It is shown that an extension of the Planck law to statistical ensembles of wave fields with a non-vanishing net heat flux makes it possible to represent the interfacial thermal resistance between media, where heat is carried by mechanical waves in terms of algebraic equations.

Soft haptic devices could enable unique applications in AR/VR, healthcare, and human-machine interface systems. Here, a versatile reconfigurable haptic system is proposed for different application scenarios at diverse anatomical locations and body shapes. Lego toys are the inspiration of the module concept where individual haptic modules can be cut, reconfigured, and reconnected as a new system in the ambient environment repeatedly. Each module consists of the basic building block of flexible self-healable ion-conducting and insulating layers. After the assembly and self-healing processes, experimental results show minor changes in their specific electrical conductivities (>90% recovery) and output actuation displacements (>80%) while the connected modules can sustain a tensile force of over 4 Newton without breakage. In a proof-of-concept demo, a soft haptic array is reconfigured to different sizes and shapes without affecting mechanical stimulation outputs. There are many possible applications in AR/VR and human-machine interfaces, we show that these modules can be readily reconfigured between two common usages in this work: (1) as virtual keyboards by working separately; (2) as a smart safety warning patch on the steering wheel of a car when assembled together.

AbstractLaser-lubricant interaction has been a critical reliability issue in a thermo-tribological system named heat-assisted magnetic recording, one of the next generation hard disk drive solutions to increasing data storage. The lubricant response under laser irradiation and the subsequent lubricant recovery are crucial to the system’s reliability and longevity, however, they cannot be diagnosed locally and timely so far. Here, we propose a thermal scheme to in-situ characterize the mechanical laser-lubricant interaction. The nanometer-thick lubricant has a thermal barrier effect on the near-field thermal transport in the system, according to which the lubricant thickness can be determined. As demonstrations, this paper reports the first quantitative in-situ measurements of the laser-induced lubricant depletion and the subsequent reflow dynamics. The proposed scheme shows a sub-angstrom resolution (~0.2 Å) and a fast response time within seconds, rendering in-situ real-time lubricant diagnosis feasible in the practical hard disk drive products.

This report investigates the kinetics of lubricant molecules in the HAMR air bearing to understand the initiation and growth of PFPE contamination on the head surface. The collisions with the air bearing induce three forces—drag, thermophoresis, and lift. Of these, we find that lift forces are negligible. Then, a sensitivity analysis of the remaining two forces reveals the conditions where they dominate. Further, a hybrid simulation strategy is utilized to track their movements. The results show that the contaminations (smear) highly depend on the interplay between the thermophoresis and drag forces. We then explain the mechanism of the formation of the various observed patterns. Finally, we offer some recommendations to exploit the air bearing to contain smear on the head.

We simulate the gas bearing in the HAMR head-disk interface using the direct simulation Monte Carlo (DSMC) method and the consistent Boltzmann algorithm. We report two observed phenomena and comment upon their implications on smear formation on the head surface. First, we observe that in very low spacing, the gas particles are more likely to collide with the head and disk surfaces rather than with each other. Second, we observe the presence of a gas velocity component toward the head in the leading edge and trailing edge. These vertical drifts can carry airborne contaminant particles to the head surface, forming smear.

In this protocol, we present a facile nanoscale thermal mapping technique for electronic devices by use of atomic force microscopy and a phase change material Ge 2 Sb 2 Te 5 . We describe steps for Ge 2 Sb 2 Te 5 thin film coating, Ge 2 Sb 2 Te 5 temperature calibration, thermal mapping by varying heater power, and thermal mapping by varying heating time. The protocol can be applied for resolving surface temperatures of various operational microelectronic devices with a nanoscale precision. For complete details on the use and execution of this protocol, please refer to Cheng et al. 1

It is demonstrated that the commonly accepted mathematical derivation of the fluctuation-dissipation theorem for electromagnetic fields in spatial domains involves restrictive conditions that essentially limit the applicability of the Theorem to the analysis of large-scale radiative heat transfer, and therefore, it may not be applicable to problems of radiative heat transfer over nano- and low-micro-scale distances.

Modeling the smear growth in the write heads of the Heat Assisted Magnetic Recording (HAMR) Disk Drives is an essential tool for understanding the mechanisms of smear formation. In this paper, we present a novel method to simulate the growth of smear under the influence of thermophoretic and drag force. First, a time-averaged force field is generated using particle mechanics. Then, using evaporation and condensation parameters, we track the trajectory of random smear particles on the disk. The resulting particles that form smear on the head are plotted and compared with experimental results. The comparison reveals good similarities.

A main challenge in Heat-Assisted Magnetic Recording technology is the build-up of contaminants called smear on the near field transducer. In this paper, we investigate the role of optical forces originating from the electric field gradient in the formation of smear. First, based on suitable theoretical approximations, we compare this force with air drag and the thermophoretic force in the head-disk interface for two smear nanoparticle shapes. Then, we evaluate the force field’s sensitivity to the relevant parameter space. We find that the smear nanoparticle’s refractive index, shape, and volume significantly impact the optical force. Further, our simulations reveal that the interface conditions, such as spacing and the presence of other contaminants, also influence the magnitude of the force.

