VTechWorks Repository :: Browsing by Author "Cheng, Jiangtao"

Browsing by Author "Cheng, Jiangtao"

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Adaptive optical beam steering and tuning system based on electrowetting driven fluidic rotor
Cheng, Weifeng; Liu, Jiansheng; Zheng, Zheng; He, Xukun; Zheng, Bowen; Zhang, Hualiang; Cui, Huachen; Zheng, Xiaoyu; Zheng, Tao; Gnade, Bruce E.; Cheng, Jiangtao (2020-01-27)
Reconfigurable beam steering components are indispensable to support optical and photonic network systems operating with high adaptability and with various functions. Currently, almost all such components are made of solid parts whose structures are rigid, and hence their functions are difficult to be reconfigured. Also, optical concentration beam steering is still a very challenging problem compared to radio frequency/microwave steering. Here we show a watermill-like beam steering system that can adaptively guide concentrating optical beam to targeted receivers. The system comprises a liquid droplet actuation mechanism based on electrowetting-on-dielectric, a superlattice-structured rotation hub, and an enhanced optical reflecting membrane. The specular reflector can be adaptively tuned within the lateral orientation of 360 degrees, and the steering speed can reach similar to 353.5 degrees s(-1). This work demonstrates the feasibility of driving a macro-size solid structure with liquid microdroplets, opening a new avenue for developing reconfigurable components such as optical switches in next-generation sensor networks.
Adsorption of Small Molecules in Advanced Material Systems
Zhang, Fei (Virginia Tech, 2019-06-10)
Adsorption is a ubiquitous phenomenon that plays key roles in numerous applications including molecule separation, energy storage, catalysis, and lubrications. Since adsorption is sensitive to molecular details of adsorbate molecule and adsorbent materials, it is often difficult to describe theoretically. Molecular modeling capable of resolving physical processes at atomistic scales is an effective method for studying adsorption. In this dissertation, the adsorption of small molecules in three emerging materials systems: porous liquids, room-temperature ionic liquids, and atomically sharp electrodes immersed in aqueous electrolytes, are investigated to understand the physics of adsorption as well as to help design and optimize these materials systems. Thermodynamics and kinetics of gas storage in the recently synthesized porous liquids (crown-ether-substituted cage molecules dispersed in an organic solvent) were studied. Gas molecules were found to store differently in cage molecules with gas storage capacity per cage in the following order: CO2>CH4>N2. The cage molecules show selectivity of CO2 over CH4/N2 and demonstrate capability in gas separation. These studies suggest that porous liquids can be useful for CO2 capture from power plants and CH4 separation from shale gas. The effect of adsorbed water on the three-dimensional structure of ionic liquids [BMIM][Tf2N] near mica surfaces was investigated. It was shown that water, as a dielectric solvent and a molecular liquid, can alter layering and ordering of ions near mica surfaces. A three-way coupling between the self-organization of ions, the adsorption of interfacial water, and the electrification of the solid surfaces was suggested to govern the structure of ionic liquid near solid surfaces. The effects of electrode charge and surface curvature on adsorption of N2 molecules near electrodes immersed in water were studied. N2 molecules are enriched near neutral electrodes. Their enrichment is enhanced as the electrode becomes moderately charged but is reduced when the electrode becomes highly charged. Near highly charged electrodes, the amount of N2 molecules available for electrochemical reduction is an order of magnitude higher near spherical electrodes with radius ~1nm than near planar electrodes. The underlying molecular mechanisms are elucidated and their implications for development of electrodes for electrochemical reduction of N2 are discussed.
Analysis of Interfacial Processes on Non-Wetting Surfaces
Hatte, Sandeep Shankarrao (Virginia Tech, 2022-10-04)
Non-wetting surfaces mainly categorized into superhydrophobic (SHS), lubricant-infused (LIS) and solid-infused surfaces (SIS), by virtue of their superior water repellant properties have wide applications in several energy and environmental systems. In this dissertation, the role of non-wetting surfaces toward the enhancement of condensation effectiveness is analyzed by taking into consideration the tube side and shell side individual interfacial energy transport processes namely, drag reduction, convection heat transfer enhancement, fouling mitigation and dropwise condensation heat transfer. First, an analytical solution is developed for effective slip length and, in turn, drag reduction and friction factor on structured non-wetting surfaces. Secondly, by combining the solution for effective slip length on structured non-wetting surfaces and the fractal characterization of generic multiscale rough surfaces, a theoretical analysis of drag reduction, friction factor, and convection heat transfer enhancement is conducted for scalable non-wetting surfaces. Next, fractal representation of rough surfaces is used to theoretical derive the dropwise condensation heat transfer performance on SHS and novel SIS surfaces. The aspect of dynamic fouling mitigation properties of non-wetting surfaces is explored by conducting systematic experiments. Using Taguchi design of experiments, this work for the first time presents a closed formed relationship of fouling mitigation quantified in terms of asymptotic fouling resistance with Reynolds number, foulant concentration and viscosity of the infusion material that represents the different surface types in a unified manner. Furthermore, it was observed that LIS and SIS offer excellent fouling mitigation compared to SHS and conventional smooth surfaces, however only SIS owing to the presence of solid-like infusion materials is observed to be robust for practical applications.
