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In this work, we explore a simplified model based on both analytical and computational methods for the study of film-boiling droplet motion on microscale ratchets. We consider a specific ratchet design with the length periods and depth of ratchets much smaller than the size of the droplet. We conclude based on our modeling that for the ratchet configuration considered in this paper, the conduction within the vapor film is the dominant means of heat transfer in comparison with convection and radiation. Furthermore, we demonstrate a more manageable two-dimensional model in which analytical approaches coupled with computational approaches yield reasonably accurate results in comparison to the actual experiments.

In this work, we explore a simplified model based on both analytical and computational methods for the study of film-boiling droplet motion on microscale ratchets. We consider a specific ratchet design with the length periods and depth of ratchets much smaller than the size of the droplet. We conclude based on our modeling that for the ratchet configuration considered in this paper, the conduction within the vapor film is the dominant means of heat transfer in comparison with convection and radiation. Furthermore, we demonstrate a more manageable two-dimensional model in which analytical approaches coupled with computational approaches yield reasonably accurate results in comparison to the actual experiments. [ABSTRACT FROM AUTHOR]

Current treatments for cancer, such as chemotherapy, radiotherapy, immunotherapy, and surgery, have positive results but are generally ineffective against metastatic tumors. Treatment effectiveness can be improved by employing bioengineered cancer traps, typically utilizing chemoattractant-loaded materials, to attract infiltrating cancer cells preventing their uncontrolled spread and potentially enabling eradication. However, the encapsulated chemical compounds can have adverse effects on other cells causing unwanted responses, and the generated gradients can evolve unpredictably. Here, we report the development of a cancer trap based on mechanical ratchet structures to capture metastatic cells. The traps use an array of asymmetric local features to mechanically attract cancer cells and direct their migration for prolonged periods. The trapping efficiency was found to be greater than isotropic or inverse anisotropic ratchet structures on either disseminating cancer cells and tumor spheroids. Importantly, the traps exhibited a reduced effectiveness when targeting non-metastatic and non-tumorigenic cells, underscoring their particular suitability for capturing highly invasive cancer cells. Overall, this original approach may have therapeutic implications for fighting cancer, and may also be used to control cell motility for other biological processes. Current cancer treatments have limitations in treating metastatic tumors, where cancer cells can invade distant organs. Biomaterials loaded with chemoattractants can be implanted to attract and capture metastatic cells preventing uncontrolled spread. However, encapsulated chemical compounds can have adverse effects on other cells, and gradients can evolve unpredictably. This paper presents an original concept of "cancer traps" based on using mechanical ratchet-based structures to capture metastatic cancer cells, with greater trapping efficiency and stability than previously studied methods. This innovative approach has significant potential clinical implications for fighting cancer, particularly in treating metastatic tumors. Additionally, it could be applied to control cell motility for other biological processes, opening new possibilities for biomedicine and tissue engineering. [Display omitted] [ABSTRACT FROM AUTHOR]

A cross-laminated timber (CLT) wall panel offers an efficient solution to resist seismic loads. Earthquake-induced vibrations can be accommodated and dampened by ductile connections that hold down a rocking wall panel. In conventional hold-downs, irreversible damage from localized crushing of timber and/or withdrawal of slender steel fasteners results in pinching behavior. The impaired stiffness and dissipation are understood to cause larger peak displacements and increase the vulnerability of structures to aftershocks. To overcome this, a pinching-free connector (PFC) was developed previously, and in the present study it was tested as hold-downs for a rocking CLT wall. Quasi-static and pseudodynamic tests demonstrated that the PFCs significantly reduced pinching and provided additional restoring forces from the hold-downs near the rocking toe by responding elastically between 0.6% and 2% drift. Below this drift, the wall recentered under self-weight, because the PFCs are ratcheting tension-only connectors that do not resist recentering. Nevertheless, restoring forces may be possible over a larger range of drifts by positioning the PFCs closer to the edges of the wall. Although minimal restoring forces allow high dissipation in every cycle, the number of cycles is limited by the elongation capacity of the mild steel rods that yield monotonically. Stainless-steel rods therefore are suggested as alternative dissipators with greater ductility. In the ratcheting mechanism (screw threads), inherent backlash of 1.38 mm manifested as slack on reloading or out-of-sync ratcheting and reloading when a pair of hold-downs was used. Consequently, the initial stiffness can decrease by as much as one-half, because one PFC engages only after the other has started to yield. Future research is recommended with roller clutches and circular friction pads to eliminate the backlash and provide large and stable ductility and dissipation. The lightweight CLT wall tested (<200 kg) demonstrates the potential of ratcheting connectors to create efficient seismic-resisting systems that self-center without any posttensioning. [ABSTRACT FROM AUTHOR]

