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- ItemBending as Key Mechanism in the Tactile Perception of Fibrillar Surfaces(Weinheim : Wiley-VCH, 2021) Gedsun, Angelika; Sahli, Riad; Meng, Xing; Hensel, René; Bennewitz, RolandThe touching of fibrillar surfaces elicits a broad range of affective reactions, which range from the adverse stinginess of a stiff bristle brush to the pleasant feel of velvet. To study the tactile perception of model fibrillar surfaces, a unique set of samples carrying dense, regular arrays of cylindrical microfibrils with high aspect ratio made from different elastomer materials have been created. Fibril length and material compliance are varied independently such that their respective influence on tactile perception can be elucidated. This work finds that the tactile perception of similarity between samples is dominated by bending of the fibrils under sliding touch. The results demonstrate that variations of material stiffness and of surface structure are not necessarily perceived independently by touch. In the case of fibrillar elastomer surfaces, it is rather the ratio of fibril length and storage modulus which determines fibril bending and becomes the dominant tactile dimension. Visual access to the sample during tactile exploration improves the tactile perception of fibril bendability. Experiments with colored samples show a distraction by color in participants’ decisions regarding tactile similarity only for yellow samples of outstanding brightness.
- ItemEngineering Micropatterned Dry Adhesives: From Contact Theory to Handling Applications(Weinheim : Wiley-VCH, 2018) Hensel, René; Moh, Karsten; Arzt, EduardReversible adhesion is the key functionality to grip, place, and release objects nondestructively. Inspired by nature, micropatterned dry adhesives are promising candidates for this purpose and have attracted the attention of research groups worldwide. Their enhanced adhesion compared to nonpatterned surfaces is frequently demonstrated. An important conclusion is that the contact mechanics involved is at least as important as the surface energy and chemistry. In this paper, the roles of the contact geometry and mechanical properties are reviewed. With a focus on applications, the effects of substrate roughness and of temperature variations, and the long-term performance of micropatterned adhesives are discussed. The paper provides a link between the current, detailed understanding of micropatterned adhesives and emerging applications.
- ItemAdhesion and Cellular Compatibility of Silicone-Based Skin Adhesives(Weinheim : Wiley-VCH, 2017) Fischer, Sarah C. L.; Kruttwig, Klaus; Bandmann, Vera; Hensel, René; Arzt, EduardPressure-sensitive adhesives based on silicone materials have emerging potential as adhesives in healthcare products, in particular for gentle skin adhesives. To this end, adhesion to rough skin and biocompatibility are crucial factors for a successful implementation. In this study, the mechanical, adhesive, and biological properties of the two-component poly(dimethylsiloxane) Soft Skin Adhesive MG 7-9800 (SSA, Dow Corning) have been investigated and compared to Sylgard 184. Different mixing ratios of SSA's components allow for tuning of the shear modulus, thereby modifying the adhesive properties of the polymer. To give a comprehensive insight, the authors have analyzed the interplay between pull-off stress, adhesion energy, and stretch of the adhesive films on smooth and rough surfaces. The focus is placed on the effects of substrate roughness and on low pressure oxygen plasma treatment of the adhesive films. SSA shows superior biocompatibility in in vitro cell culture experiments. High pull-off stresses in the range of 3 N cm−2 on a rough surface are achieved, promising broad application spectra for SSA-based healthcare products.
