They next employed a one-dimensional discretized algorithm to solve coupled Navier-Stokes and Maxwell's equations for the equation of motion. Overall, their combined experimental and theoretical approach successfully established liquid oxygen as a viable candidate for a moving part free magnetorhological system in cryogenic applications.
Real time monitoring of the rheological properties of a magnetorheological fluid are of particular importance for advancing both applied and fundamental knowledge. QCM studies were reported for both particle sedimentation under gravity and the impact of externally applied magnetic fields on the shifts of resonance frequency and dissipation factor. As expected, interactions among particles and also interactions between the iron particles and the resonator surface increased when magnetic fields were applied.
The increase was attributed to increased contact stiffness between the particles and the surface. In particular, they performed a finite element modeling study examining both cases of free settling of particles and particle alignment under the action of the magnetic field. The basic model was in agreement with the trends observed in the frequency and dissipation QCM data.
Magnetorheological fluids require at least one of the constituents of a lubricant to be magnetic, so as to response to an externally applied magnetic field. In an analogous fashion, electrically charged constituents of a lubricant will respond to externally applied electric fields. Nanoparticles in suspension are often electrically charged either intrinsically or by design.
The study of their motion through active external control by electrical fields is rapidly growing, and active colloids are emerging as an area of widespread applications. In separate studies, nanoparticles have been demonstrated as lubricant additives capable of producing significant reductions or increases in the friction coefficients of the materials that they are added to.
Acharya et al. Static, and also low frequency 0. The fields resulted in electrophoretic forces, which were employed to reposition the particles relative to a planar platinum surface of a quartz crystal microbalance, which was then used to monitor friction levels. Friction levels were reported in terms of the damping levels of the surrounding suspension on the oscillatory motion of the QCM electrode.
The field was produced by biasing the sensing electrode of the QCM with respect to a grounded Pt electrode positioned nearby within the surrounding suspension. The authors report successful active electrotunable control of friction, and studied the response as a function of the applied field's frequency.
In particular, features attributed to field-induced reorientation of water layers were identified in the friction data. They found, as anticipated, that the defects experienced frictional drag forces that depended on both the field dynamics and how the inclusion coupled to the field. Changes in friction were manifested by changes in the diffusion constants of the inclusion moving within a surrounding material. The authors used their results to predict frictional drag levels exerted on the inclusions such as proteins in lipid membranes associated with protein interactions with membrane height and composition fluctuations.
The forces on a point-like magnetic field moving within an Ising ferromagnet were also investigated, and were well-explained by these results. The theory is potentially applicable to soft materials such as those described in Example 5. The authors argue that a rich phenomenology can arise from their relatively straightforward analysis because a wide range of field theories is available to describe soft materials.
They also suggested directions for future investigations, including interacting fields, extensions to more realistic dynamics and possible experiments where their predictions could be tested. In particular, they suggested using optical tweezers to pull an inclusion, such as a colloid, through a binary fluid mixture close to the critical point. Park et al. In a first study, the researchers demonstrated electronic control of friction by employing a friction force microscope with a conductive titanium nitride coated tip in sliding contact with an oxygen passivated Si wafer.
Increased friction levels were observed only in the highly doped p regions and were observed to increase with tip-sample bias voltage, contact strain, and velocity. The increases friction levels could not be attributed to wear or surface damage. Figure 3. A Schematic of AFM measurements on a silicon pn junction device. B In the absence of an external bias voltage the friction force vs. Application of positive C or negative C,D bias voltages allows the friction to be tuned.
The excess friction in the presence of an external bias is attributed to static charge and triboelectric effects. Reprinted with permission from Park et al. The researchers also later investigated electronic contributions to friction at a semiconducting n-type GaAs surface covered by an approximately 1 nm thick oxide layer Qi et al.
The friction force microscope tip in this case had a 50 nm radius and was coated with Pt. Charge accumulation or depletion was induced by the application of forward or reverse bias voltages and substantial changes in friction levels were observed. The friction levels could be quantitatively explained by modeling the forces exerted by the trapped charges on the surfaces that were triboelectrically generated.
As demonstrated by Example 7, and confirmed by recent theory Wolloch et al. For insulating surfaces, tribo-generated static electricity moreover can be the dominant contribution to friction. An example of this is reported in Altfeder et al. The results were attributed to contact electrification and static electricity giving rise to increased friction levels in the non-superconducting phase, which for the sample studied was believed to be semiconducting in nature.