Due to its superior corrosion resistance and low coefficient of friction, polytetrafluoroethylene (PTFE) is extensively used in the aerospace, machinery, chemical, and pharmaceutical industries. However, PTFE components encounter complex alternating stresses, resulting in ratchet and creep, which will affect the component's reliability. It is therefore necessary to clarify the PTFE's resistance to ratchet and creep. In this paper, uniaxial ratchet and tensile creep experiments were conducted at five temperatures on a PTFE dog-bone tensile specimen. At various temperatures and stress levels, the effects of average stress and stress amplitude on the cyclic plastic behavior of PTFE were investigated. It is demonstrated that the ratchet strains and strain rates at 23 °C are greater than those at 50 °C. The reason for this is that the PTFE material exhibits different crystal states at these two temperatures. At temperatures above 50 °C, the ratchet strain and ratchet strain rate increase with temperature. At temperatures above 100 °C, the ratchet strain and ratchet strain rate of PTFE materials increase more rapidly due to the glass transition. By analyzing the creep strain and ratchet strain of specimens subjected to varying levels of average and amplitude stress, it was discovered that the creep strain and ratchet strain caused by the average stress under the same stress increment were greater than those caused by the amplitude stress. [ABSTRACT FROM AUTHOR]

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Three desirable features of a seismic-resisting system's hysteresis are ductile behavior, consistent dissipation during loading cycles, and negligible residual displacements after earthquakes. A self-centering, pinching-free connector (PFC) was developed to attempt to meet these demands. The tension-only PFC uses a linear ratchet made from unified/metric screw threads to absorb the slack generated when a dissipator deforms/elongates monotonically. Nondestructive ratcheting tests were conducted to support the equations derived to predict the ratcheting behavior. Although the PFC can recenter unimpeded during tension (half) cycles, the ratcheting persisted on the compressive stroke during full tension-compression cycles and subsequently hindered its ability to recenter. Therefore, additional hook and ratchet stoppers were incorporated into the PFC to control the ratcheting and allow recentering from any position. This was verified in a preliminary test of a diagonally braced frame that demonstrated self-centering when unloaded. An incremental numerical procedure was able to predict the cyclic response of the frame satisfactorily from just the backbone curve of the PFC, which is mostly a reflection of the dissipator's backbone. This paper highlights an alternate means to self-centering dissipative systems through a linear ratchet that precludes the need for buckling restraints or substantial prestressing/poststressing. One implication is that very slender braces could be used in moderate-to-high seismicity regions because the pinching is no longer an issue and the dissipative potential is restored for monotonic dissipators. [ABSTRACT FROM AUTHOR]

The collective motion of pulsating active particles with periodic size contraction is investigated in a two-dimensional asymmetric channel. Our findings reveal that changes in particle size can act as a non-equilibrium driving force, disrupting the system’s thermodynamic equilibrium and leading to the transformation of self-contraction motion into directional motion in the asymmetric channel. The specific direction of motion is dictated by the symmetrical properties of the channel. Furthermore, our study identifies an optimal degree of channel opening (or self-pulsation frequency) at which the average velocity reaches its peak value. At lower frequencies, the average velocity demonstrates a peak function in relation to the self-pulsation amplitude (or particle number density). Conversely, at higher frequencies, the average velocity increases with the self-pulsation amplitude (or particle number density). The system exhibits three distinct states: the arrested ordered state, disordered state, and cycling ordered state. Notably, particle rectification reaches its optimum in the disordered state.

