Category: Papers
Paper: Swelling effects on the adhesion
Swelling effects on localized adhesion of an elastic ribbon
M. Curatolo , P. Nardinocchi , L. Teresi and D.P. Holmes
Proceedings of the Royal Society A, 475(2225), 0067, (2019)
Abstract: We investigate the adhesion mechanism between an elastic strip of vinylpolysiloxane bent in a racquet-like shape, and a thick elastomeric substrate with the aim to understand how local swelling modifies adhesion. Using a modified loop–tack adhesion test, we place a droplet of silicone oil in between the two materials, vary the dwell time and measure the force required to separate the two interfaces. The experiments are then compared with an analytical model that describes how the critical peel force is modified as the interfacial surface energy changes over time. Our study reveals that in certain circumstances swelling can enhance adhesion. More specifically, strong adhesion is obtained when most of the droplet is absorbed by the solid. By contrast, when the droplet remains at the interface a small adhesive force is measured.
Link: https://royalsocietypublishing.org/doi/10.1098/rspa.2019.0067
Review Paper: Elasticity and Stability of Shape-Shifting Structures
Elasticity and Stability of Shape-Shifting Structures
Douglas P. Holmes
Current Opinion in Colloid and Interface Science, 40:118-137, (2019).
Abstract: As we enter the age of designer matter — where objects can morph and change shape on command — what tools do we need to create shape-shifting structures? At the heart of an elastic deformation is the combination of dilation and distortion or stretching and bending. The competition between the latter can cause elastic instabilities, and over the last fifteen years, these instabilities have provided a multitude of ways to prescribe and control shape change. Buckling, wrinkling, folding, creasing, and snapping have become mechanisms that when harmoniously combined enable mechanical metamaterials, self-folding origami, ultralight and ultrathin kirigami, and structures that appear to grow from one shape to another. In this review, I aim to connect the fundamentals of elastic instabilities to the advanced functionality currently found within mechanical metamaterials.
Link: https://www.sciencedirect.com/science/article/pii/S1359029418300839?dgcid=author
Paper: Buckling of geometrically confined shells
Buckling of geometrically confined shells
Lucia Stein-Montalvo, Paul Costa, Matteo Pezzulla and Douglas P. Holmes
Soft Matter, 15(6), 1215-1222, (2019).
Abstract: We study the periodic buckling patterns that emerge when elastic shells are subjected to geometric confinement. Residual swelling provides access to range of shapes (saddles, rolled sheets, cylinders, and spherical sections) which vary in their extrinsic and intrinsic curvatures. Our experimental and numerical data show that when these moderately thick structures are radially confined, a single geometric parameter – the ratio of the total shell radius to the amount of unconstrained material – predicts the number of lobes formed. We present a model that interprets this scaling as the competition between radial and circumferential bending. Next, we show that reducing the transverse confinement of saddles causes the lobe number to decrease with a similar scaling analysis. Hence, one geometric parameter captures the wave number through a wide range of radial and transverse confinement, connecting the shell shape to the shape of the boundary that confines it. We expect these results to be relevant for an expanse of shell shapes, and thus applicable to the design of shape-shifting materials and the swelling and growth of soft structures.
Link: https://pubs.rsc.org/en/content/articlepdf/2019/sm/c8sm02035c
Paper: Snapping of bistable, prestressed cylindrical shells
Snapping of bistable, prestressed cylindrical shells
Xin Jiang, Matteo Pezzulla, Huiqi Shao, Tushar K. Ghosh, and Douglas P. Holmes,
Europhysics Letters (EPL), 122, 6, (2018).