Analyzing the Molecular Kinetics of Water Spreading on Hydrophobic Surfaces via Molecular Dynamics Simulation
Zhao, Lei; Cheng, Jiangtao (Springer Nature, 2017-09-07)
In this paper, we report molecular kinetic analyses of water spreading on hydrophobic surfaces via molecular dynamics simulation. The hydrophobic surfaces are composed of amorphous polytetrafluoroethylene (PTFE) with a static contact angle of similar to 112.4 degrees for water. On the basis of the molecular kinetic theory (MKT), the influences of both viscous damping and solid-liquid retarding were analyzed in evaluating contact line friction, which characterizes the frictional force on the contact line. The unit displacement length on PTFE was estimated to be similar to 0.621 nm and is similar to 4 times as long as the bond length of C-C backbone. The static friction coefficient was found to be similar to 10(-3) Pa.s, which is on the same order of magnitude as the dynamic viscosity of water, and increases with the droplet size. A nondimensional number defined by the ratio of the standard deviation of wetting velocity to the characteristic wetting velocity was put forward to signify the strength of the inherent contact line fluctuation and unveil the mechanism of enhanced energy dissipation in nanoscale, whereas such effect would become insignificant in macroscale. Moreover, regarding a liquid droplet on hydrophobic or superhydrophobic surfaces, an approximate solution to the base radius development was derived by an asymptotic expansion approach.
Computing Wall Thickness and Young's Modulus of Carbon Nanotubes with Atomistic Molecular Dynamics Simulations
Ahmed, Tabassum (Virginia Tech, 2021-06-02)
Carbon nanotubes (CNTs) are tubular structure of a layer or layers of carbon atoms. CNTs serve as a prototypical nanomaterial holding great promises for various basic and applied research applications in the fields of electrical, thermal, and structural materials owing to their superlative mechanical, thermal, electrical, optical, and chemical properties. Since the discovery of CNTs by Iijima in 1991, numerous researches have been conducted to quantify and understand the atomic origin of their high strength, exceptional thermal conductivity, and unique electrical properties. CNTs are also widely used as nanofillers in composite materials to enhance their mechanical properties such as fracture toughness and to serve as sensing agents. There is thus an imperative need to deeply understand the physical properties of CNTs and their responses to various models of deformations such as stretching, bending, twisting, and combinations thereof. In this thesis, we apply all-atom molecular dynamics simulations to study in detail the behavior of several single-walled, armchair CNTs under stretching and bending deformations, realized by imposing appropriate boundary conditions on the CNTs. The simulation results reveal unique scaling properties of the stretching and bending stiffness with respect to the CNT radius and length, which indicate that a single-walled CNT is best modeled as a thin cylindrical shell with a cross-sectional radius equal to the CNT radius and a constant wall thickness much smaller than the CNT radius. By studying the thermal fluctuations of carbon atoms on the CNT wall, the wall thickness is determined to be about 0.45~AA~for all the single-walled CNTs studied in this thesis and correspondingly, Young's modulus is estimated to be about 8.78 TPa for these CNTs.
Design, Analysis, and Application of Architected Ferroelectric Lattice Materials
Wei, Amanda Xin (Virginia Tech, 2019-06-21)
Ferroelectric materials have been an area of keen interest for researchers due to their useful electro-mechanical coupling properties for a range of modern applications, such as sensing, precision actuation, or energy harvesting. The distribution of the piezoelectric coefficients, which corresponds to the piezoelectric properties, in traditional crystalline ferroelectric materials are determined by their inherent crystalline structure. This restriction limits the tunability of their piezoelectric properties. In the present work, ferroelectric lattice materials capable of a wide range of rationally designed piezoelectric coefficients are achieved through lattice micro-architecture design. The piezoelectric coefficients of several lattice designs are analyzed and predicted using an analytical volume-averaging approach. Finite element models were used to verify the analytical predictions and strong agreement between the two sets of results were found. Select lattice designs were additively manufactured using projection microstereolithography from a PZT-polymer composite and their piezoelectric coefficients experimentally verified and also found to be in agreement with the analytical and numerical predictions. The results show that the use of lattice micro-architecture successfully decouples the dependency of the piezoelectric properties on the material's crystalline structure, giving the user a means to tune the piezoelectric properties of the lattice materials. Real-world application of a ferroelectric lattice structure is demonstrated through application as a multi-directional stress sensor.
