Friday, 14 July 2006
110-3

The Forces, in Nanoscale, Involved in Adhesion of Soil Particles.

Fabio de L. Leite, Institute of Physics of São Carlos, University of São Paulo (USP), Av. Trabalhador Sãocarlense, 400, P.O. Box 369, Zip Code: 13560-970, São Carlos, SP, Brazil, Paulo Sergio de P. Herrmann, Embrapa Agricultural Instrumentation, Rua XV de Novembro 1452, P.O. Box 741, Zip Code 13560-971, São Carlos, Brazil, and Ervino C. Ziemath, Physics Department, IGCE, Universidade Estadual Paulista, P.O. Box. 178, Cip Code: 13500-970, Rio Claro, SP, Brazil.

The Atomic Force Microscopy (AFM) was used to investigate adhesion force interactions at the nanoscale level between solid surfaces through force curve measurement in air and water. The basic principle of operation is presented in the sequence. The microscope scans over the sample surface with a sharp tip, situated at the apex of a flexible cantilever that is often diving board-shaped or V-shaped and normally made of silicon. The AFM utilizes a piezoelectric scanner that moves the sample with a subnanometer displacement when a voltage is applied. AFM measurements were carried out using a Topometrix, Discoverer TMX 2010, in standard contact mode. V-shaped cantilevers (200 microm long) were used with nominal spring constants of about 0.13 +- 0.01 N/m, Si3N4 pyramidal tips, tip curvature radius 23 +- 5 nm and the images was obtained in a scan rate of 1.50Hz. Adhesion forces were carry out in Milli-Q; water, with a special cell developed by Topometrix. The characterization of adhesion forces in solids was obtained throughout 20 measurements at the same point, made in 5 distinct points of the same region, and in 3 different regions of the samples. With that it was possible to explore the variability of the adhesion forces on the surface, showing that a nano-adhesion varies with both the topography and the local physical conditions of the substrates. Curve force measurements acquired in air quantified the adhesion forces due to the capillary effect coming from the adsorbed liquid layer on the sample surface. The average values of adhesion force, in air, to quartz, silicon and mica were 20nN, 23nN and 32 nN, respectively. The measurement detected only the adhesion force due to van der Waals forces, presenting the following values: 6.0 nN (quartz), 6.0 nN (silicon) and 1.0 nN (mica). The figure 1 is showing a comparison between capillary and van der Walls forces, in silicon, quartz and mica. Adhesion maps were used to characterize the heterogeneity of atomically planar surfaces (mica) and rough surfaces (quatz). This investigation revealed that at the nanoscale the ambient condition, and not the material properties, have a important role in the adhesive interaction between solid surfaces in the materials investigated. Figure 1 – Comparison between capillary and van der Walls forces in nN (The axis X of the figure is the adhesion force), in different substrates.


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