42 results
Search Results
Now showing 1 - 10 of 42
- ItemCohesive detachment of an elastic pillar from a dissimilar substrate(Amsterdam : Elsevier, 2017) Fleck, Norman A.; Khaderi, Syed Nizamuddin; McMeeking, Robert M.; Arzt, EduardThe adhesion of micron-scale surfaces due to intermolecular interactions is a subject of in- tense interest spanning electronics, biomechanics and the application of soft materials to engineering devices. The degree of adhesion is sensitive to the diameter of micro-pillars in addition to the degree of elastic mismatch between pillar and substrate. Adhesion- strength-controlled detachment of an elastic circular cylinder from a dissimilar substrate is predicted using a Dugdale-type of analysis, with a cohesive zone of uniform tensile strength emanating from the interface corner. Detachment initiates when the opening of the cohesive zone attains a critical value, giving way to crack formation. When the cohe- sive zone size at crack initiation is small compared to the pillar diameter, the initiation of detachment can be expressed in terms of a critical value H c of the corner stress inten- sity. The estimated pull-offforce is somewhat sensitive to the choice of stick/slip boundary condition used on the cohesive zone, especially when the substrate material is much stiffer than the pillar material. The analysis can be used to predict the sensitivity of detachment force to the size of pillar and to the degree of elastic mismatch between pillar and sub- strate.
- ItemFibrillar elastomeric micropatterns create tunable adhesion even to rough surfaces(Hoboken, NJ : Wiley, 2016) Barreau, Viktoriia; Hensel, René; Guimard, Nathalie K.; Ghatak, Animangsu; McMeeking, Robert M.; Arzt, EduardBiologically inspired, fibrillar dry adhesives continue to attract much attention as they are instrumental for emerging applications and technologies. To date, the adhesion of micropatterned gecko-inspired surfaces has predominantly been tested on stiff, smooth substrates. However, all natural and almost all artificial surfaces have roughnesses on one or more different length scales. In the present approach, micropillar-patterned PDMS surfaces with superior adhesion to glass substrates with different roughnesses are designed and analyzed. The results reveal for the first time adhesive and nonadhesive states depending on the micropillar geometry relative to the surface roughness profile. The data obtained further demonstrate that, in the adhesive regime, fibrillar gecko-inspired adhesive structures can be used with advantage on rough surfaces; this finding may open up new applications in the fields of robotics, biomedicine, and space exploration.
- ItemTailored polyurethane acrylate blend for large-scale and high-performance micropatterned dry adhesives(Dordrecht [u.a.] : Springer Science + Business Media B.V, 2019) Yu, Dan; Hensel, RenĂ©; Beckelmann, Dirk; Opsölder, Michael; Schäfer, Bruno; Moh, Karsten; de Oliveira, Peter William; Arzt, EduardContinuous roll-to-roll fabrication is essential for transferring the idea of bio-inspired, fibrillar dry adhesives into large-scale, synthetic, high-performance adhesive tapes. Toward this aim, we investigated process parameters that allow us to control the morphology and the resulting adhesion of mushroom-shaped micropatterned surfaces. Flexible silicone templates enabled the replication process of the polyurethane acrylate pre-polymer involving UV-light-induced cross-linking. For this paper, we particularly tailored the polyurethane acrylate pre-polymer by adding chemical components to tune UV curing kinetics and to reduce oxygen inhibition of radicals. We found that higher intensities of the UV light and faster reaction kinetics improved the quality of the microstructures, i.e., a larger cap diameter of the mushroom tips was achieved. The polymer blend U6E4 exhibited the fastest curing kinetics, which resulted in a micromorphology similar to that of the Ni-shim master structures. Best adhesion results were obtained for adhesive tapes made from U6E4 with 116 kPa pull-off stress, 1.4 N cm−1 peel strength and 71 kPa shear strength. In addition, repeated attachment–detachment tests over 100,000 cycles demonstrated strong robustness and reusability.
- ItemSize dependent strength of bcc metal micropillars: towards high strength surfaces by micropatterning(SaarbrĂ¼cken : Leibniz-Institut fĂ¼r Neue Materialien, 2010) Schneider, Andreas; Arzt, EduardThe size effect in body-centered cubic (bcc) metals was comprehensively investigated through microcompression tests performed on focused ion beam machined tungsten (W), molybdenum (Mo) and niobium (Nb) pillars, with single slip [235] and multiple slip [001] orientations. The relationship between yield strength and pillar diameter as well as the deformation morphologies were found to correlate with a parameter specific for bcc metals, i.e. the critical temperature Tc. This finding sheds new light on the phenomenon of small-scale plasticity in largely unexplored non-fcc metals. This effect may be used in the patterning of surfaces to achieve higher strengths.
- ItemA bioinspired snap-through metastructure for manipulating micro-objects(Washington, DC [u.a.] : American Association for the Advancement of Science, 2022) Zhang, Xuan; Wang, Yue; Tian, Zhihao; Samri, Manar; Moh, Karsten; McMeeking, Robert M.; Hensel, René; Arzt, EduardMicro-objects stick tenaciously to each other—a well-known show-stopper in microtechnology and in handling micro-objects. Inspired by the trigger plant, we explore a mechanical metastructure for overcoming adhesion involving a snap-action mechanism. We analyze the nonlinear mechanical response of curved beam architectures clamped by a tunable spring, incorporating mono- and bistable states. As a result, reversible miniaturized snap-through devices are successfully realized by micron-scale direct printing, and successful pick-and-place handling of a micro-object is demonstrated. The technique is applicable to universal scenarios, including dry and wet environment, or smooth and rough counter surfaces. With an unprecedented switching ratio (between high and low adhesion) exceeding 104, this concept proposes an efficient paradigm for handling and placing superlight objects.
