Thesis (PhD)--Stellenbosch University, 2012. / This doctoral dissertation focuses on the application of Van-der-Waals’ forces in micromaterial
handling. A micro-material handling system consists of four main elements, which
include: the micro-gripper, the micro-workpart, the picking up position and the placement
position. The scientific theoretical frameworks of Van-der-Waals’ forces, presented by Van
der Waals, Hamaker, London, Lifshitz, Israelachvilli, Parsegian, Rumpf and Rabinovich, are
employed in exploring the extent to which these forces could be applied in a micromanufacturing
situation. Engineering theoretical frameworks presented by Fearing,
Bohringer, Sitti, Feddema, Arai and Fukuda, are employed in order to provide an in-depth
synthesis of the application of Van-der-Waals’ forces in micro-material handling. An
empirical or pragmatic methodology was adopted in the research.
The Electron Beam Evaporation (e-beam) method was used in generating interactive surfaces
of uniform surface roughness values. E-beam depositions of copper, aluminum and silver on
silicon substrates were developed. The deposition rates were in the range of 0.6 – 1.2
Angstrom/s, at an average vacuum pressure of 2 x 10-6 mbar. The topographies were analysed
and characterised using an Atomic Force Microscope and the corresponding rms surface
roughness values were obtained. The Rumpf-Rabinovich equation, which gives the
relationship of the exerted Van-der-Waals’ forces and the rms surface roughness values, is
used to numerically model the results. In the final synthesis it is observed that the e-beam
depositions of copper are generally suited for the pick-up position. Aluminum is suited for the
micro-gripper and silver is suited for the placement position in an optimised micro-material
handling system.
Another Atomic Force Microscope was used in order to validate the numerically modelled
results 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 in
micro-material handling operations. The measurements proved that Van-der-Waals’ forces
exerted by the samples could be used for micro-material handling purposes on condition that
they exceeded the weight of the micro-part being handled.
Three fundamental parameters, ie: material type, geometrical configuration and surface
topography 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 materials
have different Hamaker coefficients, which resulted in them experiencing a specific Van-der-
Waals’ forces’ intensity during handling. The second strategy utilised variation in the
geometrical configuration of the interacting surfaces. The guiding principle in this case was
that, the larger the contact area was, the greater the exerted Van-der-Waals’ forces would be
In the analytical modelling of Van-der-Waals’ forces with reference to geometrical
configuration, a flat surface was found to exert more force than other configurations. The
application of the design, for purposes of manufacturing and assembling (DFMA) criteria,
also proved that flat interactive surfaces have high design efficiency. The third strategy was
based on surface roughness. The rougher the topography of a given surface was, the lesser the
Van-der-Waals’ forces exerted were. It was synthesised that in order for a pick-transfer-place
cycle to be realised, the root-mean-square (rms) interactive surface roughness values of the
micro-part (including the picking position, the micro-gripper, and the placement position)
should decrease successively. Hybrid strategies were also identified in this research in order
to deal with some complex cases. The hybrids combined at least two of the aforementioned
strategies.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/71608 |
| Date | 12 1900 |
| Creators | Matope, Stephen |
| Contributors | Van der Merwe, Andre F., Stellenbosch University. Faculty of Engineering. Dept. of Industrial Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 144 p. : ill. |
| Rights | Stellenbosch University |
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