Study On The Friction Of Two-dimensional Superparamagnetic Colloids

With the improvement of technology and the development of mesoscopic physics,the friction properties of materials at mesoscopic scale,especially the interface friction characteristics of mesoscopic moving parts,have aroused extensive attention from both experimental and theoretical researchers.It is important to study the friction characteristics of the interface in mesoscopic scales for the precise control of the micro precision instruments and the biological organs.In the first chapter of this thesis,we firstly summarize the macroscopic friction,mesoscopic friction,and atomic and molecular friction.Then in the second chapter,we briefly introduce two typical superparamagnetic interface lubrication materials.One is the magnetorheological(MR)fluid.Due to the good abrasion resistance,MR fluids are widely used in the transmission components,and are used to simulate the movement of the human arm joint.Another is the active magnetic colloid.Active colloids have become a hot research topic in recent years.Investigation of the friction characteristics can help us to achieve active colloids in mesoscopic scale independent and controllable transport,and this mechanism is used in engineering application,such as accurate diagnosis,drug delivery and the simple treatment of the vessel skeleton by the micro mechanism.In the third chapter,we model two kinds of superparamagnetic interface colloid lubricating materials in mesoscopic scales with two-dimensional MR fluids and active magnetic colloids on the substrate with randomly distributed pinning centers,and systematically investigate the effects of the substrate pinning strength,magnetic field strength and the temperature on the average friction force and the maximum static friction force.Through systematic researches on the average friction force,we find:(1)For the weak pinning substrate,the systems exhibit ordered structures(the MR fluids present long chain and the active magnetic colloids show island structures),the average friction force can be neglected in such a case and the systems are in a superlubricity state.With the increasing pinning strength,the micro ordered chain structures are gradually destroyed when the average friction force appears,and the average friction force increases linearly with the pinning strength.(2)The higher magnetic field strength causes that the MR fluids maintain super lubricity even for the stronger pinning substrate,but it is the opposite case for the active magnetic colloids.(3)The increase of temperature can also lead to the superlubricity phenomenon.Even in the case of large pinning strength,it is possible to have superlubricity when the temperature is high enough.Through systematic researches on the maximum static friction force,we find:(1)The maximum static friction increases with an increase in the pinning strength,the ordered structures of the systems(long chains for the MR fluids and islands for the active magnetic colloids))are destroyed and even disappear.(2)For the MR fluids,with the increase of magnetic field strength,the maximum static friction force decreases gradually,and long chain and even bundle chain structures become to appear above the depinning.When the magnetic field increases to a certain intensity,the maximum static friction force does not change with the change of magnetic field intensity basically,and the chains throughout the entire region occur above the depinning.Further increasing the magnetic field strength will reduce the maximum static friction force to zero rapidly,and above the depinning,the chain structures are broken.For the active magnetic colloid,the maximum static friction force increases and then decreases with the increase of magnetic field strength,(i.e.the peak effect).With the increase of the magnetic field strength,the ordered island structures of the system disappear at low fields above the depinning.When the magnetic field increases to a certain intensity(i.e.,the peak value),With the further increase of magnetic field strength,the system is gradually transformed from the disordered structure into ordered crystal structure above the depinning.(3)The maximum static friction force is found to decrease with the increase of temperature,and above the depinning,more ordered microstructures(long chain structures for the MR fluids and island structures for the active magnetic colloids)are formed at the low temperature.The ordered structures are destroyed instead when the temperature is further increased.This is the results of competition between the thermal fluctuation caused by temperature and the pinning strength.The competition is especially evident when the pinning strength is small.The above results provide theoretical guidance for the study of the effects of the external fields on the friction characteristics of mesoscopic devices and the mesoscopic controllable transports.