- ItemOptoacoustically induced auditory brainstem responses in the mouse model enhanced through an absorbing film(Bellingham, Wash. : SPIE, 2021) Sorg, Katharina; Heimann, Larissa; Lana, Gabriela Moreira; Langenbucher, Achim; Schick, Bernhard; Arzt, Eduard; Wenzel, Gentiana IoanaSignificance: Optoacoustic stimulation offers an alternative stimulation strategy for the hearing organ. To serve as the base for a novel auditory prosthesis, the optoacoustic stimulation must be biocompatible and energy-saving. Aim: Enhancing the efficiency of optoacoustic stimulation while reducing the energy input in a suited animal model. Approach: Optoacoustically induced auditory brainstem responses (oABRs) were recorded after the pulsed laser irradiation of the tympanic membrane (TM) in mice. The results were compared with the ABRs induced through acoustic click stimulation. In addition, self-adhesive absorbing films were applied on the TM before the optoacoustic stimulation to investigate their effect on the resulting ABRs.Results: Using an absorbing film on the TM during optical stimulation led to considerably enhanced oABR wave I amplitude values compared with the stimulation of the bare TM. When using our stimulation strategy, we induced oABR waves in the 50% to 60% range of the acoustical stimulation reached with 80-dB SPL click stimuli. Conclusions: The mouse model can be used for certain developmental work for an optoacoustic auditory prosthesis. Using absorbing films on the TM during optical stimulation considerably enhances oABR wave I amplitude. Optimization of the stimulation strategy could further enhance the efficiency within biocompatibility margins.
- ItemAdhesion behavior of polymer networks with tailored mechanical properties using spherical and flat contacts(Cambridge : Cambridge University Press, 2013) Lakhera, Nishant; Graucob, Annalena; Schneider, Andreas S.; Kroner, Elmar; Micciché, Maurizio; Arzt, Eduard; Frick, Carl P.Four acrylate-based networks were developed such that they possessed similar glass transition temperature (~-37 °C) but varied in material stiffness at room temperature by an order of magnitude (2-12 MPa). Thermo-mechanical and adhesion testing were performed to investigate the effect of elastic modulus on adhesion profiles of the developed samples. Adhesion experiments with a spherical probe revealed no dependency of the pull-off force on material modulus as predicted by the Johnson, Kendall, and Roberts theory. Results obtained using a flat probe showed that the pull-off force increases linearly with an increase in the material modulus, which matches very well with Kendall's theory.
- ItemIndentation-induced two-way shape-memory effect in aged Ti-50.9 at.% Ni(Cambridge : Cambridge University Press, 2015) Frensemeier, Mareike; Arzt, Eduard; Qin, Enwei; Frick, Carl P.; Schneider, Andreas S.In this study, Vickers indentation was used to investigate the two-way shape-memory effect (TWSME) in an austenitic Ti-50.9 at.% Ni alloy, exposed to different heat treatments. Three aging treatments were used to manipulate the size of Ti3Ni4 precipitates. All samples were Vickers indented, and the indent depth was investigated as function of thermal cycling. The TWSME was found only in the material aged at 400 °C, which contained coherent precipitates. Thermal cycling shows stable TWSME, however, heating well above the austenite finish temperature lead to permanent austenitic protrusions. The results indicate that stabilized martensite plays a critical role in creating TWSME surfaces.
- ItemNumerical study of adhesion enhancement by composite fibrils with soft tip layers(Amsterdam : Elsevier, 2016) Balijepalli, Ram Gopal; Fischer, Sarah C.L.; Hensel, René; McMeeking, Robert M.; Arzt, EduardBio-inspired fibrillar surfaces with reversible adhesion to stiff substrates have been thoroughly investigated over the last decade. In this paper we propose a novel composite fibril consisting of a soft tip layer and stiffer stalk with differently shaped interfaces (flat vs. curved) between them. A tensile stress is applied remotely on the free end of the fibril whose other end adheres to a rigid substrate. The stress distributions and the resulting adhesion of such structures were numerically investigated under plane strain (2D) and axisymmetric (3D) conditions. The stress intensities were evaluated for different combinations of layer thickness and Young’s moduli. The adhesion strength values were found to increase for thinner layers and larger modulus ratio; these trends are also reflected in selected experimental results. The results of this paper provide a new strategy for optimizing adhesion strength of fibrillar surfaces.
- ItemBreakdown of continuum models for spherical probe adhesion tests on micropatterned surfaces(Amsterdam [u.a.] : Elsevier Science, 2021) Bettscheider, Simon; Yu, Dan; Foster, Kimberly; McMeeking, Robert; Arzt, Eduard; Hensel, René; Booth, Jamie A.The adhesion of fibrillar dry adhesives, mimicking nature's principles of contact splitting, is commonly characterized by using axisymmetric probes having either a flat punch or spherical geometry. When using spherical probes, the adhesive pull-off force measured depends strongly on the compressive preload applied when making contact and on the geometry of the probe. Together, these effects complicate comparisons of the adhesive performance of micropatterned surfaces measured in different experiments. In this work we explore these issues, extending previous theoretical treatments of this problem by considering a fully compliant backing layer with an array of discrete elastic fibrils on its surface. We compare the results of the semi-analytical model presented to existing continuum theories, particularly with respect to determining a measurement system- and procedure-independent metric for the local adhesive strength of the fibrils from the global pull-off force. It is found that the discrete nature of the interface plays a dominant role across a broad range of relevant system parameters. Accordingly, a convenient tool for simulation of a discrete array is provided. An experimental procedure is recommended for use in conjunction with this tool in order to extract a value for the local adhesive strength of the fibrils, which is independent of the other system properties (probe radius, backing layer thickness, and preload) and thus is suitable for comparison across experimental studies.