The outbreak of Covid-19 in Kenya saw a resurgence of state policing, violence and repression as the state sought ways of decisively containing the spread. This was accompanied by a discourse of "being at war" with a novel and invisible enemy. As a result, terms such as shutdown, lockdown, curfew and isolation were mobilised by state agents and organs not only to refer to the concerted efforts of dealing with the pandemic; they also served to show the state's reach and to justify the excessive exercise of power and control. Through the analysis of two hip-hop songs, "Pandemic" by Kitu Sewer featuring Robah, and "Pandemik" by the Ochungulo Family, this article examines the artists' representation of the state's idioms of closure and pandemic-mediated violence, while at the same time necessitating a reimagination of sociality, freedom and resistance in Kenya's urban margins. Analysing the songs' recourse to Kenyan anti-colonial struggles and ratchetness, I argue that the artists present Nairobi's margins as war zones where several pandemics intersect. In addition, I contend that the artists invite a rethinking of a collapsing world in the face of multiple pandemics as offering the possibility of regeneration and transformation. [ABSTRACT FROM AUTHOR]

Due to its superior corrosion resistance and low coefficient of friction, polytetrafluoroethylene (PTFE) is extensively used in the aerospace, machinery, chemical, and pharmaceutical industries. However, PTFE components encounter complex alternating stresses, resulting in ratchet and creep, which will affect the component’s reliability. It is therefore necessary to clarify the PTFE’s resistance to ratchet and creep. In this paper, uniaxial ratchet and tensile creep experiments were conducted at five temperatures on a PTFE dog-bone tensile specimen. At various temperatures and stress levels, the effects of average stress and stress amplitude on the cyclic plastic behavior of PTFE were investigated. It is demonstrated that the ratchet strains and strain rates at 23 °C are greater than those at 50 °C. The reason for this is that the PTFE material exhibits different crystal states at these two temperatures. At temperatures above 50 °C, the ratchet strain and ratchet strain rate increase with temperature. At temperatures above 100 °C, the ratchet strain and ratchet strain rate of PTFE materials increase more rapidly due to the glass transition. By analyzing the creep strain and ratchet strain of specimens subjected to varying levels of average and amplitude stress, it was discovered that the creep strain and ratchet strain caused by the average stress under the same stress increment were greater than those caused by the amplitude stress.

Self‐centering energy dissipative (SCED) braces have the potential to contribute greatly to building resiliency, because the braces can limit both maximum and residual story drifts during earthquakes. An obstacle to widespread adoption of existing SCED systems is that they require high prestressing forces, corresponding to more than half of the brace yield strength. To address this problem, a new low‐prestressed SCED (LP‐SCED) system has been proposed, which has a flag‐shaped force–deformation hysteresis but needs only a low level of prestressing force. This paper presents the results of an experimental investigation to establish the properties of the new system's key components, as well as prototypes of the full brace specimen. The tested system had a prestressing force equal to only 3∼4% of the brace yield force. The tested systems had a stable, flag‐shaped force–deformation hysteresis relationship, even after being subjected to deformations corresponding to 3.4% story drift. Eventually, when the imposed deformations were sufficiently large, the brace stiffness increased dramatically, which helps to reduce drift concentrations that may be critical in braced‐frame structures under extreme earthquakes. The yielding and damage were restricted to the mild‐steel dissipaters, which could be replaced easily after each test, and in practice, they could be replaced easily after an earthquake. [ABSTRACT FROM AUTHOR]

