Andrei Peressadko

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.

WHEN LESS IS MORE: EXPERIMENTAL EVIDENCE FOR TENACITY ENHANCEMENT BY DIVISION OF CONTACT AREA

Most recent data on hairy systems demonstrated their excellent adhesion and high reliability of contact. In contrast to smooth systems, some hairy systems seem to operate with dry adhesion and do not require supplementary fluids in the contact area. Contacting surfaces in such devices are subdivided into patterns of micro- or nanostructures with a high aspect ratio (setae, hairs, pins). The size of single points gets smaller and their density gets higher as the body mass increases. Previous authors explained this general trend by applying the JKR theory, according to which splitting up the contact into finer subcontacts increases adhesion. Fundamental importance of contact splitting for adhesion on smooth and rough substrata has been previously explained by a very small effective elastic modulus of the fibre array. This article provides the first experimental evidence of adhesion enhancement by division of contact area. A patterned surface made out of polyvinylsiloxane (PVS) has significantly higher adhesion on a glass surface than a smooth sample made out of the same material. This effect is even more pronounced on curved substrata. An additional advantage of patterned surfaces is the reliability of contact on various surface profiles and the increased tolerance to defects of individual contacts.

Material structure, stiffness, and adhesion: why attachment pads of the grasshopper (Tettigonia viridissima) adhere more strongly than those of the locust (Locusta migratoria) (Insecta: Orthoptera)

The morphology, ultrastrucure, effective elastic modulus, and adhesive properties of two different smooth-type attachment pads were studied in two orthopteran species. Tettigonia viridissima (Ensifera) and Locusta migratoria (Caelifera) have a similar structural organization of their attachment pads. They both possess a flexible exocuticle, where the cuticular fibrils are fused into relatively large rods oriented at an angle to the surface. The compliant material of the pad contributes to the contact formation with the substrate. However, the pad material structure was found to be different in these two species. L. migratoria pads bear a thick sub-superficial layer, as well as a higher density of rods. The indentation experiments showed a higher effective elastic modulus and a lower work of adhesion for L. migratoria pads. When the indentations were made at different depths, a higher effective elastic modulus was revealed at lower indentation depths in both species. This effect is explained by the higher stiffness of the superficial pad layer. The obtained results demonstrate a clear correlation between density of the fibres, thickness of the superficial layer, compliance of the pad, and its adhesive properties. Such material structures and properties may be dependent on the preferred environment of each species.

A Robot that Climbs Walls using Micro-structured Polymer Feet

Insect-inspired foot materials can enable robots to walk on surfaces regardless of the direction of gravity, which significantly increases the functional workspace of a compact robot. Previously, Mini-Whegs™, a small robot that uses four wheel-legs for locomotion, was converted to a wall-walking robot with compliant, conventional-adhesive feet. In this work, the feet were replaced with a novel, reusable insect-inspired adhesive. The reusable structured polymer adhesive has less tenacity than the previous adhesive, resulting in less climbing capability. However, after the addition of a tail, changing to off-board power, and widening the feet, the robot is capable of ascending vertical surfaces using the novel adhesive.

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.

Advanced testing of adhesion and friction with a microtribometer

To optimize the tribological performance of miniaturized systems, adequate experimental means are needed. Current microtribometers suffer from inability to ensure parallel flat-on-flat contact and often experience metrological problems related to the geometry of the force sensing element. The present work demonstrates how these limitations can be removed in either homemade or commercial microtribometers by using a self-aligning system of specimen holders and an improved force sensor based on a symmetric design principle. Preliminary tests conducted on a polyvinylsiloxane∕glass tribopair confirm the satisfactory operation of the setup and show no effect of applied load on pull-off force and friction coefficient.