Nano-adhesion

Fig. 5.3 Force-distance curve obtained by AFM (nano-adhesion). Fadhesion = K D, whete Fadhesion = adhesion force D=maxi-mum deflection fC=calibration constant.

Nanoscale measurements have been performed by AFM in contact mode. For the nano-adhesion measurement, force-distance curves have been obtained (Fig. 5.3). The maximum cantilever deflection D during separation is directly proportional to the adhesion force. 

For PDMS 6, the deflection D was quite constant before and after extraction and was close to 70 nm (with AD=5 nm). The deflection value did not depend on the contact force for PDMS 6. Measurements were not possible with PDMS 17 before extraction because the AFM tip could not be separated from the PDMS surface within the measurable cantilever deflection range. Hence the adhesion value could not be determined, but it has to be considered as important. After extraction of free chains (PDMS 17 ), measurements were possible and the deflection value D obtained was close to 500 nm (with AD=30 nm). Hence, in correlation with the macro-adhesion results, the nano-adhesion of PDMS 17 was much greater than that of PDMS 6. 

However, the nano-adhesion of PDMS 17 decreased after extraction. This result is the opposite of that for macro-adhesion, where an increase in adherence was observed after extraction. Important adsorption phenomena of the numerous and long free chains of PDMS 17 on the AFM tip could explain the higher nano-adhesion observed before extraction. The mobility of these free chains (greater than of pendant chains) allows a better adsorption on the tip, avoiding the separation (with the same experimental device, i.e., cantilever stiffness). 

NANO-ADHESION JOINING MATERIALS FOR ELECTRONIC APPLICATIONS... 

A fully automated microscale indentor known as the Nano Indentor is available from Nano Instmments (257—259). Used with the Berkovich diamond indentor, this system has load and displacement resolutions of 0.3 N and 0.16 nm, respectively. Multiple indentations can be made on one specimen with spatial accuracy of better than 200 nm using a computer controlled sample manipulation table. This allows spatial mapping of mechanical properties. Hardness and elastic modulus are typically measured (259,260) but time-dependent phenomena such as creep and adhesive strength can also be monitored. 

Kwon K, Kidoaki S, and Matsuda T. Electrospun nano- to microfiber fabrics made of biodegradable copolyesters Structural characteristics, mechanical properties and cell adhesion potential. Biomaterials, 2005, 26, 3929-3939. 

There are a number of industrial and technological areas in which nanoscale adhesion is important. One of the earliest fields concerned with adhesion on this scale was colloid science. Colloid particles lie in the intermediate region between macro and nano, with dimensions typically of the order of hundreds of nanometers up to a few microns. This means that their true contact areas he well within the nano-domain and are influenced by interactions on this length scale. Adhesion between such particles is important, due to its influence on mineral separation processes and on the aggregation of powders, for example, on the walls of machinery or in the forming of medical tablets. In an extraterrestrial context, such... 

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