- ItemIn Situ Experiments To Reveal the Role of Surface Feature Sidewalls in the Cassie–Wenzel Transition(Washington D.C. : American Chemical Society, 2014) Hensel, René; Finn, Andreas; Helbig, Ralf; Killge, Sebastian; Braun, Hans-Georg; Werner, CarstenWaterproof and self-cleaning surfaces continue to attract much attention as they can be instrumental in various different technologies. Such surfaces are typically rough, allowing liquids to contact only the outermost tops of their asperities, with air being entrapped underneath. The formed solid–liquid–air interface is metastable and, hence, can be forced into a completely wetted solid surface. A detailed understanding of the wetting barrier and the dynamics of this transition is critically important for the practical use of the related surfaces. Toward this aim, wetting transitions were studied in situ at a set of patterned perfluoropolyether dimethacrylate (PFPEdma) polymer surfaces exhibiting surface features with different types of sidewall profiles. PFPEdma is intrinsically hydrophobic and exhibits a refractive index very similar to water. Upon immersion of the patterned surfaces into water, incident light was differently scattered at the solid–liquid–air and solid–liquid interface, which allows for distinguishing between both wetting states by dark-field microscopy. The wetting transition observed with this methodology was found to be determined by the sidewall profiles of the patterned structures. Partial recovery of the wetting was demonstrated to be induced by abrupt and continuous pressure reductions. A theoretical model based on Laplace’s law was developed and applied, allowing for the analytical calculation of the transition barrier and the potential to revert the wetting upon pressure reduction.
- ItemAdhesion of a rigid punch to a confined elastic layer revisited(Milton Park : Taylor & Francis, 2017) Hensel, René; McMeeking, Robert M.; Kossa, AttilaThe adhesion of a punch to a linear elastic, confined layer is investigated. Numerical analysis is performed to determine the equivalent elastic modulus in terms of layer confinement. The size of the layer relative to the punch radius and its Poisson’s ratio are found to affect the layer stiffness. The results reveal that the equivalent modulus of a highly confined layer depends on its Poisson’s ratio, whereas, in contrast, an unconfined layer is only sensitive to the extent of the elastic film. The solutions of the equivalent modulus obtained from the simulations are fitted by an analytical function that, subsequently, is utilized to deduce the energy release rate for detachment of the punch via linear elastic fracture mechanics. The energy release rate strongly varies with layer confinement. Regimes for stable and unstable crack growth can be identified that, in turn, are correlated to interfacial stress distributions to distinguish between different detachment mechanisms.
- ItemPreventing Catastrophic Failure of Microfibrillar Adhesives in Compliant Systems Based on Statistical Analysis of Adhesive Strength(Washington, DC : ACS Publications, 2021) Hensel, René; Thiemecke, Jonathan; Booth, Jamie A.Adhesives based on fibrillar surface microstructures have shown great potential for handling applications requiring strong, reversible, and switchable adhesion. Recently, the importance of the statistical distribution of adhesive strength of individual fibrils in controlling the overall performance was revealed. Strength variations physically correspond to different interfacial defect sizes, which, among other factors, are related to surface roughness. For analysis of the strength distribution, Weibull's statistical theory of fracture was introduced. In this study, the importance of the statistical properties in controlling the stability of attachment is explored. Considering the compliance of the loading system, we develop a stability criterion based on the Weibull statistical parameters. It is shown that when the distribution in fibril adhesive strength is narrow, the global strength is higher but unstable detachment is more likely. Experimental variation of the loading system compliance for a specimen of differing statistical properties shows a transition to unstable detachment at low system stiffness, in good agreement with the theoretical stability map. This map serves to inform the design of gripper compliance, when coupled with statistical analysis of strength on the target surface of interest. Such a treatment could prevent catastrophic failure by spontaneous detachment of an object from an adhesive gripper. © 2021 The Authors. Published by American Chemical Society.
- ItemTuning the Release Force of Microfibrillar Adhesives by Geometric Design(Weinheim : Wiley-VCH, 2022) Barnefske, Lena; Rundel, Fabian; Moh, Karsten; Hensel, René; Zhang, Xuan; Arzt, EduardSwitchable micropatterned adhesives exhibit high potential as novel resource-efficient grippers in future pick-and-place systems. In contrast with the adhesion acting during the “pick” phase, the release during the “place” phase has received little research attention so far. For objects smaller than typically 1 mm, release may become difficult as gravitational and inertial forces are no longer sufficient to allow shedding of the object. A compressive overload can initiate release by elastic buckling of the fibrils, but the switching ratio (ratio between high and low adhesion force) is typically only 2–3. In this work, new microfibrillar designs are reported exhibiting directional buckling with high switching ratios in the order of 20. Their functionality is illustrated by in situ optical observation of the contact signatures. Such micropatterns can enable the successful release of small objects with high placement accuracy.
