EEFs

Notiee that the right-hand side of this equation is a funetion of r only it eontains no 0 or ( ) dependenee. Eefs eall the entire right hand side F(r) to emphasize this faet. 

In the past decade, effects of an EEF on the properties of lubrication and wear have attracted significant attention. Many experimental results indicate that the friction coefficient changes with the intensity of the EEF on tribo-pairs. These phenomena are thought to be that the EEF can enhance the electrochemical reaction between lubricants and the surfaces of tribo-pairs, change the tropism of polar lubricant molecules, or help the formation of ordered lubricant molecular layers [51,73-77]. An instrument for measuring lubricant film thickness with a technique of the relative optical interference intensity (ROII) has been developed by Luo et al. [4,48,51,78] to capture such real-time interference fringes and to study the phenomenon when an EEF is applied, which is helpful to the understanding of the mechanism of thin film lubrication under the action of the EEF. 

An interesting phenomenon that some gas microbubbles emerged in the thin liquid film in a nanogap under an EEF was observed by Luo s group [78-80]. They have investigated the influence factors on and the mechanism of the emergence of these micro-bubbles. 

Fig. 48 —Interference patterns of the film of glycerin under a load of 4 N. The film thickness in the central region was 10 nm, 105 nm in the first dark ring, and about 315 nm in the second dark ring, (a) External voltage was 0 V, which means no EEF was applied to the liquid film, (b)-(f) The external voltage was kept constant (6 V) for a time of "t," and (b) f=0 s, (c) t=0.5 s, (d) t=1.0 s, (e) t=5.0 s, (f) t=100 s.

The interference pattern without an EEF is shown in Fig. 50(a). As the external voltage was raised, the number of the places where micro-bubbles emerged increased (Fig. 

Fig. 51—Dependence of electric current on time for different EEF polarities. Liquid glycerin. Load 4 N. Line (a) and (b) represent the positive and negative cases, respectively. Positive EEF intensities of 518.6 kV/cm during the initial period of 60s then...

The chemical composition of the glycerin liquid after the EEF test was measured with a Raman microscopy as shown in Fig. 54. Curve (a) is a typical Raman spectrum of glycerin without any EEF applied, and Curve (b) is the Raman spectrum of the glycerin after the positive EEF intensity... 

Fig. 53—Dependence of the average thickness of the glycerin film in the contact region over time at positive EEF intensities of 518.6 kV/cm ( ), 666.7 kV/cm (O), and 1 MV/cm (. ).

The overheating effect is believed to play a dominant role in explaining the physical mechanism of the microbubble formation, and three main reasons could be underpinned First, it was observed that a small amount of the glycerin injected into the gap would eventually disappear after a few hours at the positive EEF intensity of 1 MV/cm. Second, no remarkable difference in the chemical composition between the glycerin before and after the EEF was applied could be found in the experiment, which indicated physical effects might predominate. Besides, a rough estimation of the temperature rise in the contact region due to the electro-thermal effect will be conducted as follows. 

Fig. 54—Raman spectra of the tested glycerin liquid, (a) before EEF applied (b) after a positive EEF intensity of 1 MV/cm applied for 60 minutes.

Fig. 55—Temperature calculation for different liquid films (glycerin and hexadecane) at different locations in the contact region Area A is the central area in the inset photo and area B is the edge area. The filled histogram represents the positive EEF intensity of 518.6 kV/cm, and the empty one of 667.7 V/cm. The solid (glycerin) and dotted (hexadecane) lines are variation curves of the boiling point along the radial direction in the contact region.

The mechanism of the EEF polarity dependence of the micro-bubble emerging is believed to be that the electrolysis of water molecules absorbed plays an important role. The deposited Cr layer is susceptible to be oxidized, and cracks tend to form and propagate due to the interfacial stress between the oxidized film and the glass disk, resulting in the damage of the electrode. 

Histones and nonhistone nuclear proteins Ribosomal protein S6 elF (eukaryotic initiation factor) eEF (eukaryotic elongation factor)... 

Table 3 Relative estrogenic potencies as determined by different bioassays (expressed as EEF - the molar-based 17/5-estradiol equivalency factor) (adapted from [51])... 

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