INTRODUCTION: Thin-film-ended hairy adhesive structures have evolved independently in insects, arachnids and reptiles to secure their locomotion on substrates of arbitrary orientation, geometry and chemical composition [2]. After it was revealed that terminal thin-film elements operate using intermolecular and capillary forces, which allowed to replicate these structures with engineering materials, much effort has been put in the development of biomimetic adhesive surfaces [3, 4]. Consequently, this led to the introduction of mushroom-shaped microstructures and these microstructures remain the most thoroughly studied to date, due to the well-established manufacture procedures [5, 6]. Besides the mushroom-shaped terminal elements, the second type of adhesive hairs, which is based on spatulate geometry, is characteristic of all hairy attachment pads regardless of the animal group. The spatula-shaped adhesive hairs are known for their ability to be actuated by contact shearing and to be easily released by peeling, which can make them useful in applications requiring dynamic short-term attachment [7]. To date, a handful of different designs inspired by spatula-shaped adhesive hairs have been suggested based on wedge and flap geometry of contact elements [8-10]. These adhesive surface designs had been studied [8-11] but an experimental verification of the very basic concept of the pulling angle effect [7, 12] has not yet been reported. To bridge this gap, here we use wall-shaped adhesive microstructures [11] of three different flap heights to systematically study the effect of pulling angle on the normal and tangential components of the pull-off force tested at different preliminary tangential displacements.
METHODS: Micro-structured surfaces were fabricated by using polyvinylsiloxane (PVS; Coltène Whaledent, Altstätten, Switzerland; Young’s modulus of about 3 MPa [13]) from a tungsten sheet of 0.15 mm in thickness prepared by laser micro-machining (Oxford Lasers, Shirley, MA). Three different heights of 140, 100 or 70 μm (high, medium and low flaps) were obtained by varying waiting time prior to pouring PVS on the template, and then the mold was gently released from the template. A glass slide of 30×5×1 mm in size was used as a counter surface. Tested surfaces were imaged in a Quanta 250 environmental scanning electron microscope (SEM; FEI, Brno, Czech Republic). The tests were conducted in a custom-built tribometer [14] that can also operate inside the SEM. A monochrome digital camera DMK 23UP1300 (Imaging Source, Charlotte, NC) mounted on a high-magnification optical lens Zoom-12X (Navitar, Rochester, NY) was used to capture images of the real contact area. After mounting a structured sample on the tribometer in perpendicular direction to the sliding/pulling, the glass slide was moved in perpendicular to the contact plane until a normal load of 10 mN was achieved. Then, the glass slide was moved in parallel to the contact plane under the same normal load and at a speed of 100 μm/s for a designated preliminary distance of 0 ~ 500 μm. Next, the glass slide was withdrawn from the contact at the speed of 100 μm/s and at a designated pulling angle of 10˚ ~ 170˚ at intervals of 10˚. The test stopped at a complete detachment of the glass slide from the structured sample. The temperature and relative humidity in the laboratory were 25-27 °C and 54-55 %, respectively.
RESULTS & DISCUSSION: Several examples of normal and tangential forces recorded during the tests are shown in Fig. 1 as a function of tangential displacement. The curves that split from the normal and tangential force envelops correspond to the forces measured during the withdrawal stage after three characteristic preliminary displacements in Fig. 1a and after three characteristic