Development of High-Performance Optofluidic Sensors on Micro/Nanostructured Surfaces
Cheng, Weifeng (Virginia Tech, 2020-01-22)
Optofluidic sensing utilizes the advantages of both microfluidic and optical science to achieve tunable and reconfigurable high-performance sensing purpose, which has established itself as a new and dynamic research field for exciting developments at the interface of photonics, microfluidics, and the life sciences. With the trend of developing miniaturized electronic devices and integrating multi-functional units on lab-on-a-chip instruments, more and more desires request for novel and powerful approaches to integrating optical elements and fluids on the same chip-scale system in recent years. By taking advantage of the electrowetting phenomenon, the wettability of liquid droplet on micro/nano-structured surfaces and the Leidenfrost effect, this doctoral research focuses on developing high-performance optofluidic sensing systems, including optical beam adaptive steering, whispering gallery mode (WGM) optical sensing, and surface-enhanced Raman spectroscopy (SERS) sensing. A watermill-like beam steering system is developed that can adaptively guide concentrating optical beam to targeted receivers. The system comprises a liquid droplet actuation mechanism based on electrowetting-on-dielectric, a superlattice-structured rotation hub, and an enhanced optical reflecting membrane. The specular reflector can be adaptively tuned within the lateral orientation of 360°, and the steering speed can reach ~353.5°/s. This work demonstrates the feasibility of driving a macro-size solid structure with liquid microdroplets, opening a new avenue for developing reconfigurable components such as optical switches in next-generation sensor network. Furthermore, the WGM sensing system is demonstrated to be stimulated along the meridian plane of a liquid microdroplet, instead of equatorial plane, resting on a properly designed nanostructured chip surface. The unavoidable deformation along the meridian rim of the sessile microdroplet can be controlled and regulated by tailoring the nanopillar structures and their associated hydrophobicity. The nanostructured superhydrophobic chip surface and its impact on the microdroplet morphology are modeled by Surface Evolver (SE), which is subsequently validated by the Cassie-Wenzel theory of wetting. The influence of the microdroplet morphology on the optical characteristics of WGMs is further numerically studied using the Finite-Difference Time-Domain method (FDTD) and it is found that meridian WGMs with intrinsic quality factor Q exceeding 104 can exist. Importantly, such meridian WGMs can be efficiently excited by a waveguiding structure embedded in the planar chip, which could significantly reduce the overall system complexity by eliminating conventional mechanical coupling parts. Our simulation results also demonstrate that this optofluidic resonator can achieve a sensitivity as high as 530 nm/RIU. This on-chip coupling scheme could pave the way for developing lab-on-a-chip resonators for high-resolution sensing of trace analytes in various applications ranging from chemical detections, biological reaction processes to environmental protection. Lastly, this research reports a new type of high-performance SERS substrate with nanolaminated plasmonic nanostructures patterned on a hierarchical micro/nanostructured surface, which demonstrates SERS enhancement factor as high as 1.8 x 107. Different from the current SERS substrates which heavily relies on durability-poor surface structure modifications and various chemical coatings on the platform surfaces which can deteriorate the SERS enhancement factor (EF) as the coating materials may block hot spots, the Leidenfrost effect-inspired evaporation approach is proposed to minimize the analyte deposition area and maximize the analyte concentration on the SERS sensing substrate. By intentionally regulating the temperature of the SERS substrate during evaporation process, the Rhodamine 6G (R6G) molecules inside a droplet with an initial concentration of 10-9 M is deposited within an area of 450 μm2, and can be successfully detected with a practical detection time of 0.1 s and a low excitation power of 1.3 mW.
The Development of High-Throughput and Miniaturized Differential Scanning Calorimeter for Thermodynamic Study of Bio-Molecules
Yu, Shifeng (Virginia Tech, 2019-02-19)
Biomolecular interactions are fundamentally important for a wide variety of biological processes. Understanding the temperature dependence of biomolecular interactions is hence critical for applications in fundamental sciences and drug discovery. Micro-Electro-Mechanical Systems (MEMS) technology holds great potential in facilitating temperature-dependent characterization of biomolecular interactions by providing on-chip microfluidic handling with drastically reduced sample consumption, and well controlled micro- or nanoscale environments in which biomolecules are effectively and efficiently manipulated and analyzed. This dissertation is focused on a high-through and miniaturized differential scanning calorimeter for thermodynamic study of bio-molecules using MEMS techniques. The dissertation firstly introduces the overall design and operation principles. This miniaturized DSC was fabricated based on a polyimide (PI) thin film. Highly temperature sensitive vanadium oxide was used as the thermistor material. A PDMS (Polydimethylsiloxane) microfluidic chamber was separately fabricated and then bonded firmly with the PI substrate by a stamp-and-stick method. Meanwhile, the micro heater design was optimized to reach better uniformity. A heating stage was constructed for fast and reliable scanning. In this study, we used syringes to deliver the 0.63 μL liquid sample into both the sample and reference chambers. All the testing processes were functionalized using the LabVIEW programs. The sensing material was also characterized. To seek a higher temperature coefficient of resistance (TCR) and less resistive behavior, explorations about various PVD (physical vapor deposition) parameters and annealing conditions were conducted for optimization. In this research, we found vanadium oxide deposited under certain conditions leads to the highest TCR value (a maximum of 2.51%/oC). To better understand the material’s property, we also did the XRD (X-ray Diffraction), SEM (Scanning electron microscope). The micro calorimeter was calibrated using a step thermal response. The time constant was around 3s, the thermal conductance was 0.6mW/K, and the sensitivity was 6.1V/W. The static power resolution of the device at equilibrium is 100 nW, corresponding to 250 nJ/K. These performances confirmed the design and material to be appropriate for both good thermal isolation and power sensitivity. We demonstrated the miniaturized DSC’s performance on several different kinds of protein samples: lysozyme, and mAb (monoclonal antibody) and a DVD IgG (double variable domain immunoglobulin G). The results were found to be reasonable by comparing it with the commercial DSC’s tests. Finally, this instrument may be ideal for incorporation into high throughput screening workflows for the relative comparison of thermal properties between large numbers of proteins when only small quantities are available. The micro-DSC has the potential to characterize the thermal stability of the protein sample with significantly higher throughput and less sample consumption, which could potentially reduce the time and cost for the drug formulation in the pharmaceutical industry.
