The fundamental interactions between deposits and surfaces at nanoscale using atomic force microscopy
Akhtar, Noreen (2010)
Ph.D. thesis, University of Birmingham.
The objective of this research was to investigate adhesion of different fouling deposits with different contact surfaces using atomic force microscopy (AFM). In this thesis, AFM has been employed to measure:
(i) The adhesive interactions between a colloidal silica microparticle and stainless steel, PTFE-coated stainless steel, glass and ceramic surfaces, in the presence of a number of solutions and suspensions of ingredients found in commercially available toothpaste.
(ii) To compare the measurements from the AFM and micromanipulation to see the differences and similarities. The micromanipulation technique was developed to measure the adhesive strength of different deposits. The method uses a T-shaped probe made of stainless steel chip, dimension 30 x 6 x 1 mm connected to the output aperture of a transducer (Model BG-1000, Kulite Semiconductor, Leonia, NJ. USA) which was itself mounted on a three dimensional micromanipulator (MicroInstruments, Oxon, UK). The two measurement methods are capable of giving quantitative results for the strength of the forces involved in adhesion; fast moving consumer goods (FMCG) deposits, toothpaste and confectionary stimulant deposits have been studied, and their interactions with stainless steel, glass and PTFE surfaces measured.
(iii) Further investigation of AFM adhesion measurements, with caramel, whey protein and sweet condensed milk (SCM) deposits after heating at 30ºC, 50ºC, 70ºC, and 90ºC. The two selected spherical microparticles used were stainless steel and PTFE, which were attached to the end of an AFM tip.
The data shows that, for removal in all cases using micromanipulation, the pulling energy increases with increasing height above the surface and the slope of the lines of pulling energy versus thickness is similar. Stainless steel shows the highest pulling energy with slightly higher energies than glass and PTFE, whilst PTFE show the lowest interaction. For the AFM data, PTFE again gives much lower adhesion forces. This is due to the different molecular interactions between different surfaces and caramel. There is thus partial agreement between the two methods. The micromanipulation method measures a range of parameters – such as the deformation and flow of the deposits, and so it might not be expected that there would be complete agreement. Here stainless steel and glass show very similar behaviour, as opposed to the differences seen using AFM; the different surface roughness of the two materials might also be expected to have an effect.
At different temperatures the results from the different contact positions on the deposits; with an approach speed to the deposits for all experiments was 3μm/s, then a 5 second pause on the deposit and then the rate of retract was 0.25μm/s. Significant (more than an order of magnitude) differences are seen between forces for the same and different deposits, and between different surfaces for the same deposits. Lower forces are seen at 90ºC in all cases; at the higher temperature, the force between surface and deposit is less. To design systems to resist fouling, these results suggest that measurements at different process temperatures are needed; data at room temperature has overpredicted the interactions.
The results suggest that the AFM force curve measurement technique could be used to study a variety of food deposits that have undergone different processing conditions. The method can help in optimising removal of food deposits in terms of food cleaning protocols. AFM could be a valuable technique in measuring surface properties, and in relating behaviour to surfaces.
The capability of the AFM to provide better understanding of materials structure, surface characteristics and the interactive forces at the meso- and nanoscale level. The AFM will enhance the understanding of large-scale engineering processes, especially as materials are increasingly being designed down to the submicrometre level.
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