Shakedown analysis is a powerful tool for the ratchet ＆ alternating plasticity prediction of the pressure piping under thermal-mechanical loads. In order to achieve the shakedown analysis containing the synergetic influence of the kinematic hardening and the temperature-dependent properties of the material,a numerical method was developed. The presenting method is an extension of the two-surface model for shakedown analysis and is based on the basis reduction method. The idea is to transfer the nonlinear programming resulting from the shakedown analysis with kinematic hardening and temperature-dependent nature into an elastic perfectly plastic one which can be solved with less effort. Numerical examples indicated that the proposed method is accurate. This method has the potential to improve the accuracy of failure prediction for the piping suffering thermal-mechanical loads and therefore is of engineering values.

We examine a charge lattice coupled to a one-dimensional asymmetric potential in the presence of an applied magnetic field, which induces gyrotropic effects in the charge motion. This system could be realized for Wigner crystals in nanostructured samples, dusty plasmas, or other classical charge-ordered states where gyrotropic motion and damping can arise. For zero magnetic field, an applied external ac drive can produce a ratchet effect in which the particles move along the easy flow direction of the substrate asymmetry. The zero field ratchet effect can only occur when the ac drive is aligned with the substrate asymmetry direction; however, when a magnetic field is added, the gyrotropic forces generate a Hall effect that leads to a variety of new behaviors, including a transverse ratchet motion that occurs when the ac drive is perpendicular to the substrate asymmetry direction. We show that this system exhibits commensuration effects as well as reversals in the ratchet effect and the Hall angle of the motion. The magnetic field also produces a nonmonotonic ratchet efficiency when the particles become localized at high fields.

Converting random motion into directional motion of charged dust particles in plasma, i.e., the rectification, can be achieved by using a dusty plasma ratchet, in which the asymmetry of system plays a very important role. Here, we use different staggered angles between the inner and outer gears to control the asymmetry of system and further study experimentally the effect of the asymmetry induced by staggered angle on the rectification of dust particles in a dusty plasma ratchet. Our results show that the spatial distribution and the velocity of dust particles along the saw channel are nonuniform. Decreasing the asymmetry can reduce the flow of dust particle in the saw channel. [ABSTRACT FROM AUTHOR]

This paper introduces the structure and working principle of a device for supporting bomb with the combination of ratchet and crank-slider. The virtual prototyping technology is used to analyze the working process of the device,and that the dynamic relationship between the components and the curve of spring force in the buffer mechanism is obtained. The results show that it is feasible to apply the one-way rotation characteristic of ratchet wheel and the quick-return characteristic of crank-slider mechanism to the device. The device for supporting bomb with the principle can realize the cushioning and the locking positioning of the projectile. It meet the projectile transport stability requirements. The device shows the features of intensive structure,small occupied space and wide applicability. It provides a new design idea for the design of the sabot device. While in the mechanical field,it also has a positive significance for extending the scope of combination organization.

This paper addresses Janos Kornai's early work on the socialist economy as summarized in his first book, Overcentralization in Economic Administration. In this context, I discuss the parallel research of the 1950s by American scholars, such as Joseph Berliner, David Granick, Gregory Grossman and Eugene Zaleski, who used similar methods and arrived at similar conclusions. Kornai found little evidence of comprehensive planning. Instead, he determined that the planning system consisted of quarterly gross output orders that could readily be manipulated by managers and had to be fulfilled at any price. Kornai's Overcentralization already contained the seeds of Kornai's later key findings of the dysfunctionalities of socialist planning; namely, soft budget constraints and the shortage economy. Kornai's most important finding was largely overlooked throughout the socialist world—that the planned economy could not be reformed by partial measures. [ABSTRACT FROM AUTHOR]

