Microvortices In Droplets: Generation & Applications

The emerging field of droplet microfluidics deals with the manipulation of nL-fL droplets encapsulated within an immiscible carrier phase. The droplets are used as reaction containers for biochemical assays, enabling drastic reduction in assay volumes needed for modern life sciences research. To achieve this, basic laboratory processes such as mixing, detection, and metering must be emulated in the droplet format. Three important unit operations relevant to high throughput screening include 1) the concentration of particles and species within droplets, which is necessary for heterogeneous assays; 2) sensing the biochemical contents of a droplet; and 3) the sorting of droplets based on physical or chemical properties, which is important for single cell and proteomic assays. Currently, particle concentration in droplets requires active components, such as on-chip electrodes or magnets, along with charged or magnetic particles. Similarly, sensing and sorting droplets by chemical composition is based on flow cytometry, which also requires on-chip electrodes, feedback control, and chemical labeling. It is desirable to avoid active field techniques due to complexity, size, and cost constraints, and replace them with more simple and passive techniques. In this thesis, we utilize microvortices, the rotational motion of fluid, to enhance the capabilities of droplet microfluidics in the above three areas. The microvortices are generated using two methods: (i) hydrodynamic recirculation drag and (ii) tensiophoresis. In the first method, species concentration is accomplished by exploiting the shear-induced vortices that occur naturally inside a droplet/plug as it moves through a microchannel. Prior studies utilized these flows for enhancing mixing or interphase mass transfer. This work exploits microvortices together with two other independent phenomena--sedimentation of particles and interfacial adsorption of proteins--to concentrate both types of species at the rear of the droplet, where they can be extracted from the drop. In the latter case, the protein localization at the rear of drop reduces the interfacial tension locally resulting in an asymmetry in the drop shape. Under laminar flow, the shape deformation is deterministic and can serve as a sensitive, label-free indicator of protein concentration in proteomic screening. In the second method, label-free sorting of droplets is accomplished by a novel droplet actuation technique termed Tensiophoresis. A microchemical gradient across the droplet is transduced into a microvortex flow which propels the droplets up the chemical gradient. Using laminar flow to precisely control the gradient, droplets can be sorted by size with 3.3% resolution over a wide turning range. Droplets can be also sorted based on chemical composition because tensiophoresis is inhibited by surface active agents adsorbed on the droplet surface. Studies conducted using Bovine Serum Albumin (BSA) show that the droplet migration velocity scales inversely with protein concentration in the droplet, and migration velocity can be correlated to protein concentration with a 1 femtomole limit of detection. As modern life sciences research becomes increasingly reliant on high throughput workflows, microdroplet technology can meet the growing demand to perform screening at ultra-high throughputs with reduced sample volume. This thesis contributes three important unit operations which expand the capabilities of droplet-based workflows in proteomics, cell biology, and other areas of biomedical research

Liquid Drop Actuation by Photoelectrowetting

In electrowetting an electric potential is applied between a droplet of electrolyte and a conductor separated by an insulator. The repulsion of like charges deforms and spreads the droplet until capillary and electric forces are in equilibrium. Photoelectrowetting is a light-triggered version of electrowetting where the conductor is replaced by a moderately-doped semiconductor. The electrolyte-insulator-semiconductor stack resembles a metal-insulator-semiconductor capacitor, which has the special property that the amount of charge that can be injected into it increases when exposed to light. Thus in photoelectrowetting the exposure of light spreads the droplet further than in unilluminated conditions. In this thesis a scheme is presented for moving drops on a surface using photoelectrowetting. In order to understand photoelectrowetting I conducted a study of electrowetting with semiconductors. Devices were constructed using moderately-doped p-type silicon wafers (Na = 8.6 × 1014 cm−3) coated with a bilayer composed of thermal oxide (100 nm) and teflon (265 nm). Electric biases (< 40 V) were applied between droplets of electrolyte (10 microliter, 10 mM NaCl) and the silicon wafer, resulting in deformations of the droplet. These changes were quantified with contact angle measurements which varied from 120◦ at zero bias to 90◦ at 40V depending on the conditions of the experiment. Three regimes were observed depending on the polarity of the bias and above-bandgap illumination impinging on the droplet, corresponding to the charge regimes of an MIS capacitor: accumulation, inversion and deep-depletion. I present a model for these wetting changes based on a balance of capillary and electrostatic forces. After accounting for various non-ideal effects, I find that the model agrees with the data. I demonstrate that it is essential to account for interface traps in our devices (1.8 × 1011 cm−2) in the deep-depletion regime, leading to a 33% (4◦) correction to the prediction at 40V. I elucidate the nature of the photoelectrowetting effect and find that contrary to reports in the literature the transition is not reversible by light alone. In the next phase of my thesis, I demonstrate how photoelectrowetting triggered with a light beam on one side moves the droplet along a surface. Comparable with traditional electrowetting-based devices, I achieve speeds of up to 12 mm/s with 10 microliter drops of electrolyte (1% w/w NaCl) with a surfactant (5 mM NaCl) using an oscillating electric potential composed of an AC bias of magnitude 32.5 Vpp and a DC offset of −7 V cycled at a frequency of 15 kHz and a laser intensity of 40 mW/cm^2 (λ = 660nm). I measure the speed for varying magnitude and frequency of the bias, laser intensity, droplet size and viscosity. The optimal cycling frequency is set by competing effects: on the low frequency side ( 15 kHz) the speed is limited by the laser intensity. I present a model for the speed incorporating these effects that compares favorably with experiment. I present results of simulations of minority charge carrier concentrations in depletion regions. These exhibit self-similarity in space and time. The front of the concentrations follows a power law in time with an exponent that depends on the dopant concentration. Predictions from the power laws compare favorably with experiment.PHDApplied PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/140843/1/czarv_1.pd