Development of Microfluidic Platforms for Electric Field-Driven Drug Delivery and Cell Migration
Moarefian, Maryam (Virginia Tech, 2020-06-02)
Recent technologies in micro-devices for investigation of functional biology in a controlled microenvironment are continually growing and evolving. In particular, electric-field mediated microfluidic platforms are evolving technologies that have significant applications in drug delivery and cell migration investigations. Although drug delivery has had several successes, in some areas, it continues to be a challenge; in recent years, the positive impact of electric fields is being explored. The primary objectives of the dissertation are to design, fabricate, and employ two novel microfluidic platforms for drug delivery and cell migration in the presence of electric fields. Description of iontophoretic carboplatin delivery into the MDA-MB-231 triple-negative breast cancer cells and investigation of neutrophil electro taxis are two main aims of the dissertation. Transdermal drug delivery systems such as iontophoresis are useful tools for delivering chemotherapeutics for tumor treatment not only because of their non-invasiveness but also due to their lower systematic toxicity compared to other drug delivery systems. While iontophoresis animal models are commonly being used for the development of new cancer therapies, there are some obstacles for precise control of the tumor microenvironment's chemoresistance and scaffold in the animal models. We employed experimental and computational approaches, the iontophoresis-on-chip and the fraction of tumor killed mathematical model, for predicting the outcome of iontophoresis treatment in a controlled microenvironment. Also, precise control over the cell electromigration is a challenging investigation which we will address in the second aim of the dissertation. Here, we developed a microfluidic platform to study the consequences of DC electric fields on neutrophil electromigration (electrotaxis), which has an application of directing neutrophils away from healthy tissue by suppressing the migration of neutrophils toward pro-inflammatory chemoattractant.
Droplet Evaporation on Hot Micro-Structured Superhydrophobic Surfaces: Analysis of Evaporation from Droplet Cap and Base Surfaces
Huang, Wenge; He, Xukun; Liu, Cong; Li, Xiaojie; Liu, Yahua; Collier, C. Patrick; Srijanto, Bernadeta R.; Liu, Jiansheng; Cheng, Jiangtao (Elsevier, 2022-04-01)
In this study, evaporation of sessile water droplets on hot micro-structured superhydrophobic surfaces is experimentally and theoretically investigated. Water droplets of 4 µL are placed on micro-pillared silicon substrates with the substrate temperature heated up to 120°C. A comprehensive thermal circuit model is developed to analyze the effects of substrate roughness and substrate temperature on the sessile droplet evaporation. For the first time, two components of heat and mass transfer, i.e., one from the droplet cap surface and the other from the droplet base surface, during droplet evaporation are distinguished and systematically studied. As such, the evaporation heat transfer rates from both the droplet cap surface and the interstitial liquid-vapor interface between micropillars at the droplet base are calculated in various conditions. For droplet evaporation on the heated substrates in the range of 40°C – 80°C, the predicted droplet cap temperature matches well with the experimental results. During the constant contact radius mode of droplet evaporation, the decrease of evaporation rate from the droplet base contributes most to the continuously decreasing overall evaporation heat transfer rate, whereas the decrease of evaporation rate from the droplet cap surface is dominant in the constant contact angle mode. The influence of internal fluid flow is considered for droplet evaporation on substrates heated above 100°C, and an effective thermal conductivity is adopted as a correction factor to account for the effect of convection heat transfer inside the droplet. Temperature differences between the droplet base and the substrate base are estimated to be about 2°C, 5°C, 8°C, 13°C and 18°C for droplet evaporation on substrates heated at 40°C, 60°C, 80°C, 100°C, and 120°C, respectively, elucidating the delayed or depressed boiling of water droplets on a heated rough surface due to evaporative cooling.