Smear is a key issue hindering the reliability of HAMR drives. Under ultra-low spacing, the random thermal motions by particles can lead to a smear generation on the head. To study this phenomenon, we began by reducing the complexity of the Head-disk interface (HDI) by considering it as a three-body problem. Then, using a Lennard-Jones potential, we calculated the interaction energy for a particle that eventually forms smear. Finally, using Kramer’s rate theory, we calculated the rate of transport of particles from the disk to the head.

Heat-assisted magnetic recording (HAMR), one of the promising hard disk drive technologies to achieve areal density > 10 Tb/in ^2 , integrates a laser across its head-disk interface to assist data writing. However, the laser brings a disk-to-head material transfer problem due to the temperature difference. The thermally-induced material buildup on the head, also known as smear, has been a crucial reliability issue in HAMR, which needs to be addressed before HAMR hard disk drive’s commercialization. In this paper, two mitigation strategies for the material buildup issue are presented: a mechanical approach and a thermal approach. The mechanical approach utilizes the thermal fly-height control heater protrusions to accomplish light head-disk contact to burnish the buildup away due to frictional interactions. The atomic force microscopy images show that the upstream/downstream buildup can be removed by use of dual heaters respectively only within several revolutions. In the thermal approach, the direction of the temperature difference across the head-disk interface is reversed compared to that during the data writing. The results show that the material buildup breaks down into small dots from traces and is mitigated by 91 ∼ 40 min. Additionally, it is found that a pulsed laser operating at a higher frequency can produce less upstream buildup. This study contributes to the material buildup management in the HAMR head-disk interface, and hence, it is important for the next generation HAMR technology.

In hard disk drives, a recording head and a recording disk constitute a high-speed sliding system: head–disk interface, where the head with a prescribed thermal protrusion on its surface flies over the rotating disk at an air gap < 1–2 nm. Currently, the magnetic recording operations involve two embedded joule heaters in the recording head to generate two thermal protrusions and thus improve the device performance in data writing and data reading. In this study, we performed touchdown experiments to induce frictional interaction between the head and the disk and characterize the total head protrusion due to the dual heaters. The experimental results demonstrate that the head protrusion can be tuned by use of the dual heaters, and that the touchdown area can be controlled precisely.

Heat assisted magnetic recording, as one of the next generation hard disk drive solutions to high areal density over 1 Tb/in.2, integrates a laser delivery system to facilitate data writing. A laser beam is launched from the recording head and is focused on the recording disk to locally heat the disk (400–500 °C), which is even hotter than the head temperature (150–250 °C). Therefore, understanding the thermal transport between the head and the disk is of great importance. In this paper, we used a non-flying test stage to exclude the strong air cooling caused by the rotating disk and performed the thermal transport experiments across a closing nanoscale air gap on two substrates (silicon wafer and AlMg-substrate disk). The experimental results show that the disk-to-head back-heating from the hot spot on the substrate can be directly measured in the case of the AlMg disk (∼2–10 °C), while the silicon case shows no back-heating due to its high thermal conductivity. It is demonstrated that the experimental setup is useful for thermal transport studies between two macroscopic surfaces and future development of such microelectronic devices.

This work presents a novel study of radiative heat transfer between closely separated plates based on an extension of Planck’s spectrum of thermal radiations to systems with a steady heat flux. This extension together with electromagnetic wave theory is chosen specifically to avoid the commonly used so-called fluctuation dissipation theory, which is also limited to equilibrium systems. The spectrum of thermal radiation with a heat flux is described by the introduction of an analog of a chemical potential, which creates a bias toward the direction of heat transfer. This is the first comprehensive study of radiative heat transfer based on the generalization of Planck’s spectrum for systems with a heat flux, which eliminates contradictions arising when a heat flux is described in terms of the laws limited to equilibrium systems. The total heat flux is split into fluxes carried by waves with different frequencies, directions of propagation, and polarizations. This simplifies the analysis because due to the stochastic independence, the energy fluxes of such waves are additive, and this also reveals that the heat carrying capacity of radiation with the parallel polarization is significantly higher than that of the perpendicularly polarized radiation. This suggests that the rate of radiative heat transfer may be noticeably increased by the control of the polarization of thermal radiation.

