Molecular-scale change in the surface properties of silica and copper in pure water with time

Effect of Audible Sounds on the Forces Acting between Charged Surfaces in Water

We aimed to determine if audible sounds could change the forces acting between charged surfaces in water and their electric double layers (EDLs). This was achieved by using an atomic force microscope to measure force-distance curves between a microsized silica particle attached to a cantilever (probe) and a silicon wafer in water in the absence and presence of sound. Sound decreased the repulsive forces acting between the probe and silicon wafer, where the range and magnitude of the forces decreased with an increase in the sound frequency from 300 to 15000 Hz. The decrease in the force range was explained by a decrease in the EDL thickness. This result was explained by (1) the shrinkage of the EDL by a high-pressure region of the sound wave, where an increased sound frequency caused the number of high-pressure regions that passed between the probe and the substrate to increase and (2) the inability of the EDL to fully re-expand to its original thickness during the time that a low-pressure region of the sound wave was applied. The decrease in the force magnitude with a sound frequency increase was explained by the increased screening of charged surfaces that accompanies a decrease in the EDL thickness. An increase in the force measurement speed caused the sound waves to reduce the repulsive forces less. A faster speed decreased the time to measure a force curve, which reduced the number of high-pressure regions of the sound wave to pass through the water between the probe and the substrate. This reduced the number of times that the EDL was compressed by the sound wave.

Time Dependence of Silica Surfaces on Their Interactions in Water and Alkaline Solutions

Spherical silica particles are widely used to study the stability of colloids. This is because the stability of silica particles is important in industrial processes and also because monodispersed particles are available in various sizes, allowing fundamental investigations on colloidal stability to be performed. However, it is often assumed that the surface properties of silica do not change during the measurements, e.g. for 60 min. In the present study, we used the atomic force microscope to determine the validity of this assumption by studying the change in the surface microstructure of silica with its immersion time in water and alkaline solutions. The microstructure of silica surfaces was found to change with the immersion time, even in water. This change was especially large for solutions of high pH. These changes are explained in terms of the hydrogen bonds between the silanol groups protruding from the silica surfaces, whose length depends on the solution pH and the immersion time.

Effect of the charge and roughness of surfaces on normal and friction forces measured in aqueous solutions

We used the atomic force microscope (AFM) to determine how the roughness and charge on a surface affect the adhesion and friction when measured against a smooth surface (colloid probe) in an aqueous solution. The effect of roughness was investigated by coating TiO2 crystal substrates with TiO2 nano- or micro-sized particles, where an increase in the particle size increased the RMS roughness of the substrate. The charge of the substrate was varied by changing the pH of the aqueous solution. Force-separation curves and friction-load data were measured for the smooth colloid probe-rough substrate systems. The adhesion and friction between two surfaces in solution were seen to depend on the surface charge and roughness. A noncharged surface gave the greatest adhesion, while a charged surface gave weaker adhesions. Increasing the roughness of the surface resulted in a stronger adhesion. The magnitude and range of the adhesions were not affected by the measuring velocity in the case of a noncharged substrate but decreased with an increasing velocity for charged surfaces. The friction was seen not to depend on roughness in the case of a noncharged surface. However, in the case of a charged surface, the friction decreased with an increased roughness for low loads and then showed no dependence on the surface roughness for high loads. The results of this experiment show that the adhesion and friction of a system can be decreased via the roughness and charge of the substrate and the ion types in the solution.