Measuring DLVO force and surface potential based on AFM colloidal probe technique at liquid-solid interfaces

Surface potential is an important parameter of the fluid flow in microfluidics. This study develops a method to measure the surface potential and surface charge density based on the DLVO force obtained by colloidal probe technique using atomic force microscope (AFM). A novel procedure of colloidal probe fabrication is proposed, and then a cantilever-to-cantilever calibration method is used to determine the spring constant of the colloidal probe. We thus measure DLVO forces and surface potentials of silicon, silica and silicon nitride substrates, in 0.1 mM to 1 mM NaCl solutions respectively. The results show that our approach could well measure the DLVO forces at the liquid-solid interfaces, which is especially sensitive to the exponential variation of the electrostatic force. The surface potential, the surface charge density and other important parameters can be obtained via fitting the force curves. Additionally, the variation of surface potentials at different silicon-based surfaces indicates that the density of silane groups plays a dominant role on the surface potential. Thus it is possible to effectively control the surface potential by changing the silane density of the silicon-based materials. The findings could be valuable to regulating electro kinetic flow intensity in microfluidic chips.