As such, friction measurements provide a means to test theories of superconductivity that predict thin superconducting or semiconducting surface regions above Tc. The density of charge carriers in a superconducting materials can be varied by applying an external magnetic field to the material so as to drive the material from a superconducting into a normal state. Highland and Krim employed this approach to study sliding friction levels in nitrogen, water, and superheated helium films adsorbed on Pb substrates with a quartz crystal microbalance by positioning the sample chamber within a solenoid coil and applying a weak magnetic field close to the superconducting transition to, respectively, drive the system in and out of the superconducting state.
Reductions in friction upon entry into the superconducting state were reported to be larger for nitrogen than helium, consistent with theory based on polarizability of the adsorbed layers and were successfully interpreted in terms of conduction electron contributions to friction.
The Highland and Krim studies were performed in conditions that suppressed competing mechanisms for friction, allowing electronic effects to be resolved. In general, however, such effects are quite small relative to alternate mechanisms phononic, static electricity, etc.
Independent efforts to observe conduction electron friction have variously encountered high background levels of phonon friction which prevented them from being resolved Pierno et al. In addition to reporting friction levels in the presence and absence of the external magnetic field, the researchers also observed that the repetitive cycling of an externally applied magnetic field in and of itself impacted friction levels as compared to measurements performed in static field conditions.
Gabureac et al. They observed significant changes in resistance depending on the angle between the applied field and the contact direction. The changes are consistent with modifications of the contact geometry induced by magnetostriction, which is a property of ferromagnetic materials whereby they change their shape or dimensions during the process of magnetization. The studies were performed at field strengths ranging from 0 to 1T for Ni-Ni nanocontacts in close proximity.
While they do not specifically report friction levels associated with the varying contact levels resulting from the applied magnetic fields, they point out that such effects are likely to be ubiquitous in systems employing externally applied fields, and must be accounted for to properly design and control mechanical systems via electronic methods. They conclude by emphasizing the importance of magnetomechanical effects in measuring magnetoresistance in nanocontacts and argue that the effects are extremely difficult to avoid.
This in turn impacts studies of the electrical resistance of nanocontacts. The same would therefore be applicable to nanoscale studies of the impact of magnetic fields on tribocontacts. Benassi et al. To circumvent this lack of versatility, they propose to tune the material properties with external control parameters, and note that the example of an external field applied to an ionic liquid is a promising example of such control.
In particular, they propose a non-contact motion control technique based on the introduction of a tunable magnetic interaction. As a specific example, they suggest coating two non-contacting bodies with ferromagnetic films.
The researchers demonstrate that wear-free motion control in these systems can be achieved by means of interacting magnetic domains arising at the surface below the Curie temperature. Their hypothesis is that the size, shape, and ordering of such domains is easily controlled with an external magnetic field, allowing a flexible and reversible means to tune system tribological properties.
Rajauria et al. Contact hysteresis between sliding interfaces is a common phenomenon that has a major impact on the tribological performance of a head-disk system. Additional methods that perform measurements at higher speed are therefore of interest. The researchers employed commercially available hard disk drives to perform the studies.
Such systems are of particular interest, since the sliding speeds are high, while the gap sizes between the contacts are of nm extent. The primary result was to demonstrate that out-of-plane oscillations induced by AC voltages applied between the head and the disk are able to completely suppress contact hysteresis, and thus tune the friction and adhesion levels.
They noted that this is a dynamic effect associated with the time for contacts to grow and adhere, since the electrostatic force arising from the applied DC or AC voltage is attractive. It should also be noted here that a large number of studies have examined active control of friction through externally applied vibrations Socoliuc et al. The two approaches to tuning friction therefore have some overlap. Bias potential dependence of friction is a common phenomenon in the literature for rubbing contacts that contain iron, particularly in the presence of water.
In a study, Zhu et al. They employed impedance spectroscopy and Fourier transform infrared micro spectroscopy to explore friction and wear of the rubbing contacts in well-defined conditions as a function of the applied bias voltage. The researchers measured friction coefficients with a macroscale tribometer comprised of a reciprocating hemispherical pin with stroke length 0. The pin was pressed into contact with a flat surface that was fully immersed in a cell filled with an aqueous electrolyte employed as the lubricant.
The pin and flat were both connected to an external potentiostat and otherwise electrically grounded. A reference electrode and Pt counterelectrode were also immersed in the cell within close proximity to the contact, to facilitate potential control in cases where both the pin and flat were conducting. The researchers concluded that the observed changes in friction and wear were attributable to two primary mechanisms: 1 Changes in electrostatic repulsion associated with changes to the double layer charge and 2 surface redox reactions.
