Application of Van-der-Waals forces in micro-material handling

[摘要] This doctoral dissertation focuses on the application of Van-der-Waals' forces in micromaterialhandling. A micro-material handling system consists of four main elements, whichinclude: the micro-gripper, the micro-workpart, the picking up position and the placementposition. The scientific theoretical frameworks of Van-der-Waals' forces, presented by Vander Waals, Hamaker, London, Lifshitz, Israelachvilli, Parsegian, Rumpf and Rabinovich, areemployed in exploring the extent to which these forces could be applied in a micromanufacturingsituation. Engineering theoretical frameworks presented by Fearing,Bohringer, Sitti, Feddema, Arai and Fukuda, are employed in order to provide an in-depthsynthesis of the application of Van-der-Waals' forces in micro-material handling. Anempirical or pragmatic methodology was adopted in the research.The Electron Beam Evaporation (e-beam) method was used in generating interactive surfacesof uniform surface roughness values. E-beam depositions of copper, aluminum and silver onsilicon substrates were developed. The deposition rates were in the range of 0.6 – 1.2Angstrom/s, at an average vacuum pressure of 2 x 10-6 mbar. The topographies were analysedand characterised using an Atomic Force Microscope and the corresponding rms surfaceroughness values were obtained. The Rumpf-Rabinovich equation, which gives therelationship of the exerted Van-der-Waals' forces and the rms surface roughness values, isused to numerically model the results. In the final synthesis it is observed that the e-beamdepositions of copper are generally suited for the pick-up position. Aluminum is suited for themicro-gripper and silver is suited for the placement position in an optimised micro-materialhandling system.Another Atomic Force Microscope was used in order to validate the numerically modelledresults of the exerted Van- der-Waals' forces. The aim was to measure the magnitude of Vander-Waals' forces exerted by the e-beam depositions and to evaluate their applicability inmicro-material handling operations. The measurements proved that Van-der-Waals' forcesexerted by the samples could be used for micro-material handling purposes on condition thatthey exceeded the weight of the micro-part being handled.Three fundamental parameters, ie: material type, geometrical configuration and surfacetopography were used to develop strategies of manipulation of micro-materials by Van-der-Waals' forces. The first strategy was based on the material type variation of the interactive surfaces in a micro-material handling operation. This strategy hinged on the fact that materialshave different Hamaker coefficients, which resulted in them experiencing a specific Van-der-Waals' forces' intensity during handling. The second strategy utilised variation in thegeometrical configuration of the interacting surfaces. The guiding principle in this case wasthat, the larger the contact area was, the greater the exerted Van-der-Waals' forces would beIn the analytical modelling of Van-der-Waals' forces with reference to geometricalconfiguration, a flat surface was found to exert more force than other configurations. Theapplication of the design, for purposes of manufacturing and assembling (DFMA) criteria,also proved that flat interactive surfaces have high design efficiency. The third strategy wasbased on surface roughness. The rougher the topography of a given surface was, the lesser theVan-der-Waals' forces exerted were. It was synthesised that in order for a pick-transfer-placecycle to be realised, the root-mean-square (rms) interactive surface roughness values of themicro-part (including the picking position, the micro-gripper, and the placement position)should decrease successively. Hybrid strategies were also identified in this research in orderto deal with some complex cases. The hybrids combined at least two of the aforementionedstrategies.

[发布机构] Stellenbosch University