Liran Ma

Origins of hydration lubrication

Why is friction in healthy hips and knees so low? Hydration lubrication, according to which hydration shells surrounding charges act as lubricating elements in boundary layers (including those coating cartilage in joints), has been invoked to account for the extremely low sliding friction between surfaces in aqueous media, but not well understood. Here we report the direct determination of energy dissipation within such sheared hydration shells. By trapping hydrated ions in a 0.4-1 nm gap between atomically smooth charged surfaces as they slide past each other, we are able to separate the dissipation modes of the friction and, in particular, identify the viscous losses in the subnanometre hydration shells. Our results shed light on the origins of hydration lubrication, with potential implications both for aqueous boundary lubricants and for biolubrication.

Superlubricity achieved with mixtures of acids and glycerol.

In this work, superlubricity between glass and Si(3)N(4) surfaces lubricated by mixtures of acid solutions and glycerol solutions has been found by using a traditional tribometer. Ultralow friction coefficients of between 0.004 and 0.006 were obtained after a running-in period. Related experiments indicate that the hydrogen ions in the mixtures play an important role in achieving superlubricity. Moreover, the ultralow friction is also closely related to the pH value of the acid and the concentration of glycerol. According to these results, the possible superlubricity mechanism has been revealed, which is attributed to a fluid-hydrated water layer between the hydrogen-bonded networks of glycerol and water molecules on the positively charged surfaces.

Hydration lubrication: exploring a new paradigm

Lubrication by hydration shells that surround, and are firmly attached to, charges in water, and yet are highly fluid, provide a new mode for the extreme reduction of friction in aqueous media. We report new measurements, using a mica surface-force balance, on several different systems which exhibit hydration lubrication, extending earlier studies significantly to shed new light on the nature and limits of this mechanism. These include lubrication by hydrated ions trapped between charged surfaces, and boundary lubrication by surfactants, by polyzwitterionic brushes and by close-packed layers of phosphatidylcholine vesicles. Sliding friction coefficients as low as 10(-4) or even lower, and mean contact pressures of up to 17 MPa or higher are indicated. This suggests that the hydration lubrication mechanism may underlie low-friction sliding in biological systems, in which such pressures are rarely exceeded.

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.

Investigation of the film formation mechanism of oil-in-water (O/W) emulsions

To reveal the film formation mechanism of oil-in-water (O/W) emulsions, the thicknesses of nanofilms of an aqueous paraffin oil emulsion, stabilized with the nonionic Tween 80 and Span 80 surfactants, were measured in confinement between two solid surfaces, by the use of the relative optical interference intensity (ROII) approach. Such films’ thicknesses were found to be sensitive to the rolling speed. In contrast to a single-phase oil lubricant, which as an elastohydrodynamic film had a thickness that always increased with speed when there was a sufficient supply of new material, the film formation of oil-in-water emulsions normally has a hill-like appearance (the film undergoes a collapse to a relatively low thickness at a critical speed after the evident rise in thickness with increasing rolling speed). The critical speeds for film formation of O/W emulsions with various emulsifiers and oil concentrations were focused on to gain an insight into the film formation mechanism of oil-in-water emulsions. Droplets can be observed to concentrate and break up before the contact at a low speed, which induces an oily pool. The oily pool seemed to act as the provider of the lubrication of the contact during the rolling process. The re-emulsification effect was employed to explain the collapse of the film thickness as the speed exceeded the critical value. A theoretical model was proposed to describe the re-emulsification effect, which established a relationship between the critical speed and the concentrations of either the oil or the emulsifiers.

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.

Investigations on the mechanism of superlubricity achieved with phosphoric acid solution by direct observation

In this work, the contact region between a Si3N4 ball and a SiO2 plate with the lubrication of phosphoric acid solution is observed directly by an optical microscope combined with a Raman microscope to understand the superlubricity mechanism. It is found that the wear on the friction surfaces mainly occurs at the beginning of the test and nearly disappears after the friction coefficient reduces to 0.05. When the superlubricity appears (μ = 0.004), there is only a limited amount of solution available to the contact (forming starvation state), resulting in an “H” distribution surrounding the contact region. Moreover, it is observed that the hydrogen bond effect in the solution is enhanced with time going by, and finally a thin film with hydrogen bond network among H3PO4, H2PO4−, and H2O is formed on the friction surfaces, leading to the superlubricity. By employing this direct observation approach, the structure of the confined solution and the superlubricity mechanism of phosphoric acid solution are finall...

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.

Analysis of the effects of evaporative cooling on the evaporation of liquid droplets using a combined field approach.

During liquid evaporation, the equations for the vapor concentration in the atmosphere and for the temperature in the liquid are coupled and must be solved in an iterative manner. In the present paper, a combined field approach which unifies the coupled fields into one single hybrid field and thus makes the iteration unnecessary is proposed. By using this approach, the influences of the evaporative cooling on the evaporation of pinned sessile droplets are investigated, and its predictions are found in good agreement with the previous theoretical and experimental results. A dimensionless number Ec which can evaluate the strength of the evaporative cooling is then introduced, and the results show that both the evaporation flux along the droplet surface and the total evaporation rate of the droplet decrease as the evaporative cooling number Ec increases. For drying droplets, there exists a critical value EcCrit below which the evaporative cooling effect can be neglected and above which the significance of the effect increases dramatically. The present work may also have more general applications to coupled field problems in which all the fields have the same governing equation.

Reemulsification effect on the film formation of O/W emulsion

Worldwide efforts to illuminate the behavior of multiple-phase liquids are frequently focused on oil-in-water emulsions. With the aim to reveal the mechanism of the film formation of aqueous emulsion in confinement, we investigated the reemulsification effect on the formation of the confined film in a nanogap. By employing two different feeding modes, we have demonstrated a surprising finding dramatically different from the traditional concept. The film formation has been detected to be distinctly enhanced under an insufficient feeding condition compared to the situation under a sufficient feeding condition. Moreover, we have observed the oil pool formed surrounding the contact area, derived from the oil film adhered on the solid surfaces, as well as the research into how the oil pool changes with rolling speed and feeding mode. The unusual performance resulted from different feeding modes leads to an strong evidence of the reemulsification concept. Here, we demonstrate, directly from experimental observations of emulsion behaviors under different feeding modes, that the confined film of emulsion is significantly affected by the reemulsificiation effect.