Patterning Of Surfaces To Control The Storage, Mobility And Transport Of Liquids For Microfluidic Applications

Systems and methods to pattern surfaces to create regions of variable adhesive force on a superhydrophobic paper surface. By taking advantage of high surface energy sticky islands on a non-sticky superhydrophobic surface, microliter water drops can be registered or confined at specific locations; selected drops can then be transferred to another patterned substrate and the drops mixed and/or allowed to react without the need for pipettes or other fluid transfer tool.Georgia Tech Research Corporatio

Droplet microfluidics: technologies and applications

液滴微流控系统是微流控芯片领域的一个新的分支,由于其诸多独特的优势而得到了广泛的研究和报道。本文对液滴的制备和相关的操控技术,包括液滴的分裂、融合、混合、分选、存储和编码等进行了介绍,对液滴技术近年来在化学与生物化学分析等领域中的应用进行了综述,并展望了液滴微流控技术的发展前景。Droplet microfluidics is a new area in microfluidics research and has been extensively studied and reported due to its various unique advantages.In this paper,we review the preparation and wide range of manipulation technologies of microdroplets including splitting,coalescence,mixing,sorting,storage,coding,etc.,as well as their applications in chemical and biochemical analyses;we also envision the future developments of droplet microfluidic technologies.国家自然科学基金项目(No．21005065);教育部高等学校博士点专项科研基金项目(No．20100121120006

Advances in Optofluidics

Optofluidics a niche research field that integrates optics with microfluidics. It started with elegant demonstrations of the passive interaction of light and liquid media such as liquid waveguides and liquid tunable lenses. Recently, the optofluidics continues the advance in liquid-based optical devices/systems. In addition, it has expanded rapidly into many other fields that involve lightwave (or photon) and liquid media. This Special Issue invites review articles (only review articles) that update the latest progress of the optofluidics in various aspects, such as new functional devices, new integrated systems, new fabrication techniques, new applications, etc. It covers, but is not limited to, topics such as micro-optics in liquid media, optofluidic sensors, integrated micro-optical systems, displays, optofluidics-on-fibers, optofluidic manipulation, energy and environmental applciations, and so on

Microdevices and Microsystems for Cell Manipulation

Microfabricated devices and systems capable of micromanipulation are well-suited for the manipulation of cells. These technologies are capable of a variety of functions, including cell trapping, cell sorting, cell culturing, and cell surgery, often at single-cell or sub-cellular resolution. These functionalities are achieved through a variety of mechanisms, including mechanical, electrical, magnetic, optical, and thermal forces. The operations that these microdevices and microsystems enable are relevant to many areas of biomedical research, including tissue engineering, cellular therapeutics, drug discovery, and diagnostics. This Special Issue will highlight recent advances in the field of cellular manipulation. Technologies capable of parallel single-cell manipulation are of special interest

10th EASN International Conference on Innovation in Aviation & Space to the Satisfaction of the European Citizens

This Special Issue book contains selected papers from works presented at the 10th EASN (European Aeronautics Science Network) International Conference on Innovation in Aviation & Space, which was held from the 2nd until the 4th of September, 2020. About 350 remote participants contributed to a high-level scientific gathering providing some of the latest research results on the topic, as well as some of the latest relevant technological advancements. Eleven interesting articles, which cover a wide range of topics including characterization, analysis and design, as well as numerical simulation, are contained in this Special Issue

Bostonia

Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs