Study On Inner Surface Potential Modulation Of The Electric Double Layer Based On AFM Colloid Probe Technique

In recent years,micro and nano technology has a wide range of applications in many areas related to people's livelihood.Microfluidic technology,as an important branch of micro and nano technology,also plays an important role,especially in the fields of Biochemistry,medicine,environment and energy.With the further reduction of the size of the micro fluid,the flow control has attracted much attention.In order to realize the control of nano scale,the surface charge in the double layer of liquid solid interface plays an important role in the flow control.The energy conversion in the flow control can be realized by controlling the surface charge flow in the eletric double layer.A novel idea is to control the surface charge by applying a polarized external voltage,outside the wall insulating layer,to release the charge bound on the wall.Therefore,it is necessary to study the influence factors of the surface potential,the external conditions of modulating surface potential and develop a reliable method to measure the surface potential.The main contents of this paper include:(1)The theoretical model of the surface potential modulation under the polarization voltage is established.The simulation results show that the surface potential can be effectively controlled by the polarization voltage.Surface potential regulation range decreases as the surface hydroxyl group density increases and the pH value increases.The surface potential modulation of silicon is more efficient than that of silicon nitride.(2)A method is developed to measure the surface potential and surface charge density based on the DLVO force obtained by colloidal probe technique using atomic force microscope(AFM).The DLVO forces and surface potentials of silicon,silica and silicon nitride substrates is measured in 0.1 mM ~ 1 mM NaCl solutions respectively.The results show that the method precision of measuring DLVO force based on the colloidal probe technique can reach 10-11 N.The electrostatic force is especially sensitive away from the interface in 100 nm.And the sensitivity is up to 0.15 nN / nm.The greater the concentration,the smaller the absolute value of surface potential.Additionally,the variation of surface potentials at different silicon-based surfaces show that the surface potential of silicon and silica is close in the same conditions,and the silicon nitride is smaller about 10~20 mV.(3)The surface potential of silicon and silicon nitride is controlled by polarized external voltage.The results show that the surface potential of liquid-solid interface can be effectively modulated by polarization voltage and the experimental results are in agreement with the simulation results.The higher the solution concentration is,the lower the surface potential modulation efficiency is.The main reason is that the higher the concentration,the greater the capacitance of the double layer,and the greater the applied voltage if the same surface charge density needs to be changed.Compared with the same concentration of salt and acid solution,the surface potential modulation efficiency in acidic solution is higher.The main reason is that the surface of silicon based material is negatively charged by the ionization of silanol groups.The surface potential is determined by the adsorption of hydrogen ions.The adsorption of cations in salt solution is much weaker than that of hydrogen ions.In summary,the DLVO force and the surface potential in the double layer of the liquid solid interface can be measured effectively based on the AFM colloid probe technique.The surface potential in microfluidic can be modulated effectively by external voltage,solution concentration,pH and different substrate materials,so as to adjust the electrokinetic flow.The results will be helpful to elucidate the mechanism of energy conversion and provide experimental basis for the study.