Bistable shells can reversibly change between two stable configurations with very little energetic input. Understanding what governs the shape and snap-through criteria of these structures is crucial for designing devices that utilize instability for functionality. Bistable cylindrical shells fabricated by stretching and bonding multiple layers of elastic plates will contain residual stress that will impact the shell's shape and the magnitude of stimulus necessary to induce snapping. Using the framework of incompatible elasticity, we first predict the mean curvature of a nearly cylindrical shell formed by arbitrarily prestretching one layer of a bilayer plate with respect to another. Then, beginning with a residually stressed cylinder, we determine the amount of the stimuli needed to trigger the snapping between two configurations through a combination of numerical simulations and theory. We demonstrate the role of prestress on the snap-through criteria, and highlight the important role that the Gaussian curvature in the boundary layer of the shell plays in dictating shell stability.
Link: http://iopscience.iop.org/article/10.1209/0295-5075/122/64003
Paper: Elastogranular Mechanics
Elastogranular Mechanics: Buckling, Jamming, and Structure Formation
David J. Schunter, Jr., Martin Brandenbourger, Sophia Perriseau, and Douglas P. Holmes,
Physical Review Letters, 120, 078002, (2018).
Confinement of a slender body into a granular array induces stress localization in the geometrically nonlinear structure, and jamming, reordering, and vertical dislodging of the surrounding granular medium. By varying the initial packing density of grains and the length of a confined elastica, we identify the critical length necessary to induce jamming, and demonstrate how folds couple with the granular medium to localize along grain boundaries. Above the jamming threshold, the characteristic length of elastica deformation is shown to diverge in a manner that is coupled with the motion and rearrangement of the grains, suggesting the ordering of the granular array governs the deformation of the slender structure. However, overconfinement of the elastica will vertically dislodge grains, a form of stress relaxation in the granular medium that illustrates the intricate coupling in elastogranular interactions.
Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.078002
Paper: Curvature-Induced Instabilities of Shells
Curvature-Induced Instabilities of Shells
Matteo Pezzulla, Norbert Stoop, Mark P. Steranka, Abdikhalaq J. Bade, and Douglas P. Holmes, Physical Review Letters, 120, 048002, (2018).
Induced by proteins within the cell membrane or by differential growth, heating, or swelling, spontaneous curvatures can drastically affect the morphology of thin bodies and induce mechanical instabilities. Yet, the interaction of spontaneous curvature and geometric frustration in curved shells remains poorly understood. Via a combination of precision experiments on elastomeric spherical shells, simulations, and theory, we show how a spontaneous curvature induces a rotational symmetry-breaking buckling as well as a snapping instability reminiscent of the Venus fly trap closure mechanism. The instabilities, and their dependence on geometry, are rationalized by reducing the spontaneous curvature to an effective mechanical load. This formulation reveals a combined pressurelike term in the bulk and a torquelike term in the boundary, allowing scaling predictions for the instabilities that are in excellent agreement with experiments and simulations. Moreover, the effective pressure analogy suggests a curvature-induced subcritical buckling in closed shells. We determine the critical buckling curvature via a linear stability analysis that accounts for the combination of residual membrane and bending stresses. The prominent role of geometry in our findings suggests the applicability of the results over a wide range of scales.
Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.048002
Paper: Extended Lubrication Theory
Extended lubrication theory: improved estimates of flow in channels with variable geometry
Behrouz Tavakol, Guillaume Froehlicher, Douglas P. Holmes, Howard A. Stone, Proceedings of the Royal Society A, 0234, (2017).
Abstract: Lubrication theory is broadly applicable to the flow characterization of thin fluid films and the motion of particles near surfaces. We offer an extension to lubrication theory by starting with Stokes equations and considering higher-order terms in a systematic perturbation expansion to describe the fluid flow in a channel with features of a modest aspect ratio. Experimental results qualitatively confirm the higher-order analytical solutions, while numerical results are in very good agreement with the higher-order analytical results. We show that the extended lubrication theory is a robust tool for an accurate estimate of pressure drop in channels with shape changes on the order of the channel height, accounting for both smooth and sharp changes in geometry.
Link: http://rspa.royalsocietypublishing.org/content/473/2206/20170234
Paper: Kirigami Actuators
Kirigami Actuators
Marcelo A. Dias, Michael P. McCarron, Daniel Rayneau-Kirkhope, Paul Z. Hanakata, David K. Campbell, Harold S. Park and Douglas P. Holmes, Soft Matter, 13, 9087-9802, (2017).
