Experimental Study Of The Wettability Of Conformal Contct Surfaces On Liquid Film Lubrication

The classical lubrication theory is not able to well describe the lubrication features in mircro/nano scale.In such situations,as a dominating factor,the solid/liquid interface affinity effect should not be ignored.However,little is known about this effect,and as a result,there are yet few scientific knowledge for the applications in engineering design,e.g.liquid lubrication in MEMS.In the present project,rubbing samples with different surface wettabilities are prepared via surface treatment.Using fluorescence microscopy system,the velocity across film thickness and oil reservoir surrounding the slider-on-disc contacts are observed.Thus the relationship between film formation and affinity effect is established.The main work includes:(1)On the relationships between interface wettability and lubricant film thickness.The slider surfaces with different affinity were prepared via surface tension modification.The variations of film thickness with speed at different loads were observed.The film thickness vs.contact angle and the film thickness vs.contact angle hysteresis were analyzed and their correlations were shown.The results show that the film formation can be better characterized in terms of contact angle hysteresis.(2)On the relationships between fluorescence intensity and shear rate in fluorescence measurements.A new experimental system was designed,and a suitable combination of lubricants and fluorescent probe was obtained by screening tests.The fluorescence intensity of lubricants is independent of shear rate.(3)Establishment of calibration relationship between film thickness and fluorescence intensity.By changing the inclination of the slider,the relationship between light intensity and film thickness in the contact area was established.The results show that there is a single linear relationship between light intensity and film thickness.The results show that the film thickness measured by fluorescence method are consistent with that measured by interference method.(4)On the relationships between oil reservoir and speed under fully flooded conditions.The variations of oil reservoir evolutions around the slider-on-disc contacts with speed were measured via fluorescence method.The lubricant states of loss and replenishment can be recognized from the evolutions of oil reservoirs.(5)On the influence of surface wettabilities of slider and disc on oil supply to the contact.In the fully flooded occasions,only oil reservoirs around the contacts were affected by the surface wettabilities,and there is no influence on the contact region.However,in the case of limited lubricant supply,both the distributions of lubricants around the contacts and the spreading of the liquid at slider edges are enhanced by surface wettability.It is further validated that the inlet lubricant supplying and hence film formation are improved by good wettability.(6)Measurement of velocity continuity at the interface of contacts with different wettabilities.The velocity distributions of Couette flow were acquired from the fluorescence bleaching images over time.The effects of lubricant viscosity and oil film thickness on the flow velocity distributions were analyzed.