Dynamics and Statics of Three-Phase Contact Line
Zhao, Lei (Virginia Tech, 2019-09-17)
Wetting, which addresses either spontaneous or forced spreading of liquids on a solid surface, is a ubiquitous phenomenon in nature and can be observed by us on a daily basis, e.g., rain drops falling on a windshield and lubricants protecting our corneas. The study of wetting phenomena can be traced back to the observation of water rising in a capillary tube by Hauksbee in 1706 and still remains as a hot topic, since it lays the foundation for a wide spectrum of applications, such as fluid mechanics, surface chemistry, micro/nanofluidic devices, and phase change heat transfer enhancement. Generally, wetting is governed by the dynamic and static behaviors of the three-phase contact line. Therefore, a deep insight into the dynamics and statics of three-phase contact line at nanoscale is necessary for the technological advancement in nanotechnology and nanoscience. This dissertation aims to understand the dynamic wetting under a molecular kinetic framework and resolve the reconfiguration of liquid molecules at the molecular region of contact line. Water spreading on polytetrafluoroethylene surfaces is selected as a classical example to study the dynamic behaviors of three-phase contact line. To accommodate the moving contact line paradox, the excess free energy is considered to be dissipated in the form of molecular dissipation. As-formed contact line friction/dissipation coefficient is calculated for water interacting with PTFE surfaces with varying structures and is found to be on the same order of magnitude with dynamic viscosity. From an ab initio perspective, contact line friction is decomposed into contributions from solid-liquid retarding and viscous damping. A mathematical model is established to generalize the overall friction between a droplet and a solid surface, which is able to clarify the static-to-kinetic transition of solid-liquid friction without introducing contact angle hysteresis. Moreover, drag reduction on lotus-leaf-like surface is accounted for as well. For the first time, the concept of contact line friction is used in the rational design of a superhydrophobic condenser surface for continuous dropwise condensation. We focus on the transport and reconfiguration of liquid molecules confined by a solid wall to shed light on the morphology of the molecular region of a three-phase contact line. A governing equation, which originates from the free energy analysis of a nonuniform monocomponent system, is derived to describe the patterned oscillations of liquid density. By comparing to the Reynolds transport theorem, we find that the oscillatory profiles of interfacial liquids are indeed governed in a combined manner by self-diffusion, surface-induced convection and shifted glass transition. Particularly for interfacial water, the solid confining effects give rise to a bifurcating configuration of hydrogen bonds. Such unique configuration consists of repetitive layer-by-layer water sheets with intra-layer hydrogen bonds and inter-layer defects. Molecular dynamics simulations on the interfacial configuration of water on solid surfaces reveal a quadratic dependence of adhesion on solid-liquid affinity, which bridges the gap between macroscopic interfacial properties and microscopic parameters.
The Effects of Surface Topography on Droplet Evaporation and Condensation
He, Xukun (Virginia Tech, 2021-06-02)
Droplet evaporation and condensation are two important topics of interest, since these two phase-change phenomena not only occur in the cycle of global water, e.g., the formation of rain, fog, dew, and snow in nature, but also play a critical role in a variety of applications including phase-change heat transfer enhancement, surface chemistry and energy system optimization. Especially, in the past two decades, the rapid development of the nature-inspired non-wetting surfaces has promoted the applications of droplet-based phase change phenomena in various scenarios. However, most previous studies focused on the sessile droplets on one flat surface in the open space, and the effects of surface topography, i.e., surface curvature or configurations, on droplet evaporation and dropwise condensation are still elusive. This dissertation aims to explore droplet-based evaporation and condensation in more complex spaces and to elucidate how the surface topography affects the evaporating or coalescing droplet dynamics during these phase-change processes. The coalescence-induced jumping of nanodroplet on curved superhydrophobic surface is modeled via molecular dynamic simulations. As the surface curvature increases from 0 to 2, the corresponding energy conversion efficiency of jumping droplet during the coalescence process could be significantly improved about 20 times. To explain this curvature-enhanced jumping effect, the contact line dissipation, i.e., an important source of energy dissipation in nanoscale, is considered in our scaling energy analysis. And this energy-effective jumping of coalesced droplet could be mainly attributed to the reduction of contact line dissipation due to the decrease of contact line length and contact time on curved surface. As the droplets are confined between two parallel or non-parallel low-energy surfaces, i.e., hydrophobic or superhydrophobic surfaces, with a narrow gap, the total evaporation time of the squeezed droplets would be dramatically prolonged about two times. An ellipsoidal segment diffusion-driven model is established to successfully predict the evolution of contact radius and volume of the squeezed droplets during the evaporation process and to clarify it is the vapor enrichment inside the confined space giving rise to the mitigated evaporation. If two hydrophobic surfaces are configured as non-parallel, the confined droplet inside the V-shaped grooves would be self-transported towards the cusp/corner during the evaporation. Based on our energy and force analyses, the asymmetrically confined droplet would move towards an equilibrium location le, where the Laplace pressure induced force is balanced with normal adhesion force, to minimize its Gibbs surface energy. As le decreases during the evaporation, this equilibrium location would directionally shift towards the cusp, which could be regarded as the origin of this evaporation-triggered unidirectional motion. For the first time, the solvent transport and colloidal extraction could be accurately controlled in a combined manner.