Molecular machines are ubiquitous in nature and function away from equilibrium by consuming fuels to produce appropriate work. Chemists have recently excelled at mimicking the fantastic job performed by natural molecular machines with synthetic systems soluble in organic solvents. In efforts toward analogous systems working in water, we show that guest molecules can be exchanged in the synthetic macrocycle cucurbit[7]uril by involving kinetic traps, and in such a way as modulating energy wells and kinetic barriers using pH, light, and redox stimuli. Ditolyl‐viologen can also be exchanged using the best kinetic trap and interfaced with alginate, thus affording pH‐responsive blue, fluorescent hydrogels. With tunable rate and binding constants toward relevant guests, cucurbiturils may become excellent ring molecules for the construction of advanced molecular machines working in water. [ABSTRACT FROM AUTHOR]

In this open letter, I ask the editors of the Journal and its readers, to reflect on the Journal's relationship to studies of language and Black sexuality, and consider new ways to reach scholars of Black life, culture, and language. Studies of Black language practices rarely deal with the ways that Black language practices are often complicated by gender/sexuality. And yet, there are scholars doing this work, but like Queer Linguistics, it often doesn't "look" that way that typical studies of language are supposed to look. This is because linguistics and linguistic anthropology as disciplines have often failed to capture the imagination and attention of these scholars; it is not because studies of Black sexuality and language do not exist. I encourage the Journal then to seek out these studies and to do so with a sense of urgency. [ABSTRACT FROM AUTHOR]

Using continuum simulations, we show that under a sinusoidal electric field, liquid crystal skyrmions undergo periodic shape oscillations which produce controlled directed motion. The speed of the skyrmion is non-monotonic in the frequency of the applied field, and exhibits multiple reversals of the motion as a function of changing frequency. We map out the dynamical regime diagram of the forward and reverse motion for two superimposed ac driving frequencies, and show that the reversals and directed motion can occur even when only a single ac driving frequency is present. Using pulsed ac driving, we demonstrate that the motion arises due to an asymmetry in the relaxation times of the skyrmion shape. We discuss the connection between our results and ratchet effects observed in systems without asymmetric substrates.

The biological roles of the three natural F1FO-ATPase inhibitors, ε, ζ, and IF1, on cell physiology remain controversial. The ζ subunit is a useful model for deletion studies since it mimics mitochondrial IF1, but in the F1FO-ATPase of Paracoccus denitrificans (PdF1FO), it is a monogenic and supernumerary subunit. Here, we constructed a P. denitrificans 1222 derivative (PdΔζ) with a deleted ζ gene to determine its role in cell growth and bioenergetics. The results show that the lack of ζ in vivo strongly restricts respiratory P. denitrificans growth, and this is restored by complementation in trans with an exogenous ζ gene. Removal of ζ increased the coupled PdF1FO-ATPase activity without affecting the PdF1FO-ATP synthase turnover, and the latter was not affected at all by ζ reconstitution in vitro. Therefore, ζ works as a unidirectional pawl-ratchet inhibitor of the PdF1FO-ATPase nanomotor favoring the ATP synthase turnover to improve respiratory cell growth and bioenergetics.

An infinitely variable transmission (IVT) is a system that allows for a continuous (non-discrete) variation (including zero) in transmission ratio between two rotating elements. In this paper, a novel ratcheting-type IVT mechanism is presented and its geometrical design and kinematic analysis are studied in details. The proposed system contains two identical units. Each unit includes a cam with a follower, oscillatory slotted links pivoted at a shaft that can be moved vertically by a hydraulic ram (alterable transmission ratio), and a grooved wheel with an actuating rod. The input rotational motion is converted through each unit to an oscillatory angular motion of controlled amplitude. This resulting motion is rectified using a ratchet to get a unidirectional output rotational motion. Therefore, the system output motion will have a different velocity and acceleration than those of the system input. The kinematic analysis revealed that the transmission ratio can be varied continuously in a range from zero to infinity. The analysis also showed that, for particular transmission ratios, the system gives uniform output (angular velocity and acceleration) for a corresponding uniform input. [ABSTRACT FROM AUTHOR]

