Debonding energy of PDMS: A new analysis of a classic adhesion scenario

Adhesion study at the interface of a PDMS-elastomer and borosilicate glass-slide: effect of modulus and thickness of the elastomer

Adhesion control at the interface of two surfaces is crucial in many applications. Examples are the design of micro and nanodevices such as microfluidic devices, biochips, and electronic sensors. Adhesion at the interface of two materials can be controlled by various methods, such as chemical treatment on the surface of the materials, modification of the surface texture of the materials, and change of the mechanical properties of the materials. The main idea of this study is to control the adhesion by changing the mechanical properties (modulus) of the polydimethylsiloxane (PDMS) elastomer. We vary the modulus of the PDMS elastomer by changing mixing ratio (w/w) of the silicone elastomer base and its curing agent (SylgardTM 184, Dow Corning). Our study also includes the effect of the thickness of the PDMS elastomer sheet on its adhesion behavior. Adhesion measurements at the interface of the borosilicate glass slide and different PDMS elastomer specimens were performed using a wedge test. This method inserts a glass coverslip at the interface to create a wedge. We observe a significant decrease in the work of adhesion and an increase in equilibrium crack length with an increase in elastic-modulus and thickness of the PDMS elastomer samples. We present and discuss the effect of modulus and specimen-thickness on the adhesion behavior of the PDMS elastomer against a glass slide.

Predicting the Electrical, Mechanical, and Geometric Contributions to Soft Electroadhesives through Fracture Mechanics

Electroadhesion is the modulation of adhesive forces through electrostatic interactions and has potential applications in a number of next-generation technologies. Recent efforts have focused on using electroadhesion in soft robotics, haptics, and biointerfaces that often involve compliant materials and nonplanar geometries. Current models for electroadhesion provide limited insight on other contributions that are known to influence adhesion performance, such as geometry and material properties. This study presents a fracture mechanics framework for understanding electroadhesion that incorporates geometric and electrostatic contributions for soft electroadhesives. We demonstrate the validity of this model with two material systems that exhibit disparate electroadhesive mechanisms, indicating that this formalism is applicable to a variety of electroadhesives. The results show the importance of material compliance and geometric confinement in enhancing electroadhesive performance and providing structure-property relationships for designing electroadhesive devices.

Geometry-controlled instabilities for soft-soft adhesive interfaces

Soft materials interfaces can develop complex morphologies, such as cavities or finger-like features, during separation as a result of a mechanical instability. While the onset and growth of these instabilities have been investigated previously for interfaces between rigid and soft materials, no existing predictive model provides insight for controlling the separation morphology associated with these instabilities when both "sides" of the interface are soft. Here, we expand previous models to account for the geometry and materials properties of two soft materials that form an interface. The total compliance of the system, which depends nonlinearly on the thickness of each contacting soft material, plays a primary role in governing the morphology of the separating interface. We validate this model with experimental measurements using a series of soft elastomers with varying layer thicknesses and fixed materials properties, in order to emphasize the geometry alone can give rise to the observed differences in the interface separation process. This model also demonstrates that the degree of geometric asymmetry, or the ratio of the layer thicknesses that form an interface, influences the stress experienced in either layer, thus providing a rich means of controlling how unstable interface separations develop and propagate. This framework is a powerful tool to understand and control adhesion mechanisms in fields ranging from biology to soft robotics, and provides intuition for engineering a separation mode for a desired end result.

Adhesion of a tape loop

In this work, we revisit experimentally and theoretically the mechanics of a tape loop. Using primarily elastic materials (polydimethylsiloxane, PDMS, or polycarbonate, PC) and confocal microscopy, we monitor the shape as well as the applied forces during an entire cycle of compression and retraction of a half-loop compressed between parallel glass plates. We observe distinct differences in film shape during the cycle; points of equal applied force or equal plate separation differ in shape upon compression or retraction. To model the adhesion cycle in its entirety, we adapt the 'Sticky Elastica' of [T. J. W. Wagner et al., Soft Matter, 2013, 9, 1025-1030] to the tape loop geometry, which allows a complete analytical description of both the force balance and the film shape. We show that under compression the system is generally not sensitive to interfacial interactions, whereas in the limit of large separation of the confining parallel plates during retraction the system is well described by the peel model. Ultimately, we apply this understanding to the measurement of the energy release rate of a wide range of different cross-linker ratio PDMS elastomer half-loops in contact with glass. Finally, we show how the model illuminates an incredibly simple adhesion measurement technique, which only requires a ruler to perform.

Clogging of microfluidic constrictions by monoclonal antibody aggregates: role of aggregate shape and deformability

The formation of aggregates in solutions of monoclonal antibodies is difficult to prevent. Even if the occurrence of large aggregates is rare, their existence can lead to partial or total clogging of constrictions in injection devices, with drastic effects on drug delivery. Little is known on the origin and characteristics of such clogging events. Here we investigate a microfluidic model system to gain fundamental understanding of the clogging of constrictions by monoclonal antibody aggregates. Highly concentrated solutions of monoclonal antibodies were used to create protein aggregates (larger than 50 microns) using mechanical or heat stress. We show that clogging occurs when aggregates reach the size of the constriction and that clogs can in some cases be released by increasing the applied pressure. This indicates the important role of protein aggregate deformability. We perform systematic experiments for different relative aggregate sizes and applied pressures, and measure the resulting flow-rate. This allows us to present first in situ estimates of an effective Young's modulus. Despite their different shapes and densities, we can predict the number of clogging events for a given constriction size from the aggregate size distribution measured by Flow Imaging Microscopy (MFI). In addition our device can detect the occurrence of very rare big aggregates often overlooked by other detection methods.

