Ashish Kumar Thokchom

Bio: Ashish Kumar Thokchom is an academic researcher from Ulsan National Institute of Science and Technology. The author has contributed to research in topics: Particle image velocimetry & Particle deposition. The author has an hindex of 9, co-authored 15 publications receiving 247 citations. Previous affiliations of Ashish Kumar Thokchom include Indian Institute of Technology Guwahati & Shiv Nadar University.

Transparent-flexible-multimodal triboelectric nanogenerators for mechanical energy harvesting and self-powered sensor applications

Abstract: Triboelectric nanogenerators (TENGs) harvest and convert mechanical energy to electrical energy. TENGs that are transparent and flexible can be applied to various (opto-)electronic devices supporting finger- or pen-based touchscreen inputs. This paper presents a transparent, flexible TENG that harvests mechanical tapping energy (typically discarded) by simple placement on touchscreen devices. The developed TENG consists of flexible and transparent conducting electrodes (FTCE) with high transmittance (> 93%) and low sheet resistance (18.5 Ω/sq), and transparent 3D-hierarchical polydimethylsiloxane (PDMS) with porous pyramid-patterns. In this study, the developed TENG directly powered eight light-emitting diodes (LEDs) by harvesting the mechanical energy produced by tapping with a touch pen while playing a smartphone game. We also used the transparent TENG as a transparent single-electrode-based, self-powered raindrop detection sensor on a window for a smart home. Our results indicate that the proposed TENG can be used not only as an effective mechanical energy harvester for transparent, flexible, and next-generation optoelectronics devices but also as a self-powered sensor for future Internet-of-Things applications.

Nanochannel-Assisted Perovskite Nanowires: From Growth Mechanisms to Photodetector Applications

Abstract: Growing interest in hybrid organic–inorganic lead halide perovskites has led to the development of various perovskite nanowires (NWs), which have potential use in a wide range of applications, including lasers, photodetectors, and light-emitting diodes (LEDs). However, existing nanofabrication approaches lack the ability to control the number, location, orientation, and properties of perovskite NWs. Their growth mechanism also remains elusive. Here, we demonstrate a micro/nanofluidic fabrication technique (MNFFT) enabling both precise control and in situ monitoring of the growth of perovskite NWs. The initial nucleation point and subsequent growth path of a methylammonium lead iodide–dimethylformamide (MAPbI3·DMF) NW array can be guided by a nanochannel. In situ UV–vis absorption spectra are measured in real time, permitting the study of the growth mechanism of the DMF-mediated crystallization of MAPbI3. As an example of an application of the MNFFT, we demonstrate a highly sensitive MAPbI3-NW-based photod...

Analysis of fluid flow and particle transport in evaporating droplets exposed to infrared heating

Abstract: Analysis of fluid flow and particle transport inside evaporating droplets exposed to external radiation was carried out by experiments and numerical simulations. In this study, we have shown that by altering the free surface temperature we can modify the fluid flow profile inside the droplet and hence the deposition pattern of solute particles on the substrate. The fluid velocity and particle concentration profiles inside the evaporating droplet were measured by Particle Image Velocimetry (PIV) technique. Experiments were carried out on a small sessile water droplet containing dispersed polystyrene particles. To avoid problem of image correction encountered in PIV measurements with 3D droplets, our experiments were performed on an equivalent disc shaped 2D drop sandwiched between two non-wetting surfaces, while the base of the droplet was pinned to a wetting surface. The top surface of the droplet was heated by Infrared (IR) light. The temperature of droplet surface was measured by thermocouples. The velocity field, particle concentration profile and particle deposition patterns were studied during evaporation process. We have also performed numerical simulations by solving continuity, momentum and energy transport equations. The computed velocity profiles resulting from buoyancy and Marangoni convection are in qualitative agreement with the experiments.