In this paper, we develop a modified salary scale for NBA rookies that adjusts player financial incentives to enter the draft early. Our proposed "ratcheting" salary schedule pays different salary amounts depending on draft pick and the number of years the player delayed entry to the draft to alter the financial incentives of declaring early. We analyze this proposed new salary scale using a put option analogy to determine early exercise boundary positions in which there is no value in the option to delay entry into the draft. The results show that under a ratchet salary system the incentives for early entry to the draft can be greatly altered, especially for freshmen and sophomores unless they are expected to be lottery picks. This information may help players make better decisions of when to enter the draft and can address unraveling in the NBA labor market. [ABSTRACT FROM AUTHOR]

Highlights • An underwater gas bubble may unidirectional move while impact on hydrophilic a ratchet surface with gradient inclination. • Dynamics of underwater gas bubbles were analyzed and a new mechanism was developed. • The motions of underwater gas bubbles on a ratchet with gradient inclination can be manipulated. Abstract An underwater gas bubble may have a unidirectional movement, after impacting with a ratchet, which consists of asymmetric teeth with gradient inclination. In this work, the corresponding movements were first experimentally investigated. Small bubbles are easily pinned on a ratchet surface. In contrast, large bubbles have directional transportation from large to small inclination areas. Since the detected motions could not be interpreted using existing propulsive mechanisms, a new propulsive mechanism was then developed. Based on the new mechanism, two approaches to manipulate bubbles have been investigated experimentally. [ABSTRACT FROM AUTHOR]

The ATP synthase is a ubiquitous nanomotor that fuels life by the synthesis of the chemical energy of ATP. In order to synthesize ATP, this enzyme is capable of rotating its central rotor in a reversible manner. In the clockwise (CW) direction, it functions as ATP synthase, while in counter clockwise (CCW) sense it functions as an proton pumping ATPase. In bacteria and mitochondria, there are two known canonical natural inhibitor proteins, namely the ε and IF1 subunits. These proteins regulate the CCW F1FO-ATPase activity by blocking γ subunit rotation at the αDP/βDP/γ subunit interface in the F1 domain. Recently, we discovered a unique natural F1-ATPase inhibitor in Paracoccus denitrificans and related α-proteobacteria denoted the ζ subunit. Here, we compare the functional and structural mechanisms of ε, IF1, and ζ, and using the current data in the field, it is evident that all three regulatory proteins interact with the αDP/βDP/γ interface of the F1-ATPase. In order to exert inhibition, IF1 and ζ contain an intrinsically disordered N-terminal protein region (IDPr) that folds into an α-helix when inserted in the αDP/βDP/γ interface. In this context, we revised here the mechanism and role of the ζ subunit as a unidirectional F-ATPase inhibitor blocking exclusively the CCW F1FO-ATPase rotation, without affecting the CW-F1FO-ATP synthase turnover. In summary, the ζ subunit has a mode of action similar to mitochondrial IF1, but in α-proteobacteria. The structural and functional implications of these intrinsically disordered ζ and IF1 inhibitors are discussed to shed light on the control mechanisms of the ATP synthase nanomotor from an evolutionary perspective. [ABSTRACT FROM AUTHOR]

