Michael Varenberg

Biomimetic mushroom-shaped fibrillar adhesive microstructure.

To improve the adhesive properties of artificial fibrillar contact structures, the attachment systems of beetles from the family Chrysomelidae were chosen to serve as a model. Biomimetic mushroom-shaped fibrillar adhesive microstructure inspired by these systems was characterized using a variety of measurement techniques and compared with a control flat surface made of the same material. Results revealed that pull-off force and peel strength of the structured specimens are more than twice those of the flat specimens. In contrast to the control system, the structured one is found to be very tolerant to contamination and able to recover its adhesive properties after being washed in a soap solution. Based on the combination of several geometrical principles found in biological attachment devices, the presented microstructure exhibits a considerable step towards the development of an industrial dry adhesive.

An improved wedge calibration method for lateral force in atomic force microscopy

An improved wedge calibration method for quantitative lateral force measurement in atomic force microscopy is presented. The improved method differs from the original one in several aspects. It utilizes a much simpler, commercially available, calibration grating and can be performed at any single specified applied load. It enables calibration of all types of probes, both integrated with sharp tips, and colloidal with any radius of curvature up to 2 μm. The improved method also simplifies considerably the calculation of the calibration factor by using flat facets on the calibration grating to cancel out system errors. A scheme for the data processing for on-line calibration of the lateral force is also presented.

Different aspects of the role of wear debris in fretting wear

Two different aspects of the role of oxide wear debris in fretting wear are studied by allowing them to escape from the interface during sliding. This is accomplished by laser surface texturing that forms regular micro-pores topography on the friction surfaces which enables this escape. It is found that the role of oxide wear debris depends on the dominant fretting wear mechanism. Their presence in the interface protects the friction surfaces when the dominant wear mechanism is adhesive and harms the friction surfaces when this mechanism is abrasive. The escape of oxide wear debris into the micro-pores results in up to 84% reduction in the electrical contact resistance of the textured fretting surfaces.

Shearing of fibrillar adhesive microstructure: friction and shear-related changes in pull-off force

To characterize the effect of shearing on function of fibrillar adhesive microstructure, friction and shear-related changes in pull-off force of a biomimetic polyvinylsiloxane mushroom-shaped fibrillar adhesive microstructure were studied. In contrast to a control flat surface, which exhibited pronounced stick–slip motion accompanied with high friction, the fibrillar microstructure demonstrated a stable and smooth sliding with a friction coefficient approximately four times lower. The structured contact also manifested zero pull-off force in a sheared state, while the flat surface exhibited highly scattered and unreliable pull-off force when affected by contact shearing. It appears that the fibrillar microstructure can be used in applications where a total attachment force should be generated in a binary on/off state and, most surprisingly, is suitable to stabilize and minimize elastomer friction.

Spatulate structures in biological fibrillar adhesion

To provide an explanation of why most biological hairy adhesive systems involved in locomotion rely on spatulate structures, we have studied the contact formed between a smooth substrate and individual thin-film terminal elements of attachment pads evolved in insects, arachnids and reptiles. The data obtained were analyzed using the Kendall peeling model, which demonstrated that an animal’s attachment ability grows with an overall length of the peeling line, which is the sum of widths of all thin-film elements participating in contact. This robust principle is found to manifest itself across 8 orders of magnitude in an overall peeling line ranging from 64 micrometres for a red spider mite to 1.8 kilometres for a Tokay gecko, generalizing the critical role of terminal elements in biological fibrillar adhesion.

Mushroom-shaped geometry of contact elements in biological adhesive systems

It is well known that geometry of contact element is an extremely important factor affecting adhesion. Comparing various biological adhesive systems, we find that mushroom-shaped geometry of contact elements is widely spread in nature. Based on experimental data obtained with an artificial model system implementing a mushroom-shaped geometry of contact element, we discuss the principles responsible for particularly strong adhesion in this type of contact. Finally, we draw a general relationship between functional types of different biological adhesive systems and geometry of contact elements.

Close-up of mushroom-shaped fibrillar adhesive microstructure: contact element behaviour

To analyse the performance of mushroom-shaped fibrillar adhesive microstructure, its behaviour was studied during different stages of attachment–loading–detachment cycle. Visualizing the evolutions of real contact area of single microfibres, it is shown that the mushroom-shaped geometry of contact elements promotes fast and simple generation of reliable adhesion. The mushroom-shaped geometry seems to transform fibrillar contact elements into passive suction devices and makes them tolerant to overload, thus enhancing their robustness and stability. These findings may also be extrapolated to biological fibrillar attachment devices sharing the same geometry.

A beetle-inspired solution for underwater adhesion

Glue-free reversible adhesion was achieved underwater using a beetle-inspired mushroom-shaped fibrillar microstructure. Structured surfaces reveal a 25% increase in pull-off force when immersed in water and their underwater attachment is 20 times more effective than that of flat surfaces. The van der Waals interaction that underlies the adhesion of the mushroom-shaped fibrillar microstructure is significantly enhanced by a suction effect when underwater. This results in a higher adhesive capability of the material, with potential in medicine, bio- and marine technologies and a range of applications in liquid-dominated environments.

Effect of real contact geometry on adhesion

The effect of real contact geometry on adhesion was studied by measurement of pull-off force, real contact area, and real contact perimeter of fibrillar and dimpled flat surfaces. The structures, made of polyvinylsiloxane, were brought in contact with a flat glass substrate. These experiments demonstrate that adhesion does not correlate with the real contact area. The real contact perimeter is found to be the main geometrical factor governing adhesion. This results in a natural scaling effect with finer structures exhibiting stronger adhesion.

Tuning elastomer friction by hexagonal surface patterning

Frictional behavior of hexagonal elastomeric surface texture mimicking the pattern evolved in attachment pads of bush crickets has been investigated as a function of aspect ratio and area density of the texture elements. We show that this texture not only stabilizes the sliding behavior of elastomer surface, but also allows tuning its friction force from as low as 50% to nearly 100% of that of unmodified surface by adjusting the aspect ratio of the texture elements. Changing the texture area density does not affect the friction force, which allows choosing this parameter independently to support different normal loads. Both effects are explained using an in situscanning electron microscopy of steady-sliding surfaces.