Characterizing self-assembly and deposition behavior of nanoparticles in inkjet-printed evaporating droplets

Abstract: The self-assembly and deposition mechanisms of nanoparticles in droplets on a substrate are of significant importance in many inkjet printing-based industrial applications such as microelectronics, display systems, and paint manufacturing. However, a comprehensive investigation into the velocity field of fluid and its accompanying particle transport behavior in injected droplets undergoing immediate evaporation has not been conducted. In this study, we describe the underlying mechanisms of the self-assembly and deposition behavior of nanoparticles in inkjet-printed, evaporating droplets by visualizing the internal fluid flows. We additionally characterize the relationship between the internal fluid flows and nanoparticle patterns by changing not only the wettability and temperature of the substrate, but also the chemical composition of nanoparticle suspensions. We verify that Marangoni flow generated on a hydrophobic PDMS substrate with a contact angle (CA) of >90° helps the formation of dome-shaped nanoparticle structures, while radially outward flow generated on a hydrophilic glass substrate with a CA of

Fluid Flow and Particle Dynamics Inside an Evaporating Droplet Containing Live Bacteria Displaying Chemotaxis

Abstract: Evaporation-induced particle deposition patterns like coffee rings provide easy visual identification that is beneficial for developing inexpensive and simple diagnostic devices for detecting pathogens. In this study, the effect of chemotaxis on such pattern formation has been realized experimentally in drying droplets of bacterial suspensions. We have investigated the velocity field, concentration profile, and deposition pattern in the evaporating droplet of Escherichia coli suspension in the presence and absence of nutrients. Flow visualization experiments using particle image velocimetry (PIV) were carried out with E. coli bacteria as biological tracer particles. Experiments were conducted for suspensions of motile (live) as well as nonmotile (dead) bacteria. In the absence of any nutrient gradient like sugar on the substrate, both types of bacterial suspension showed two symmetric convection cells and a ring like deposition of particles after complete evaporation. Interestingly, the droplet containing...

Recent advancements in supercapacitor technology

Abstract: Supercapacitors (SCs) are attracting considerable research interest as high-performance energy storage devices that can contribute to the rapid growth of low-power electronics (e.g., wearable, portable electronic devices) and high-power military applications (e.g., guided missile techniques and highly sensitive naval warheads). The performance of SCs can be assessed in terms of the electrochemical properties determined through a combination between the electrode and the electrolyte materials. Likewise, the charge storage capacities of SCs can be affected significantly by selection of such materials (e.g., via surface redox mechanisms). Enormous efforts have thus been put to make them more competitive with existing options for energy storage such as rechargeable batteries. This article reviews recent advances in SC technology with respect to charge storage mechanisms, electrode materials, electrolytes (e.g., particularly paper/fiber-like 3D porous structures), and their practical applications. The challenges and opportunities associated with the commercialization of SCs are also discussed.

Bio-Integrated Wearable Systems: A Comprehensive Review

Abstract: Bio-integrated wearable systems can measure a broad range of biophysical, biochemical, and environmental signals to provide critical insights into overall health status and to quantify human performance. Recent advances in material science, chemical analysis techniques, device designs, and assembly methods form the foundations for a uniquely differentiated type of wearable technology, characterized by noninvasive, intimate integration with the soft, curved, time-dynamic surfaces of the body. This review summarizes the latest advances in this emerging field of “bio-integrated” technologies in a comprehensive manner that connects fundamental developments in chemistry, material science, and engineering with sensing technologies that have the potential for widespread deployment and societal benefit in human health care. An introduction to the chemistries and materials for the active components of these systems contextualizes essential design considerations for sensors and associated platforms that appear in f...