Abstract: Thin elastic sheets bend easily and, if they are patterned with cuts, can deform in sophisticated ways. Here we show that carefully tuning the location and arrangement of cuts within thin sheets enables the design of mechanical actuators that scale down to atomically-thin 2D materials. We first show that by understanding the mechanics of a single non-propagating crack in a sheet, we can generate four fundamental forms of linear actuation: roll, pitch, yaw, and lift. Our analytical model shows that these deformations are only weakly dependent on thickness, which we confirm with experiments on centimeter-scale objects and molecular dynamics simulations of graphene and MoS2 nanoscale sheets. We show how the interactions between non-propagating cracks can enable either lift or rotation, and we use a combination of experiments, theory, continuum computational analysis, and molecular dynamics simulations to provide mechanistic insights into the geometric and topological design of kirigami actuators.
Link: http://pubs.rsc.org/en/content/articlelanding/2017/sm/c7sm01693j#!divAbstract
Paper: Generalized Scaling Law of Adhesion
Revisiting the generalized scaling law for adhesion: role of compliance and extension to progressive failure
Ahmad R. Mojdehi. Douglas P. Holmes. David A. Dillard, Soft Matter, 13, 7529-7536, (2017).
Abstract: A generalized scaling law, based on the classical fracture mechanics approach, is developed to predict the bond strength of adhesive systems. The proposed scaling relationship depends on the rate of change of debond area with compliance, rather than the ratio of area to compliance. This distinction can have a profound impact on the expected bond strength of systems, particularly when the failure mechanism changes or the compliance of the load train increases. Based on the classical fracture mechanics approach for rate-independent materials, the load train compliance should not affect the force capacity of the adhesive system, whereas when the area to compliance ratio is used as the scaling parameter, it directly influences the bond strength, making it necessary to distinguish compliance contributions. To verify the scaling relationship, single lap shear tests were performed for a given pressure sensitive adhesive (PSA) tape specimens with different bond areas, number of backing layers, and load train compliance. The shear lag model was used to derive closed-form relationships for the system compliance and its derivative with respect to the debond area. Digital image correlation (DIC) is implemented to verify the non-uniform shear stress distribution obtained from the shear lag model in a lap shear geometry. The results obtained from this approach could lead to a better understanding of the relationship between bond strength and the geometry and mechanical properties of adhesive systems.
Link: http://pubs.rsc.org/en/content/articlelanding/2017/sm/c7sm01098b#!divAbstract
Paper: Friction of extensible strips
Friction of extensible strips: An extended shear lag model with experimental evaluation
Ahmad R. Mojdehi. Douglas P. Holmes. David A. Dillard, International Journal of Solids and Structures, 124, 125-134, (2017).
Abstract: The role of effective axial compliance on the frictional response of extensible strips is investigated, both experimentally and theoretically. A translational actuator pulled a steel sled resting on top of an elastic strip, bonded only at the leading edge of the sled, across a glass substrate. The friction force and local deformation along the length of the strips were measured using a force sensor and a camera, respectively. By increasing the effective axial compliance of the strip, the static friction force was found to decrease dramatically, while the kinetic friction force increased significantly. For sufficiently soft strips, there was no observable static peak, although there was a slope change in the force-displacement curve at the point where progressive slippage initiated at the leading edge. Possible mechanisms for permanent increase in the kinetic friction are discussed that could be implemented in systems where the kinetic friction is of significant importance. A theoretical model, somewhat analogous to an extension of the classical shear lag model to incorporate elastic-plastic interlayers, is proposed to predict the friction response as a function of effective compliance. The results obtained from the theoretical model are compared with experimental results and shown to be in good agreement. This study provides a better understanding of 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.
Link: http://www.sciencedirect.com/science/article/pii/S0020768317302858