Experimental Investigations on Non-Wetting Surfaces
Stoddard, Ryan Manse (Virginia Tech, 2021-05-24)
Superhydrophobic (SHS) and lubricant-infused surfaces (LIS) exhibit exceptional non-wetting characteristics that make them attractive for energy production applications including steam condensation and fouling mitigation. The dissertation work focuses on application of non-wetting surfaces to energy production using a systematic approach examining each component of surface fabrication in three functional areas. First, SHS and LIS are fabricated using robust, scalable methods and tested for durability in heated, wet conditions and under high-energy water jet impingement. Clear performance differences are shown based on surface texturing, functionalizing agent, and infused lubricant. Second, SHS and LIS are applied to tube exteriors and evaluated for their ability to produce sustained dropwise condensation in a typical power plant condenser environment. The surfaces are shown to produce heat transfer coefficients up to 7-10 times that of film-wise condensation, with condenser effectiveness of 0.92 or better compared to effectiveness of about 0.6 in conventional condensers. Third, LIS on the interior of tubes are assessed in accelerated mineral fouling conditions. LIS are shown to mitigate calcium sulfate and calcium carbonate fouling under laminar conditions. The results of the study bear profound benefits to reducing the levelized cost of condensers and water uptake in thermoelectric power plants, that currently consume about 50% of the total water use in the U.S.
Exploiting Interfacial Phenomena to Expel Matter from its Substrate
Mukherjee, Ranit (Virginia Tech, 2021-09-02)
Spontaneous expulsion of various forms and types of matter from their solid substrates has always been an integral part of interfacial physics problems. A thorough understanding of such interactions between a solid surface and different soft materials not only expands our theoretical knowledge, but also has applications in self-cleaning, omniphobic surfaces and phase-change heat transfer. Although there is a renewed interest in the design of robust functional surfaces which can passively remove highly viscous liquids or dew, or retard ice accretion or frost formation, the physics of several dewetting and/or deicing mechanisms are yet to be fully understood. Even though we know how jumping-droplet condensation offers significantly better heat transfer performance than regular dropwise condensation and can liberate foreign particles, fundamental questions on the effect of surface orientation on jumping-droplet condensation or how it helps in large-scale fungal disease epidemic in plants are still unanswered. Thus, we first try to fill the knowledge gap in jumping-droplet condensation by characterizing their orientation-dependence and their role in a large-scale pathogenic rust disease dissemination among wheat. Unfortunately, understanding of such dewetting mechanisms does not necessarily translates to prevention or removal of ice and frost on subzero surfaces. Use of superhydrophobic structures or hygroscopic materials to retard the growth of frost was found to be limiting. Therefore the search for an efficient, inexpensive, and environmentally favorable anti-icing or de-icing mechanism is still underway. Here we give a framework for making a novel de-icing construct by analyzing a peculiar jumping frost phenomena where frost particles spontaneously jump off the surface when a polar liquid is brought above. Lastly, we demonstrate a simple and cost-effective technique to design a slippery liquid-infused surface from low-density hydrocarbon-based polymers, which is able to effectively remove a wide variety of soft materials. The main all-encompassing theme of this dissertation is to enhance our understanding of several dewetting phenomena, which might enable better design and/or mitigation strategies to control the expulsion of various forms of matter from a wide variety of surfaces.
Fog Harps: Elastocapillarity, Droplet Dynamics, and Optimization
Kowalski, Nicholas Gerald (Virginia Tech, 2021-05-18)
Fog harvesting is emerging as a promising means to ease the water shortage crisis in arid regions of the world with ample fog. The current state-of-the-art for fog harvesting is mesh netting, which is accessible yet struggles from a dual constraint: a course mesh lets most microscopic fog droplets pass through it, while a fine mesh clogs. In recent years, fog harps have been gaining attention as a superior alternative to meshes, bypassing these inherent constraints. In this work, we expand upon previous fog harp research with a focus on optimization. First, we analyze wire tangling in a harp due to capillary forces, resulting in a mathematical model that is able to predict when wire tangling will occur. Second, we systematically vary three key parameters of a fog harp (wire material, center-to-center wire pitch, and wire length), arriving at an optimal combination. Finally, we develop a numerical model to describe the dynamics of a fog droplet sliding down a harp wire while coalescing with others littered along it. By applying all knowledge acquired through these studies, the next generation of fog harps will push the performance ceiling of practical fog harvesters higher than ever.
