Capillary-force measurement on SiC surfaces

Capillary forces have been measured by atomic force microscopy in the sphere-plate geometry, in a controlled humidity environment, between smooth silicon carbide and borosilicate glass spheres. The force measurements were performed as a function of the rms surface roughness ∼4–14 nm mainly due to sphere morphology, the relative humidity (RH) ∼0%–40%, the applied load on the cantilever, and the contact time. The pull-off force was found to decrease by nearly two orders of magnitude with increasing rms roughness from 8 to 14 nm due to formation of a few capillary menisci for the roughest surfaces, while it remained unchanged for rms roughness <8 nm implying fully wetted surface features leading to a single meniscus. The latter reached a steady state in less than 5 s for the smoothest surfaces, as force measurements versus contact time indicated for increased RH∼40%. Finally, the pull-off force increases and reaches a maximum with applied load, which is associated with plastic deformation of surface asperities, and decreases at higher loads.

Figure 1

AFM topography and height distribution of the borosilicate spheres roughness obtained by scanning ($800\times 800\mathrm{n}{\mathrm{m}}^2$ scan size) on top of the sphere (right) and top view scanning electron microscopy (SEM) image of a sphere attached on a cantilever (left).

Figure 2

(a) AFM topography and height distribution of the SiC sample ($4\times 4\mu {\mathrm{m}}^2$ scan size). (b) Height distribution and distance upon conduct $d_{\mathrm{o},\mathrm{SiC}}$ due to SiC surface.

Figure 3

Schematic representation of a force-distance curve. Red line represents approach and blue line retraction.

Figure 4

Contact angle measurement of a water droplet on the SiC surface.

Figure 5

(a) Schematic of water meniscus between a sphere and a plate. (b) Schematic of capillary condensation between rough surfaces, where menisci form at contact of asperities.

Figure 6

Force measurements versus rms surface roughness of both sphere and sample (in air) using spheres with varying roughness as the AFM inspection also indicated. The theory prediction via Eq. (1) for the upper (using the sphere tip radius $R=10\mu \mathrm{m}$) and lower (using an asperity radius $R_{\mathrm{sph}}\approx 135\mathrm{nm}$) force limits (horizontal lines).

Figure 7

Adhesion force measured as a function of the contact time at different relative humidity for (a) smoothest sphere, and (b) roughest sphere. The inset shows the capillary force versus piezo speed for two different values of RH as indicated.

Figure 8

Adhesion force measurements as a function of applied load (with the maximum indicated) at different relative humidities for (a) smoothest sphere, and (b) roughest sphere.