The head-disk interface (HDI) in heat-assisted magnetic recording (HAMR) hard disk drives is a system where a laser beam is launched from a recording head and focused on a recording disk to facilitate data writing. During the laser exposure, material transfers from the disk to the head surface due to the high temperature field and steep thermal gradient. The material accumulation on the head surface, also known as smear, is a challenging reliability issue for HAMR. In this paper, we experimentally investigated the effect of disk temperature and laser exposure time on the smear formation and studied the smear removal by frictional interactions between the head and the disk. In the experiments, the disk temperature and the laser exposure time were controlled separately to generate the smear, which was later characterized by atomic force microscopy (AFM). The AFM images show that the smear forms when the lubricant evaporation occurs for a certain time, and that the smear amount increases with the disk temperature and the laser exposure time. Furthermore, touchdown experiments were performed using the heads with smear. The results indicate that the smear is mostly removed by friction from the head-disk contact. This study reveals the mechanism of the smear formation in HAMR and presents a mechanical approach to mitigate the smear without damaging the head.

One grand challenge in haptic human-machine interface devices is to electromechanically stimulate sensations on the human skin wirelessly by thin and soft patches under a low driving voltage. Here, we propose a soft haptics-feedback system using highly charged, polymeric electret films with an annulus-shape bump structure to induce mechanical sensations on the fingertip of volunteers under an applied voltage range of 5-20 V. As an application demonstration, a 3 x 3 actuators array is used for transmitting patterned haptic information, such as letters of 'T', 'H', 'U' letters and numbers of '0', '1', '2'. Moreover, together with flexible lithium batteries and a flexible circuit board, an untethered stimulation patch is constructed for operations of 1 h. The analytical model, design principle, and performance characterizations can be applicable for the integration of other wearable electronics toward practical applications in the fields of AR (augmented reality), VR (virtual reality) and robotics.

One grand challenge in haptic human-machine interface devices is to electromechanically stimulate sensations on human skins wirelessly by thin and soft patches under a low driving voltage. Here, we propose a soft haptics-feedback system using highly charged, polymeric electret films with the annulus-shape bump contact structure to induce mechanical sensations on volunteers under an applied voltage as low as 5 volts. Together with bendable lithium ion batteries and a flexible circuit board, an untethered stimulation patch is constructed for active operations of at least 2 hours with low power consumptions. As an application example, a “silent haptic communication” system is demonstrated to transmit English alphabet letters via the mechanical beating patterns from a patch onto the fingertip of a receiving volunteer. The analytical model, design principle, and performance characterizations can be applicable for the integrations with other devices in wearable electronics toward various applications, including AR (augmented reality) and VR (virtual reality).

To study the nanoscale heat transfer and laser-related protrusions in heat-assisted magnetic recording (HAMR), we performed static touchdown experiments between HAMR waveguide heads and non-rotating media such as a silicon wafer and a recording disk with an AlMg substrate. During the static touchdown, the laser element is energized with DC current and the embedded contact sensor (ECS) is used to monitor the head temperature. The experimental results show that the thermal fly-height control (TFC) touchdown power decreases with increasing laser current. Meanwhile, the head temperature increases due to the laser heating. From this the ECS resistance rise induced by the laser is extracted. The results show that the silicon wafer dissipates heat effectively under the laser exposure, while the AlMg-substrate disk undergoes a higher temperature rise, which in turn heats the head.

Understanding the heat transfer behavior at the nanoscale head-disk interface (HDI) in hard disk drives is crucial for head design, media design, and failure analysis of the current hard disk drive (HDD) industry, especially for the emerging technologies including heat-assisted magnetic recording (HAMR), microwave-assisted magnetic recording (MAMR), and 2-D magnetic recording (TDMR). Previous experimental studies of both static touchdown technique and theoretical developments of the wave-based phonon conduction have shown enhanced heat transfer at the HDI. To better understand the heat transfer behavior across the HDI, a series of the simulation is necessary to connect the theory and the detailed geometric model of the HDI. In this article, we developed a finite-element model to explain the temperature change of the head during a static touchdown experiment. The wave-based phonon conduction theory is integrated into the simulation through iteration. The simulation results trend in agreement with the theoretical development and the experimental results. This simulation strategy can also be implemented for flying heads to predict the heat transfer behavior under HAMR conditions.

In heat-assisted magnetic recording (HAMR), a laser is introduced to create a hot spot on the media and locally heat the magnetic layer to its Curie temperature. Besides the optical power that the laser provides to the media, thermal energy diffuses inside the slider and induces an extra protrusion, which is called laser-induced protrusion (LIP). The LIP needs to be considered and compensated during flying in the HAMR conditions. In this study, we focus on long timescale (milliseconds) of laser heating during the flying condition. When the laser is switched from OFF to ON, the touchdown power, indicated by an acoustic emission (AE) sensor, decreases due to spacing loss and the touchdown power change (DTDP) is used as the measure of the LIP. A component-level spinstand stage for HAMR heads and media is used to study the LIP as a function of laser-on time, laser current and linear velocity. Our experimental results show that it takes around 20 ms for the LIP to reach steady state and the protrusion size is proportional to the square of laser current. As the operating linear velocity increases from 12 m/s to 24 m/s, the LIP decreases by approximately 52%.