Measuring DLVO force and surface potential based on AFM colloidal probe technique at liquid-solid interfaces
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摘要: 表面电势是微纳流控芯片中流体流动的重要参数。本文介绍了基于AFM胶体探针技术测量液固界面DLVO力并进一步测量表面电势及表面电荷密度的方法。本文改进了胶体探针制作的技术手段，并提出用双探针法测量胶体探针的弹性系数。在0.1~1mM浓度范围内的NaCl溶液中，测量了硅、二氧化硅和氮化硅液固界面双电层内的DLVO力及表面电势。实验结果表明胶体探针技术可以很好地测量液固界面的DLVO力，尤其对静电力指数变化段非常敏感。通过DLVO力曲线可以间接测量表面电势、表面电荷密度等重要参数，是微纳流动及界面属性测量的有效手段。此外，在不同硅基材料表面的测量结果显示了硅烷醇基密度对表面电势起主导作用，可以通过选用不同硅烷醇基密度的材料来有效调控表面电势，从而在硅基材料制作的微流控芯片中调控电动流动的强弱。Abstract: 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.
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Key words:
- AFM /
- colloidal probe /
- liquid-solid interfaces /
- DLVO force /
- surface potential
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图 4 双探针法测胶体探针弹性系数示意图
Figure 4. Schematic of measuring colloidal probe spring constant based on cantilever to cantilever device
图 5 二氧化硅基底上胶体探针在4种不同浓度氯化钠溶液下的对数坐标力曲线，黑线为连续电势假设下的PB方程拟合线
Figure 5. Force curves between the colloidal probe and silica in different NaCl concentrations. The black curves are fitted based on PB equation under the assumption of constant potential
图 6 二氧化硅表面电势测量结果(红点)及与Ducker(圆圈)和Horn(矩形)在相似测量环境下的结果比较。pC=-lgC(C为溶液浓度)
Figure 6. Comparison of the surface potential on silica (red symbols) between Duckers' (circles) and Horns' (squares) under similar experiment conditions. pC=-lgC.(C is concentration)
图 7 硅、二氧化硅和氮化硅表面电势对比
Figure 7. Comparison of surface potentials on silicon, silica and silicon nitride
表 1 不同氯化钠浓度下的电导率与德拜长度对照表
Table 1. Conductivity and Debye length in different NaCl solutions
Conductivity(μs·cm-1) Concentration/mM Theoretical κ-1/nm Experimental κ-1/nm 11.2 0.09 30 28 35.4 0.28 17 19 94.1 0.76 11 12 115.2 0.93 10 9 
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