TRIBOLOGICAL PROPERTIES AND MODELING OF LIQUID THIN FILM

利用新型纳米流变仪研究了弯月形液桥的静态和动态特性。在两个直径为20μm的硼硅酸盐玻璃球之间建立了液体纳米弯桥，并进行了两种实验：1）OMCTS润湿的两个玻璃球之间的拉脱力和吸引力（八甲基环四硅氧烷）分别作为后退和接近速度的函数进行测量，并将拉脱力与理论值进行比较; 2）对两个带有PFPE（Perfluoropolyether）Zdol弯月形桥的玻璃微球进行了正态振荡实验。实验结果表明：（1）在很慢的后退或接近速度下，所测得的拉脱力或吸引力几乎为常数。而当流速大于2.70μm/s时，由于粘性力开始作用，测得的力偏离常值。2)对于直径为20μm的小基底，出现了源于液体弯月面桥弹性的新动力学行为：系统的弹性分量与代表粘弹性固体特性的调制频率无关;系统在高频下由粘性分量主导，在低频下由弹性分量主导;在高频率和小分离下，液体表现为具有恒定粘度的牛顿流体。另一方面，在低频和大分离下，系统的动态粘度增强。这与表面力仪在浸水或浸没条件下的实验结果有很大的不同。

Static and dynamic behavior of liquid meniscus bridges have been investigated with newly developed nanorheometer. The liquid nanomeniscus bridges were established between two borosilicate glass spheres of 20μm in diameter and two kinds of experiment were performed : 1) The pull-off and attracting forces between two glass spheres wetted by OMCTS (Octamethylcyclotetrasiloxane) were measured as a function of receding and approaching velocity respectively and the pull-off force is compared with the theoretical value ; 2) Normal oscillatory experiment between two glass spheres with meniscus bridge of PFPE (Perfluoropolyether) Zdol was conducted. Results of each experiment are as follows : 1) The measured pull-off or attracting forces show almost constant value at very slow receding or approaching velocity. Whereas, the measured forces deviate from the constant value at higher velocity than 2.70μm/s because the viscous force may begin to operate. 2) With such small substrates of 20μm in diameter, new dynamic behavior originated from the elasticity of the liquid meniscus bridge emerges : The elastic components of the system are independent of the modulation frequency representing the characteristic of vicoelastic solid ; The system is dominated by the viscous component at high frequency and the elastic component at low frequency ; At high frequencies and small separations, the liquid behaves as a Newtonian fluid with a constant viscosity. On the other hand, at low frequencies and large separations, the dynamic viscosity of the system is enhanced. These are quite different from the experimental results of the surface forces apparatus in immersed or submerged condition.