Tech proposed the concept of reducing the weight of aircraft wing using unconventional design of the internal structure consisting of curvilinear spars and ribs (known as SpaRibs) for enhanced performance. A research code, EBF3GLWingOpt, was developed by the Kapania Group. at Virginia Tech to find the best configuration of SpaRibs in terms of weight saving for given flight conditions. However, this software had a number of limitations and it can only create and analyze limited number of SpaRibs configurations. In this work, the limitations of the EBF3GLWingOpt code has been identified and new algorithms have been developed to make is robust and analyze larger number of SpaRibs configurations. The code also has the capability to create cut-outs in the SpaRibs for passage of fuel pipes and wirings. This new version of the code can be used to find best SpaRibs configuration for multiple objectives such as reduction of weight and increase flutter velocity. The code is developed in Python language and it has parallel computational capabilities. The wing is modeled using commercial FEA software, MSC.PATRAN and analyzed using MSC.NASTRAN which are from within EBF3GLWingOpt. Using this code a significant weight reduction for a transport aircraft wing has been achieved.
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    Swelling-Induced Deformations: A Materials-Defined Transition from Macroscale to Microscale Deformations
    Pandey, Anupam; Holmes, Douglas P. (The Royal Society of Chemistry, 2013-05-10)
    Swelling-induced deformations are common in many biological and industrial environments, and the shapes and patterns that emerge can vary across many length scales. Here we present an experimental study of a transition between macroscopic structural bending and microscopic surface creasing in elastomeric beams swollen non-homogeneously with favorable solvents. We show that this transition is dictated by the materials and geometry of the system, and we develop a simple scaling model based on competition between bending and swelling energies that predicts if a given solvent droplet would deform a polymeric structure macroscopically or microscopically. We demonstrate how proper tuning of materials and geometry can generate instabilities at multiple length scales in a single structure.