The handling of micro components continues to pose a challenge in handling technology. Spherical geometries in particular make gripping more difficult, as if the sphere is imperfectly adapted to its size, it only comes into contact with the gripper at a point. This results in high surface pressures, which further increase the risk of damage. In addition, micro-components have a large surface area compared to their volume. As a result, their weight may not be sufficient to overcome the adhesive forces and they may not detach from the gripper. Research is therefore being carried out in the field of microhandling to find suitable alternatives to conventional grippers. One way of overcoming these obstacles is to use adhesive grippers. Since the turn of the millennium, great progress has been made in this area in bionics, which attempts to imitate the adhesive microstructures of living organisms. A subgroup of these structures are the mushroom-head-shaped adhesive microstructures, which are inspired by the adhesive organs of beetles. In contrast to conventional adhesive products, these are not adhesives in the original sense, as they do not require any physical setting or chemical curing. Commercial products based on this technology exist, which are often simply referred to as gecko film.

The use of gecko film for gripping microspheres reduces the risk of damage, but at the same time poses new challenges. Due to the spherical shape, the number of adhering microstructures cannot be determined from the contact surface between the gecko film and the component as is the case with a flat component. However, the force at which the microsphere detaches from the gecko film is essential for the design of a handling system. On the one hand, the ball should be held and transported securely, but on the other hand, it must also be possible to release it again. In order to gain insights into the suitability of a gripper with regard to these criteria before it is developed, a model of the microsphere and microstructure system is required that can be used to predict the contact behavior between microstructures and microspheres.

Simulation determines the required force

A simulation model developed at the bime makes it possible to calculate the force required to detach microspheres of different sizes from a gecko film. It is also possible to determine the required indentation force at which the maximum adhesive force is reached. The simulation is based on modeling the mushroom-head-shaped adhesive elements as independent springs.

Validation can be carried out by comparing the simulation results with experimental observations. For this purpose, an experimental setup is constructed with which the number of adhering elements can be determined at a defined penetration depth. The functional principle of the setup is the observation of the mushroom-head-shaped microstructure while it is in contact with a microsphere. Adherent and non-adherent mushroom heads can be distinguished by their shape and brightness. These are caused on the one hand by the change in the refractive index at the interface between the sphere and the mushroom head and on the other hand by the stretching, compression and bending of the adhesive elements. The comparison of the simulation results with the measurements carried out on the experimental setup shows good agreement with regard to the number of microstructure elements adhering in the equilibrium state. It also shows that increasing the indentation force from a limit value that varies depending on the ball size does not lead to any further increase in the required release force.