Probing the intriguing frictional behavior of hydrogels during alternative sliding velocity cycles

Understanding the friction behavior of hydrogels is critical for the long-term stability of hydrogelrelated bioengineering applications.Instead of maintaining a constant sliding velocity,the actual motion of bio-components(e.g.,articular cartilage and cornea)often changes abruptly.Therefore,it is important to study the frictional properties of hydrogels serving under various sliding velocities.In this work,an unexpected low friction regime(friction coefficientμ<10^(-4) at 1.05×10^(-3) rad/s)was observed when the polyacrylamide hydrogel was rotated against a glass substrate under alternative sliding velocity cycles.Interestingly,compared with the friction coefficients under constant sliding velocities,the measuredμdecreased significantly when the sliding velocity changed abruptly from high speeds(e.g.,105 rad/s)to low speeds(e.g.,1.05×10^(-3) rad/s).In addition,μexhibited a downswing trend at low speeds after experiencing more alternative sliding velocity cycles:the measuredμat 1.05 rad/s decreased from 2×10^(-2) to 3×10^(-3) after 10 friction cycles.It is found that the combined effect of hydration film and polymer network deformation determines the lubrication and drag reduction of hydrogels when the sliding velocity changes abruptly.The observed extremely low friction during alternative sliding velocity cycles can be applied to reduce friction at contacted interfaces.This work provides new insights into the fundamental understanding of the lubrication behaviors and mechanisms of hydrogels,with useful implications for the hydration lubrication related engineering applications such as artificial cartilage.

Integrated Sliding Mode Velocity Control of Linear Permanent Magnet Synchronous Motor with Thrust Ripple Compensation

In this paper,a compound sliding mode velocity control scheme with a new exponential reaching law(NERL)with thrust ripple observation strategy is proposed to obtain a high performance velocity loop of the linear permanent magnet synchronous motor(LPMSM)control system.A sliding mode velocity controller based on NERL is firstly discussed to restrain chattering of the conventional exponential reaching law(CERL).Furthermore,the unavoidable thrust ripple caused by the special structure of linear motor will bring about velocity fluctuation and reduced control performance.Thus,a thrust ripple compensation strategy on the basis of extend Kalman filter(EKF)theory is proposed.The estimated thrust ripple will be introduced into the sliding mode velocity controller to optimize the control accuracy and robustness.The effectiveness of the proposal is validated with experimental results.

Numerical analysis of submarine landslides using a smoothed particle hydrodynamics depth integral model

Submarine landslides can cause severe damage to marine engineering structures. Their sliding velocity and runout distance are two major parameters for quantifying and analyzing the risk of submarine landslides.Currently, commercial calculation programs such as BING have limitations in simulating underwater soil movements. All of these processes can be consistently simulated through a smoothed particle hydrodynamics（SPH） depth integrated model. The basis of the model is a control equation that was developed to take into account the effects of soil consolidation and erosion. In this work, the frictional rheological mode has been used to perform a simulation study of submarine landslides. Time-history curves of the sliding body＇s velocity, height,and length under various conditions of water depth, slope gradient, contact friction coefficient, and erosion rate are compared; the maximum sliding distance and velocity are calculated; and patterns of variation are discussed.The findings of this study can provide a reference for disaster warnings and pipeline route selection.

Fretting wear behaviour of high-nitrogen stainless bearing steel under lubrication condition

The fretting wear performance of high-nitrogen stainless bearing steel(40Cr15Mo2VN)under lubrication conditions was researched.Lithium-based grease was preparedusing MoS2 and carbon nanotubes(CNTs)as additives.AISI 52100 steel ball was used in four-ball test to evaluate the extreme pressure property and wear resistance of grease.After four-ball test,the grease adding 0.8 mass%MoS2 and 0.8 mass%CNTs,respectively,was chosen and used for fretting test.AISI 52100 ball and 40Cr15Mo2VN steel disc were used as the upper and lower samples for fretting test.The results showed that wear power consumption increased with the increase in both sliding velocity and contact stress.When initial contact stress was 2.047 GPa,the main wear mechanisms were abrasive wear and plastic deformation as the velocity increased 0.028 to 0.112 m/s.When the velocity was 0.028 m/s,the main wear mechanisms changed abrasive wear to adhesion wear and finally to abrasive wear and adhesion wear as the initial contact stress increased 1.788 to 2.579 GPa.The volume loss grew sharply becaof the changes in wear mechanisms.In this condition,the volume loss growth rate can be divided into three regions according to different wear power consumption ranges corresponding to different wear mechanisms.

Analytical study of friction coefficients of pomegranate seed as essential parameters in design of post-harvest equipment

Friction coefficients(static friction coefficient(SFC)and dynamic friction coefficient(DFC))of pomegranate seed on different structural surfaces(glass,aluminum,plywood,galvanized steel and rubber)as affected by moisture content(4-21.9%(d.b.))and sliding velocity(1.4-16(cm/s))were investigated.Analysis of variance(ANOVA)was performed to determine the effect of main treatments and their interactions on SFC and DFC.Significance of single or multiple effect of the main treatments with five levels was assessed using Duncan’s multiple range test(DMRT).To predict SFC and DFC,multiple linear regression(MLR)modeling technique was applied for each type of structural surface.The goodness of fit of each MLR model was evaluated using statistical parameters:coefficient of determination,root mean square error and mean relative deviation modulus.Results showed that the minimum and maximum SFC or DFC were in minimum and maximum moisture content on glass and rubber surface,respectively.ANOVA table indicated the significant effect of main treatments and their interactions on SFC and DFC at significance level of 1%(P<0.01).According to DMRT results,SFC linearly increased as moisture content increased and DFC increased also linearly as individual or simultaneous increment of moisture content and sliding velocity occurred,for all experimental conditions.According to the obtained statistical parameters,both SFC and DFC were properly predicted by means of MLR modeling technique.

Nano friction behaviour between magnetic materials and copper considering the inter-diffusion effect

Copper,permalloy,cobalt,and silicon are the materials that have been widely utilised in magnetic devices.When the size of interest is down to the nanoscale,the inter-diffusion between certain materials becomes influential.This paper studies the nanoscale friction characteristics between frictional pairs with and without inter-diffusion properties through the atomic force microscope.The distinct evolution features of nanoscale friction force when inter-diffusion is involved are discovered experimentally,which is also confirmed through theoretical analysis.Firstly,through the thin film deposition method,four pairs of contact materials(Cu–Ni_(81)Fe_(19),Si–Ni_(81)Fe_(19),Cu–Co,Cu–Si)are designed for friction tests,in which diffusion occurs at the interface of Cu–Ni_(81)Fe_(19)pair.Then,the effects of sliding velocity and loading force on the nano friction of each pair are measured.It is found that regardless of the diffusion phenomenon:(1)the adhesion force values exhibit a notable correlation to the values of the friction force;(2)the friction force in all four material pairs consistently increases with the growth of the normal loading force,although the growth rate may differ.In terms of the sliding velocity effect,the friction forces of immiscible materials(Si–Ni_(81)Fe_(19),Cu–Co,and Cu–Si)are found to increase with the increasing sliding velocity.However,the friction force of Cu–Ni_(81)Fe_(19),decreases with the increasing sliding velocity.Furthermore,a compositive friction model considering both the velocity and the normal force effect was proposed,which shows good agreement with the experimental results and explains the nano friction behaviour of both miscible and immiscible metals.