They also modeled the electric double layer effect and found a significant dependence of the friction coefficient on surface bias potential for ratios of the applied contact pressure to inter-surface pressure falling below an approximate factor of In addition, they also observed that the decreased friction levels at positive electrode potentials were attributable to the formation of FeOOH on the metal surfaces.
Positive electrode potentials also produced iron II carboxylate on the surfaces when octanoic acid was present, causing a significant reduction in friction. Ismail et al. In particular, friction coefficients were collectively reported to drop from approximately 0. They observed that friction was reduced in octanoate solutions under negative electrochemical bias due to a production of a low friction tribochemical layer iron octanoate.
They noted the usefulness for industrial applications of mastering the capability of switching friction between two known values by varying the bias voltage, namely high and low values for cathodic and anodic potentials. They also reported that the tribochemical layer reduced wear of both pin and disk and that in general NaOH tests yielded much higher friction coefficients and more corrosion for the same applied potentials. These include the vitreous body of eyes and synovial joints such as hip, knees, and fingers.
The exceptional tribological performance of such systems is associated with the presence of phospholipids, hyaluronan, lubricins, and bottle-brush-like glycoproteins in synovial fluids. Hyalluronan and glycoproteins are biopolyelectrolytes whose side chains contain large amounts of sulfonic and carboxylic groups. The charged nature of these biomacromolecules and their brush-like structures are thought to underlie their superior lubricating properties Kreer, In particular, they respond to electrostatic interactions via swelling—collapse transitions of end-tethered polymer chains and osmotic pressures within the brushes.
Polyelectrolyte brushes are fully stretched when they are immersed in pure water, allowing water molecules to fully penetrate them. But when they are immersed in an electrolyte solution, strong electrostatic screening occurs causing the water to be expelled and the brushes to collapse Figure 4 The various configurations are associated with significantly different friction levels, and have been the topic of much interest in the nanotribology community.
Figure 4. Incidentally, this value is the basis of the operational definition of the ampere. Privacy Policy. Skip to main content. Search for:. Magnetic Fields, Magnetic Forces, and Conductors. The Hall Effect When current runs through a wire exposed to a magnetic field a potential is produced across the conductor that is transverse to the current.
Learning Objectives Express Hall voltage for a a metal containing only one type of charge carriers. Thus, those charges accumulate on one face of the material. On the other face, there is left an excess of opposite charge. Thus, an electric potential is created.
It is a factor of current I , magnetic field B , thickness of the conductor plate t , and charge carrier density n of the carrier electrons. Key Terms elementary charge : The electric charge on a single proton. Magnetic Force on a Current-Carrying Conductor When an electrical wire is exposed to a magnet, the current in that wire will experience a force—the result of a magnet field.
Learning Objectives Express equation used to calculate the magnetic force for an electrical wire exposed to a magnetic field. Key Takeaways Key Points Magnetic force on current can be found by summing the magnetic force on each of the individual charges that make this current. The direction of the magnetic force can be determined using the right hand rule , as in fig [[]].
Key Terms drift velocity : The average velocity of the free charges in a conductor. Torque on a Current Loop: Rectangular and General A current-carrying loop exposed to a magnetic field experiences a torque, which can be used to power a motor. Learning Objectives Identify the general quation for the torque on a loop of any shape. Although the forces acting upon the loop are equal and opposite, they both act to rotate the loop in the same direction.
What matters is the area of the loop. Key Terms torque : A rotational or twisting effect of a force; SI unit newton-meter or Nm; imperial unit foot-pound or ft-lb. Learning Objectives Express the relationship between the strength of a magnetic field and a current running through a wire in a form of equation.
In this equation, partial magnetic field dB is expressed as a function of current for an infinitesimally small segment of wire dl at a point r distance away from the conductor. This curious phenomenon has lately been examined by M. Thus M. The author mentions that the effects observed cannot be due to self-induction, or they would occur when the bismuth is not in a magnetic field.
In a note on the above paper in the Journal de Physique , M. Sagnac considers what would happen if the same series of experiments were repeated with an iron wire. For weak magnetic fields in which K has a large value, the difference between the value of the apparent resistance for steady currents and for increasing currents may amount to several hundredths of the value of the resistance for steady currents.
Reprints and Permissions. This magnetic field can deflect the needle of a magnetic compass. The strength of the magnetic field is greater closer to the wire, and increases if the current increases.
The direction of the current and magnetic field can be found using the right hand grip rule. Coil the fingers of the right hand as if holding the handlebars of a bicycle, with the thumb pointing away from the hand.
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