Multifunctional Skin-Inspired Flexible Sensor Systems for Wearable Electronics

Abstract: DOI: 10.1002/admt.201800628 applications (e.g., soft robotics, medical devices).[1–6] Despite state-of-the-art bulkbased planar integrated-circuit devices, their rigid and brittle nature gives rise to the incompatibility with curvilinear and soft human bodies, restricting the development of newborn human-friendly interactive electronics. In contrast, the bendable and flexible wearable electronics could be conformally attached onto human bodies almost without discomfort and succeed in performing a great deal of sensing functionalities. Realization of such promising goals requires the flexible sensor platforms provided with crucial characteristics of light weight, ultrathinness, superior flexibility, stretchability, high sensitivity as well as rapid response.[7–10] Inspired by the perceptive features of human skins, the wearable sensor systems are capable of acquiring abundant information from the external environment with the assistance of sensing modules, such as pressure sensors, strain sensors, temperature sensors, etc.[11] A typical example is their application in prosthetics that could afford the capacity to perceive touch or temperature for the disabled.[12] Additionally, the wearable sensor systems are able to identify physical or chemical signals produced by the human body, providing promising opportunities to evaluate health states.[5,13–15] Conventional skin-like sensor platforms primarily comprise one or two sensing modules, data processing units, and power supplies. Their unitary functionality, however, cannot satisfy the increasing demands of IoTs. Recently, the rapid advances in novel sensing materials, fabrication strategies, and innovative electronic constitution contribute to the development of versatile integration of multimodal sensors, which could synchronously distinguish diverse stimuli from the complex environment and monitor multiple vital signs from the human body.[16,17] In spite of several attempts done in terms of such multimodal sensor systems, one of the cumbersome issues originates from the crosscoupling effect among different categories of signals simultaneously generated by various sensors. Furthermore, the skin-like multiple sensor systems usually suffer from the limited number of repeated use, resulting in their high use-cost. The development of separable versatile devices may address this issue with one layer realized by costeffective materials and fabrication manners for disposable use and the other composed of relatively expensive components for repeatable applications.[18] Additionally, the multiple bending or Skin-inspired wearable devices hold great potentials in the next generation of smart portable electronics owing to their intriguing applications in healthcare monitoring, soft robotics, artificial intelligence, and human–machine interfaces. Despite tremendous research efforts dedicated to judiciously tailoring wearable devices in terms of their thickness, portability, flexibility, bendability as well as stretchability, the emerging Internet of Things demand the skininterfaced flexible systems to be endowed with additional functionalities with the capability of mimicking skin-like perception and beyond. This review covers and highlights the latest advances of burgeoning multifunctional wearable electronics, primarily including versatile multimodal sensor systems, self-healing material-based devices, and self-powered flexible sensors. To render the penetration of human-interactive devices into global markets and households, economical manufacturing techniques are crucial to achieve large-scale flexible systems with high-throughput capability. The booming innovations in this research field will push the scientific community forward and benefit human beings in the near future.

Synergistic effect of graphene nanosheet and BaTiO3 nanoparticles on performance enhancement of electrospun PVDF nanofiber mat for flexible piezoelectric nanogenerators

Abstract: Piezoelectric nanogenerators (PENGs) with good flexibility and high outputs have promising and outstanding applications for harvesting mechanical energy and powering electronics In this work, PENGs are fabricated by using electrospun nanocomposite fiber mats comprising barium titanate (BT) nanoparticles, graphene nanosheets and poly(vinylidene fluoride) (PVDF) When the nanocomposite fiber mats are composed of 015 wt% graphene nanosheets and 15 wt% BT nanoparticles, the open-circuit voltage and electric power of the PENG can reach as high as 11 V and 41 μW under a loading frequency of 2 Hz and a strain of 4 mm, and no apparent decline of the open-circuit voltage is observed after 1800 cycles in the durability test In addition, the PENG generate a peak voltage as high as 112 V during a finger pressing-releasing process, which can light up 15 LEDs and drive an electric watch The improved output of the PENG is ascribed to the synergistic contribution of the BT nanoparticles and graphene nanosheets to the piezoelectric performance enhancement of the nanocomposite fibers This study shows that the nanocomposite fiber based flexible PENGs are promising mechanical energy harvesters and effective power sources for portable electronic and wearable devices