Capillary-Force-Assisted Clean-Stamp Transfer of Two-Dimensional Materials

A simple and clean method of transferring two-dimensional (2D) materials plays a critical role in the fabrication of 2D electronics, particularly the heterostructure devices based on the artificial vertical stacking of various 2D crystals. Currently, clean transfer techniques rely on sacrificial layers or bulky crystal flakes (e.g., hexagonal boron nitride) to pick up the 2D materials. Here, we develop a capillary-force-assisted clean-stamp technique that uses a thin layer of evaporative liquid (e.g., water) as an instant glue to increase the adhesion energy between 2D crystals and polydimethylsiloxane (PDMS) for the pick-up step. After the liquid evaporates, the adhesion energy decreases, and the 2D crystal can be released. The thin liquid layer is condensed to the PDMS surface from its vapor phase, which ensures the low contamination level on the 2D materials and largely remains their chemical and electrical properties. Using this method, we prepared graphene-based transistors with low charge-neutral concentration (3 × 10¹⁰ cm^-2) and high carrier mobility (up to 48 820 cm² V^-1 s^-1 at room temperature) and heterostructure optoelectronics with high operation speed. Finally, a capillary-force model is developed to explain the experiment.

The effect of surface roughness and viscoelasticity on rubber adhesion

Adhesion between silica glass or acrylic balls and silicone elastomers and various industrial rubbers is investigated. The work of adhesion during pull-off is found to strongly vary depending on the system, which we attribute to the two opposite effects: (1) viscoelastic energy dissipation close to an opening crack tip and (2) surface roughness. Introducing surface roughness on the glass ball is found to increase the work of adhesion for soft elastomers, while for the stiffer elastomers it results in a strong reduction in the work of adhesion. For the soft silicone elastomers a strong increase in the work of adhesion with increasing pull-off velocity is observed, which may result from the non-adiabatic processes associated with molecular chain pull-out. In general, the work of adhesion is decreased after repeated contacts due to the transfer of molecules from the elastomers to the glass ball. Thus, extracting the free chains (oligomers) from the silicone elastomers is shown to make the work of adhesion independent of the number of contacts. The viscoelastic properties (linear and nonlinear) of all of the rubber compounds are measured, and the velocity dependent crack opening propagation energy at the interface is calculated. Silicone elastomers show a good agreement between the measured work of adhesion and the predicted results, but carbon black filled hydrogenated nitrile butadiene rubber compounds reveal that strain softening at the crack tip may play an important role in determining the work of adhesion. Additionally, adhesion measurement under submerged conditions in distilled water and water + soap solutions are also performed: a strong reduction in the work of adhesion is measured for the silicone elastomers submerged in water, and a complete elimination of adhesion is found for the water + soap solution attributed to an osmotic repulsion between the negatively charged surface of the glass and the elastomer.

Ultimate Control of Rate-Dependent Adhesion for Reversible Transfer Process via a Thin Elastomeric Layer

Adhesion between a stamp with an elastomeric layer and various devices or substrates is crucial to successfully fabricate flexible electronics using a transfer process. Although various transfer processes using stamps with different adhesion strengths have been suggested, the controllable range of adhesion is still limited to a narrow range. To precisely transfer devices onto selected substrates, however, the difference in adhesion between the picking and placing processes should be large enough to achieve a high yield. Herein, we report a simple way to extend the controllable adhesion range of stamps, which can be achieved by adjusting the thickness of the elastomeric layer and the separation velocity. The adhesion strength increased with decreasing layer thickness on the stamp due to a magnification of the confinement and rate-dependent effects on the adhesion. This enabled the controllable range of the adhesion strength for a 15 μm-thick elastomeric layer to be extended up to 12 times that of the bulk under the same separation conditions. The strategy of designing stamps using simple adhesion tests is also introduced, and the reversible transfer of thin Si chips was successfully demonstrated. Tuning and optimizing the adhesion strength of a stamp according to the design process suggested here can be applied to various materials for the selective transfer and replacement of individual devices.

Fracture of a model cohesive granular material

We study experimentally the fracture mechanisms of a model cohesive granular medium consisting of glass beads held together by solidified polymer bridges. The elastic response of this material can be controlled by changing the cross-linking of the polymer phase, for example. Here we show that its fracture toughness can be tuned over an order of magnitude by adjusting the stiffness and size of the polymer bridges. We extract a well-defined fracture energy from fracture testing under a range of material preparations. This energy is found to scale linearly with the cross-sectional area of the bridges. Finally, X-ray microcomputed tomography shows that crack propagation is driven by adhesive failure of about one polymer bridge per bead located at the interface, along with microcracks in the vicinity of the failure plane. Our findings provide insight into the fracture mechanisms of this model material, and the mechanical properties of disordered cohesive granular media in general.

Adhesion-induced instabilities and pattern formation in thin films of elastomers and gels

A hydrostatically stressed soft elastic film circumvents the imposed constraint by undergoing a morphological instability, the wavelength of which is dictated by the minimization of the surface and the elastic strain energies of the film. While for a single film, the wavelength is entirely dependent on its thickness, a co-operative energy minimization dictates that the wavelength depends on both the elastic moduli and thicknesses of two contacting films. The wavelength can also depend on the material properties of a film if its surface tension has a pronounced effect in comparison to its elasticity. When such a confined film is subjected to a continually increasing normal displacement, the morphological patterns evolve into cracks, which, in turn, govern the adhesive fracture behavior of the interface. While, in general, the thickness provides the relevant length scale underlying the well-known Griffith-Kendall criterion of debonding of a rigid disc from a confined film, it is modified non-trivially by the elasto-capillary number for an ultra-soft film. Depending upon the degree of confinement and the spatial distribution of external stress, various analogs of the canonical instability patterns in liquid systems can also be reproduced with thin confined elastic films.