Fog Harvesting: Inspired by Spider Silk
Cen, Yijia (Virginia Tech, 2020-01-29)
The water crisis has been an increasing challenge in some places in the world. One proposed solution that has drawn lots of attention is fog harvesting. A commonly used fog collector is a vertical mesh, usually made of poly materials. Small water droplets can easily get pinned and quick evaporation is the major common challenge for vertical meshes. Coating the fog mesh with superhydrophobic chemicals is one of the solutions. However, superhydrophobicity is not durable and it may contaminate the collected water. In addition, it requires a high professional maintenance and laboratory operation standard. As a result, it is impractical to set such fog collectors in regions and countries with water crisis. Low cost, harmless, easily fabricated, higher coalesce rate and low maintenance are the five pillars for this research. This thesis topic is inspired by spider silk's ability to direct water droplets to certain locations to further enhance water collecting rate. This directional droplet movement is caused by spindle-knot and joint structure on the biomimetic silk. The spindle-knot is randomly porous, and the joint is stretched porous. In addition, the spindle-knot has a tilted angle β above the joint region. Due to these unique structures, there are three droplet movement controlling forces – surface tension force, hysteresis force, and Laplace pressure force. This thesis presents detailed equation derivations for each driving force in the introduction section. Spindle-knot is the pivot point to direct water, forming the spindle-knot structure is another focus of this thesis. Fluid coating and dip-coating with dimethylformamide (DMF), a solvent with a low evaporation rate, is the highly used methods to form the spindle-knot structures due to its simple setup and low cost. However, DMF is an extremely hazardous organic compound, and it requires high laboratory operation standards. In the second section of this thesis, DMF has been replaced with water/ethanol and photocurable materials to construct the spindle-knots. Furthermore, Additive manufacturing (3D printing method) was adopted to synthesize bionic spider web with spindle-knot structures.
Investigations on Multiscale Fractal-textured Superhydrophobic and Solar Selective Coatings
Jain, Rahul (Virginia Tech, 2017-08-21)
Functional coatings produced using scalable and cost-effective processes such as electrodeposition and etching lead to the creation of random roughness at multiple length scales on the surface. The first part of thesis work aims at developing a fundamental mathematical understanding of multiscale coatings by presenting a fractal model to describe wettability on such surfaces. These surfaces are described with a fractal asperity model based on the Weierstrass-Mandelbrot function. Using this description, a model is presented to evaluate the apparent contact angle in different wetting regimes. Experimental validation of the model predictions is presented on various hydrophobic and superhydrophobic surfaces generated on several materials under different processing conditions. Superhydrophobic surfaces have myriad industrial applications, yet their practical utilization has been severely limited by their poor mechanical durability and longevity. Toward addressing this gap, the second and third parts of this thesis work present low cost, facile processes to fabricate superhydrophobic copper and zinc-based coatings via electrodeposition. Additionally, systematic studies are presented on coatings fabricated under different processing conditions to demonstrate excellent durability, mechanical and underwater stability, and corrosion resistance. The presented processes can be scaled to larger, durable coatings with controllable wettability for diverse applications. Apart from their use as superhydrophobic surfaces, the application of multiscale coatings in photo-thermal conversion systems as solar selective coatings is explored in the final part of this thesis. The effects of scale-independent fractal parameters of the coating surfaces and heat treatment are systematically explored with respect to their optical properties of absorptance, emittance, and figure of merit (FOM).
Leveraging Capillarity
Murphy, Kevin Robert (Virginia Tech, 2022-09-20)
Surface tension is an essential force for the functioning of the world and life. Centuries of study, and still, new applications and limits of surface tension are being explored. Water has always drawn attention for its high surface tension value, 72mN/m compared to ethanol's 20mN/m. The high surface tension allows for numerous applications, superhydrophobic surfaces being one that takes heavy advantage of that value. Superhydrophobicsurfaceshave a high surface energy cost with water, resulting in small contact areas with high advancing and receding contact angles and low contact angle hysteresis. This results in very low adhesion on the surfaces. Here we study the ability of superhydrophobic surfaces with their low adhesion to shed meltwater from frost, showing a decrease in frost thickness to below 3mm for the meltwater to shed. We then take another approach to removing water from a surface, rather than increasing the surface energy cost, we introduce a difference in surface energy cost. Introducing a porous surface across from a solid one, droplets transfer from the solid to the porous, removing over 90% of the volume of the droplet from the solid surface. We thoroughly examine and model the hydrodynamics of the transfer process, varying the solid surface, the donor surface, and the liquid. This bridging between surfaces is then applied to fog harps, examining the efficiencies of large-form fog harps. Fog harps have shown a 3 to 5 times increase in water collection compared to the industry-standard mesh collector. However, droplets from fog collected on the wires eventually grow large enough to touch neighboring wires. Tominimizetheirsurfaceenergy, they begin pulling wires together, "tangling" them. This can potentially reduce efficiency, but has not been applied to large-scale harps until here. Another application of surface tension is then examined, using lower surface tension oils, but trapping them in microstructures to make slippery liquid-infused porous surfaces (SLIPS). The oil coats the microstructure, due to its lower surface tension. This creates a lubricating layer on the surface, along with potential air pockets reducing friction further. These surfaces have been studied extensively with liquids being placed on them, but here we begin to examine them when solids are used instead, showing some interesting cases where increasing the viscosity of the oil actually decreases the friction force.
