Shuhai Liu

Nanoconfined ionic liquids under electric fields

The effect of external electric fields (EEFs) on ionic liquid films confined within a nanogap has been investigated by measuring the film thickness with the thin film interferometry and calculating the effective viscosity. Experimental results indicated that the film thickness of ionic liquids could be increased obviously by the application of EEFs with strengths weaker than the electric interactions between cationic head groups and anions. The effect of EEFs on the confined ionic liquid film with a shorter alkyl side chain is more noticeable. It is thought that the charged anions and headgroups of the cations are structured near electrified walls to form ordered layers and short alkyl side chains at the interfaces are aligned along the EEF direction due to induced dipoles.

Effect of surface charge on water film nanoconfined between hydrophilic solid surfaces

The effect of surface charge on the water film confined between hydrophilic solid surfaces has been investigated using thin film interferometry. Experimental results indicate that the higher surface charge density induces the thicker water film for model surfaces, silica, and alumina. It is thought that the strong surface field induced by the surface charge establishes a more ordered hydrogen-bonding network that promotes the forming of thicker water lubrication film between hydrophilic solid surfaces.

Film Forming Characteristics of Oil-in-Water Emulsion with Super-Low Oil Concentration

The oil in water emulsion has been widely used in many fields such as rolling operations. The mechanism and characteristics of film forming have been widely investigated before. However, the mechanism is still dubious and film forming characteristics are seldom discussed under an oil concentration of 0.05%. In this paper, a lubricating film testing apparatus is used to investigate the film forming characteristics and tribological behaviors under different speed of oil-in-water emulsion between a steel ball and a glass disc. By carrying out experiments under an extremely low concentration of oil, some new phenomena are found in our experiments. Oil concentration is even low to 0.0005%. The results indicate that the speed-thickness curves are changed as the condition changes. The effect of droplet size and the stability of emulsion are both considered to be important. The frictional behaviors are investigated under different conditions. The film forming mechanism of oil-in-water emulsion is also discussed by direct observations of emulsion in the contact area. A new viewpoint on the lubrication of emulsion is put forward in this paper.

Effect of external electric field on liquid film confined within nanogap

A strong and reproducible effect of an external electric field (EEF) on liquid films confined within a nanogap between a highly polished steel ball and a smooth glass disk is described. Induced by the EEF, microbubbles were observed at the edge of the contact region. This phenomenon is more obvious in polar liquid films than that in nonpolar ones. A stronger EEF causes a decrease in the film thickness in the contact region initially, and then the variation becomes much smaller. When the ball is applied with a positive EEF, the emergence of microbubbles is stable over time, while it becomes much less as time progresses. Different mechanisms of these experimental phenomena have been discussed.

Tribological characteristics of aloe mucilage

Abstract Increased concerns about environmental damage caused by many lubricants, has created a growing worldwide trend of promoting new environmentally friendly lubricants. The tribological characteristics of aloe mucilage as a kind of original biolubricant have been investigated in the present work. The experimental results indicate that the variation of the film thickness of aloe mucilage is not the same as that in traditional elastohydrodynamic lubrication, but conforms to the lubrication regime of thin film lubrication under the present experimental conditions. The coefficient of friction (COF) of the aloe mucilage among different tribological pairs is significantly decreased by the increase in velocity, while there is little variation when the normal load is increased. The COF of aloe mucilage between WC and DLC surfaces is very small with a value of 0˙04, and the wear resistance of the aloe mucilage between WC/DLC is better than that between WC/Si and WC/steel.

Effect of microcontent of oil in water under confined condition

The lubricant film thickness between two contacting surfaces is important for the evaluation of lubrication effectiveness. According to the elastohydrodynamic lubrication theory, the film thickness of pure water is usually on the order of a few nanometers. It was found, however, that microcontent of oil contamination can cause a lubricant film more than 100 nm, much thicker than the predicted. The effect of micro-oil content in water between a smooth plate and a highly polished steel ball was investigated. The film forming characteristic of such films was presented. The film formation mechanism and influencing factors were discussed.

Water film confined in a nanoscale gap: Surface polarity and hydration effects

The water film nanoconfined between solid surfaces was investigated using thin film interferometry. Experimental results indicate that surface polarity and hydration effects can strongly influence on the property of water film. It is thought that water molecules strongly bind to surface containing hydrophilic groups and a preferential slip plane is created against the adjacent layer between the hydration sheath and bulk water. The slip plane dramatically weakens the entrainment effect in the inlet zone of lubrication by breaking the hydrogen-bonding network.

Tribological Behaviours of Wax-in-Oil Gel Deposition in Orthogonal Cleaning Process

Tribological behaviour of wax-in-oil gel deposition at orthogonal cleaning process has been directly observed with a custom-built experimental setup. Experimental results show that the chip formation of wax scraping is due to rubber tribology of elastomer disc. The scraping chip instability arises from a competition between scraping forces and the adhesion forces. An analysis of debris-material removal rate in wax scraping with negative rake face has been carried out. The study has also shown that the wax-scraping force component is sensitive to rake angle, which may be used as primary basis for developing wax-removal estimating strategies. Flank face wear results in a substantial increase in the wax-scraping force, which should be incorporated into the wax-cleaning efficiency predictive model. The behaviours of the residual phenomenon depend on the inlet aggregation in orthogonal wax-cleaning process.

Frictional behaviors of rough soft contact on wet and dry pipeline surfaces：With application to deepwater pipelaying

Elastic rubbers are widely used in nature and engineering. In order to better understand soft rough contact under dry and wet conditions during deepwater pipelaying process, various polyurethane (PU) discs both in PU-pipe and PU-steel sliding combinations have been investigated by using self-developed tribometer. The tribological test results show that the friction coefficient can be clearly seen: PU90< PU80 < PU70, under dry conditions. Experimental results also indicate that the two dramatic increases of the frictional coefficient exist in the rough soft PU-pipe contact. The potential influence of asperities and thin liquid film is discussed.

Electrospreading of dielectric liquid menisci on the small scale

The effects of an external electric field (EEF) on dielectric liquid menisci formed in a small gap between a smooth plate and a high precision steel ball have been investigated. It was found that thin spreading films were pulled out and moved away from the menisci of low permittivity dielectric liquids after exposure to the EEF. During the initial period of spreading, a strong “electric wind” due to the gas discharge in the vicinity of the three-phase contact line (TCL) of the liquid meniscus was observed. However, such electrospreading phenomena were absent for the menisci of conductive liquids which were commonly used in classical electrowetting studies. It is deduced that the spreading of liquid menisci under EEFs is driven by the thermocapillary force near the TCL.