- ItemSliding Mechanism for Release of Superlight Objects from Micropatterned Adhesives(Weinheim : Wiley-VCH, 2022) Wang, Yue; Zhang, Xuan; Hensel, René; Arzt, EduardRobotic handling and transfer printing of micrometer-sized superlight objects is a crucial technology in industrial fabrication. In contrast to the precise gripping with micropatterned adhesives, the reliable release of superlight objects with negligible weight is a great challenge. Slanted deformable polymer microstructures, with typical pillar cross-section 150 µm × 50 µm, are introduced with various tilt angles that enable a reduction of adhesion by a switching ratio of up to 500. The experiments demonstrate that the release from a smooth surface involves sliding of the contact during compression and subsequent peeling of the object during retraction. The handling of a 0.5 mg perfluorinated polymer micro-object with high accuracy in repeated pick-and-place cycles is demonstrated. Based on beam theory, the forces and moments acting at the tip of the microstructure are analyzed. As a result, an expression for the pull-off force is proposed as a function of the sliding distance and a guide to an optimized design for these release structures is provided.
- ItemIn Situ Observation Reveals Local Detachment Mechanisms and Suction Effects in Micropatterned Adhesives(Weinheim : Wiley-VCH, 2019) Tinnemann, Verena; Hernández, Luissé; Fischer, Sarah C.L.; Arzt, Eduard; Bennewitz, Roland; Hensel, RenéFibrillar adhesion pads of insects and geckoes have inspired the design of high-performance adhesives enabling a new generation of handling devices. Despite much progress over the last decade, the current understanding of these adhesives is limited to single contact pillars and the behavior of whole arrays is largely unexplored. In the study reported here, a novel approach is taken to gain insight into the detachment mechanisms of whole micropatterned arrays. Individual contacts are imaged by frustrated total internal reflection, allowing in situ observation of contact formation and separation during adhesion tests. The detachment of arrays is found to be governed by the distributed adhesion strength of individual pillars, but no collaborative effect mediated by elastic interactions can be detected. At the maximal force, about 30% of the mushroom structures are already detached. The adhesive forces decrease with reduced air pressure by 20% for the smooth and by 6% for the rough specimen. These contributions are attributed to a suction effect, whose strength depends critically on interfacial defects controlling the sealing quality of the contact. This dominates the detachment process and the resulting adhesion strength. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemBioinspired Underwater Adhesion to Rough Substrates by Cavity Collapse of Cupped Microstructures(Weinheim : Wiley-VCH, 2021) Wang, Yue; Hensel, RenéUnderwater or wet adhesion is highly desirable for numerous applications but is counteracted by the liquids in the contact which weaken intermolecular attraction. The problem is exacerbated in conjunction with surface roughness when liquids partially remain in grooves or dimples of the substrate. In the present study, a cupped microstructure with a cavity inspired by suction organs of aquatic animals is proposed. The microstructures (cup radius of 100 µm) are made from polyurethane using two-photon lithography followed by replica molding. Adhesion to rough substrates is emulated experimentally by a micropatterned model substrate with varying channel widths. Pull-off stresses are found to be about 200 kPa, i.e., twice atmospheric pressure. Evaluation of force–displacement curves together with in situ observations reveal the adhesion mechanism, which involves adaptation to surface roughness and an elastic force induced by the collapse of the cavity that holds sealed contact with the substrate during retraction. This new microarchitecture may pave the way for next generation microstructures applicable to real, rough surfaces under wet conditions.