Modeling liquid droplet impact on a micropillar-arrayed viscoelastic surface via mechanically averaged responses
Li, Yang; Cheng, Jiangtao (Taylor & Francis, 2023)
Droplet impact on a substrate is an intriguing phenomenon that widely exists in our daily life and a broad range of industrial processes. However, droplet impact dynamics on soft textured surfaces are less explored and the underlying mechanisms remain elusive. Here, we report numerical simulation of droplet impact dynamics on a micropillar-arrayed soft surface using BASILISK, which involves a multiscale geometric domain containing the micropillars and droplet that are in the order of mu m and mm, respectively. As such, the volume of fluid (VOF) method is coupled with the finite volume method (FVM) to build the fluid fields and track their interface. From a conceptual point of view, the micropillared substrate is formed by imposing interstitial gaps into the otherwise intact soft material, whose viscoelastic properties can be quantified by gap density epsilon. Via a five-parameter generalized Maxwell model, the viscoelastic properties of the micropillared substrate can be approximated by its equivalent elastic response in the Laplace-Carson (LC) space, and the averaged bulk strain of the micropillared substrate in the real space is obtained by the inverse LC transform. Moreover, through parametric studies of splash extent, it turns out that for a specific epsilon, the splash is dramatically intensified with increasing impact velocity U-i. The splash also turns more violent with increasing ambient pressure P-a, which is evidenced by a larger splash angle of 114.44 degrees between the ejected sheet and the horizontal substrate at 5 atm. Conversely, the splash becomes more depressed with increasing surface tension sigma. Overall, the splash magnitudes of our simulations agree well with those predicted by the Kelvin-Helmholtz instability theory. By leveraging the LC transform in the fluid-viscoelastic solid interactions, our simulation methodology captures the main features of droplet impact dynamics on microstructured viscoelastic surfaces by means of the mechanically averaged responses while avoiding the predicament of domain scale inconsistency.
Multiphase Interfacial Phenomena for Liquid Manipulation and Defrosting
Lolla, Venkata Yashasvi (Virginia Tech, 2024-10-07)
Interfacial phenomena are prevalent in various natural and engineered systems. A thorough understanding of these phenomena is essential for a complete understanding of processes such as phase transitions and interaction of liquid droplets with different surfaces. The insights gained from understanding interfacial behavior are pivotal in fields such as pharmaceuticals, microfluidics, material sciences, and environmental engineering. This dissertation aims to advance our understanding of interfacial behaviors, thereby facilitating the development of innovative technologies for applications in health, defrosting, and omniphobic surfaces. In Chapters 1 and 2, relevant background information and goals are provided to contextualize the research being presented in this dissertation. Chapter 3 introduces a novel metal-free alternative to conventional antiperspirants (containing aluminum salts and zirconium salts). We leverage the composition of human sweat (97% water and 3% minerals) and employ a hygroscopic substance near the outlet of an artificial sweat duct rig. This leads to complete diffusion and dehydration of sweat, forming a natural mineral plug within the artificial sweat duct that halts the flow. Chapter 4 examines the behavior of room temperature water droplets spreading on a flat icy substrate. The use of flat ice, as opposed to cold substrates, eliminates the nucleation energy barrier, enabling freeze front initiation as soon as the bulk temperature of the spreading drop reaches 0 C. Through scaling analysis, we identify distinct thermo-hydrodynamic regimes with varying Weber numbers. Chapter 5 presents a novel construct for lubricant-impregnated surfaces (LIS). To date, most of the investigations characterizing the wettability of LIS have focused on droplet mobility. We pioneer a lubricant-impregnated fiber (LIF) which exhibits unique droplet dynamics due to simultaneous exploitation of both, high mobility and high adhesion. Chapter 6 proposes an innovative approach for defrosting by exploiting the polarizability and natural thermo-voltage of frost sheets. By placing an actively charged electrode near the frost sheet, we observe that frost dendrites migrate towards the electrode. This technique, termed Electrostatic Defrosting (EDF), effectively removes up to 75% of the frost mass for superhydrophobic surfaces and 50% of the frost mass for untreated surfaces in less than 100 s.

Browsing by Author "Cheng, Jiangtao"

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