CFCH-LB films

LB films were also formed on the silicon plate after the asperity array was processed by FIB. Before depositing the LB film, we cleaned the silicon wafers in a mixture of benzene and ethanol, rinsed them in pure water, and then exposed them to a UV-ozone atmosphere. Then the plate was immersed in ultrapure water where a monolayer of stearic acid (Ci7H35COOH CH) or fluorocarboxylic acid (QFi3CiiH22COOH CFCH) was confined at a pressure of 30 mN/m. The monolayer on the water migrated onto the silicon surface and formed the LB film (CH-/CFCH-LB film) when the plate was removed from the ultrapure water. The temperature of the ultrapure water was 20°C. Table 2.3 shows the chemical modifications and the geometries of the asperity. 

Figure 2.25 shows the pull-off force measured for the asperity arrays covered with two kinds of LB films. The curvature radius on the jc-axis was shown in table 2.3. The data for each plate with CHCCnHjjCOOHl-LB or CFCH(C6F,3C H22COOH)-LB film were fitted with a line passing through the origin. The pull-off force decreased with smaller curvature radius and was roughly proportional to the curvature radius. The pull-off force on the CH-LB film was about l/5th of the pull-off force on the CFCH-LB film for the same curvature radius. 

Figure 2.26 shows the pull-off forces on two kinds of asperity arrays as a function of the relative humidity. The average curvature radius of each asperity array was 150 and 440 nm for the CH-LB film and 95 and 370 nm for the CFCH-LB film. Each plot in fig. 2.26 is the average of 256 pull-off force measurements. The error bar shows the standard deviation for each data point. The average pull-off force clearly increased with higher relative humidity for the asperity array of 370-nm radius with the CFCH-LB film... 

FIG U RE 2.26 Relation between pull-off forces and relative humidity measured on asperity arrays. Each plot shows the average of pull-off forces from 256 measurements on (a) CH-LB film and on (b) CFCH-LB film. Two asperity arrays were selected from each plate, and their curvature radii are shown in the inset boxes. 

Assuming that the CFCH-LB film had lower stiffness than the CH-LB film, the real contact area on the asperities with the CFCH-LB film was larger than that for the CH-LB film. Thus, the CFCH-LB film would exhibit a higher friction force than... 

Cq), the friction coefficients for the SAM-coated plates were much lower than that for the uncoated plate. The molecular layer probably prevented a direct contact between solids. The friction coefficients for the SAM-coated plates C14 and Cjg were about half that for the other SAM-coated plates. Some reports showed that the friction coefficient was inversely correlated with the alkyl-chain length of the SAM [28, 29]. Similar to the difference in the friction coefficients between CH-LB and CFCH-LB films, the difference in stiffness of the SAMs also might affect the friction force. The stiffness of a SAM is probably correlated with the alkyl-chain length because the van der Waals interaction between alkyl chains increases with longer chain length. The real contact area where the friction force operates is inversely correlated with the stiffness. Based on these assumptions, a longer alkyl chain tends to show a lower friction coefficient. 

FIGURE 2.25 Relation between the pull-off force and the curvature radius of the asperity peaks for silicon plates coated with CHlCpHjjCOOHl-LB and CFCH(C6Fi3CuH22COOH)-LB films. 

FIGURE2.30 Friction force vs. pull-off force measured on asperity arrays on silicon plates coated with CH(C H35COOH)-LB and CFCH(C F,3CiiH22COOH)-LB films. Measured data were fitted with a line that passes through the origin. The gradient of each approximated line is shown in the parentheses in the inset box. 

FIGURE 2.31 Friction coefficients for an uncoated plate (Cq) and SAM-coated plates (C -Cig) and for plates with LB films (CH-LB and CFCH-LB). Friction coefficients were calculated from the slopes of fitted lines in figs. 2.28 and 2.30. 

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