The ATP synthase is a reversible nanomotor that gyrates its central rotor clockwise (CW) to synthesize ATP and in counter clockwise (CCW) direction to hydrolyse it. In bacteria and mitochondria, two natural inhibitor proteins, namely the ε and IF 1 subunits, prevent the wasteful CCW F 1 F O -ATPase activity by blocking γ rotation at the α DP /β DP /γ interface of the F 1 portion. In Paracoccus denitrificans and related α-proteobacteria, we discovered a different natural F 1 -ATPase inhibitor named ζ. Here we revise the functional and structural data showing that this novel ζ subunit, although being different to ε and IF 1 , it also binds to the α DP /β DP /γ interface of the F 1 of P. denitrificans . ζ shifts its N-terminal inhibitory domain from an intrinsically disordered protein region (IDPr) to an α-helix when inserted in the α DP /β DP /γ interface. We showed for the first time the key role of a natural ATP synthase inhibitor by the distinctive phenotype of a Δζ knockout mutant in P. denitrificans . ζ blocks exclusively the CCW F 1 F O -ATPase rotation without affecting the CW-F 1 F O -ATP synthase turnover, confirming that ζ is important for respiratory bacterial growth by working as a unidirectional pawl-ratchet PdF 1 F O -ATPase inhibitor, thus preventing the wasteful consumption of cellular ATP. In summary, ζ is a useful model that mimics mitochondrial IF 1 but in α-proteobacteria. The structural, functional, and endosymbiotic evolutionary implications of this ζ inhibitor are discussed to shed light on the natural control mechanisms of the three natural inhibitor proteins (ε, ζ, and IF 1 ) of this unique ATP synthase nanomotor, essential for life. [ABSTRACT FROM AUTHOR]

This study considers the failed attempts at unifying Black women with Black men and white women after the #BlackGirlsRock mantra was criticized on social media in 2013. I examine the discursive labor of a Black women’s counterpublic in an online comments section and argue that its constituents rejected the behavioral expectations of harmony and decorum in favor of agitation, disrespectability, and ratchetness (Cooper, 2012a, 2012b, 2012c). Qualitative discourse analysis (Gee, 2014a, 2014b) revealed that Black women’s language challenged misconceptions of singular identity narratives through an oppositional discourse and celebrated Black womanhood concurrently. I warrant these claims by discussing the language functions according to two larger discourses: (1) disparaging Black men and white women and (2) fortifying the sistahood. [ABSTRACT FROM AUTHOR]

A new mechanical energy harvester (EH) has been proposed, which can harvest, store, and deliver energy in mechanical form. This harvester can be exploited in bioimplantable devices. The structure composed of three individual parts, namely, the actuator, storage, and latch, accumulates mechanical energy in a conventional spiral spring in terms of stepwise rotation. A ratchet mechanism latches the spring and releases it when energy delivery is required. Various cells are defined and their performances are compared under sequential mechanical forces of up to 7 N amplitude, which models heart vein vibration force. The storage part will store 3.21 mJ energy in a single-step rotation. The total area of the structure was 30.06 mm² with a thickness of 0.7 mm. The proposed mechanical microstructure shows an effective solution in bioimplantable EHs. [ABSTRACT FROM AUTHOR]

Active control of friction by ultrasonic vibration is a well-known effect with numerous technical applications ranging from press forming to micromechanical actuators. Reduction of friction is observed with vibration applied in any of the three possible directions (normal to the contact plane, in the direction of motion and in-plane transverse). In this work, we consider the multi-mode active control of sliding friction, where phase-shifted oscillations in two or more directions act at the same time. Our analysis is based on a macroscopic contact-mechanical model that was recently shown to be well-suited for describing dynamic frictional processes. For simplicity, we limit our analysis to a constant, load-independent normal and tangential stiffness and two superimposed phase-shifted harmonic oscillations, one of them being normal to the plane and the other in the direction of motion. As in previous works utilizing the present model, we assume a constant local coefficient of friction, with reduction of the observed force of friction arising entirely from the macroscopic dynamics of the system. Our numerical simulations show that the resulting law of friction is determined by just three dimensionless parameters. Depending on the values of these parameters, three qualitatively different types of behavior are observed: (a) symmetric velocity-dependence of the coefficient of friction (same for positive and negative velocities), (b) asymmetric dependence with respect to the sign of the velocity, but with zero force at zero velocity, and (c) asymmetric dependence with nonzero force at zero velocity. The latter two cases can be interpreted as a "dynamic ratchet" (b) and an actuator (c). [ABSTRACT FROM AUTHOR]