Individual Microdroplets Research Articles

Enhancing Double-Beam Laser Tweezers Raman Spectroscopy (LTRS) for the Photochemical Study of Individual Airborne Microdroplets.

A new device and methodology for vertically coupling confocal Raman microscopy with optical tweezers for the in situ physico- and photochemical studies of individual microdroplets (Ø ≤ 10 µm) levitated in air is presented. The coupling expands the spectrum of studies performed with individual particles using laser tweezers Raman spectroscopy (LTRS) to photochemical processes and spatially resolved Raman microspectroscopy on airborne aerosols. This is the first study to demonstrate photochemical studies and Raman mapping on optically levitated droplets. By using this configuration, photochemical reactions in aerosols of atmospheric interest can be studied on a laboratory scale under realistic conditions of gas-phase composition and relative humidity. Likewise, the distribution of photoproducts within the drop can also be observed with this setup. The applicability of the coupling system was tested by studying the photochemical behavior of microdroplets (5 µm < Ø < 8 µm) containing an aqueous solution of sodium nitrate levitated in air and exposed to narrowed UV radiation (254 ± 25 nm). Photolysis of the levitated NaNO3 microdroplets presented photochemical kinetic differences in comparison with larger NaNO3 droplets (40 µm < Ø < 80 µm), previously photolyzed using acoustic traps, and heterogeneity in the distribution of the photoproducts within the drop.

Motile Artificial Chromatophores: Light‐Triggered Nanoparticles for Microdroplet Locomotion and Color Change

AbstractGold nanoparticles coated with a poly(N‐isopropylacrylamide) (pNIPAM) shell undergo reversible dis/assembly below and above the critical temperature of 32 °C. Loading these particles into microdroplets at high density creates light‐driven artificial chromatophores. Triggering the nanoparticle assembly gives dramatic color changes from nanoparticle localization at the base of the droplets, resembling zebrafish melanophores. These reversible chromatophore states can be switched by both bulk and optical heating, explored here in individual microdroplets and in large cm2 areas of close‐packed droplets. Illuminating chromatophores off‐center with a tightly focused beam results in droplet locomotion via two separate mechanisms, Marangoni interfacial shear and gas bubble propulsion, depending on optical power.

Building Dynamic Cellular Machineries in Droplet-Based Artificial Cells with Single-Droplet Tracking and Analysis.

Although the application of droplet microfluidics has grown exponentially in chemistry and biology over the past decades, robust universal platforms for the routine generation and comprehensive analysis of droplet-based artificial cells are still rare. Here we report using microfluidic droplets to reproduce a variety of types of cellular machinery in in vitro artificial cells. In combination with a unique image-based analysis method, the system enables full automation in tracking single droplets with high accuracy, high throughput, and high sensitivity. These powerful performances allow broad applicability evident in three representative droplet-based analytical prototypes that we develop for (i) droplet digital detection, (ii) in vitro transcription and translation reactions, and (iii) spatiotemporal dynamics of cell-cycle oscillations. The capacities of this platform to generate, incubate, track, and analyze individual microdroplets via real-time, long-term imaging unleash its great potential in accelerating cell-free synthetic biology. Moreover, the wide scope covering from digital to analog to morphological detections makes this droplet analysis technique adaptable for many other divergent types of droplet-based chemical and biological assays.

Novel Method to Simultaneously Adjust the Size and pH Value of Individual Microdroplets in Silicone Oil

In this paper, a strategy to modify each micro-droplet's volume and synchronously adjust its pH value as required based on the electrolysis reaction in silicone oil is demonstrated. A pair of platinum electrodes fixed onto the jaws of a vernier caliper was used to modify the micro-droplet's volume and adjust its pH value by simply adjusting the distance between the two electrodes. To get a micro-droplet with desirable volume and pH values, three models, the relationship between the droplet's volume and its diameter when the droplet was placed on a fluorinated ethylene propylene (FEP)-covered glass substrate, the relationship between the distance of the two electrodes and the size of the resulted micro-droplet, and the relationship between the pH value and the micro-droplet's consuming rate, were built through the least square method. In our experiments, a droplet (5% sodium chloride solution, 1.4 uL, pH = 7) could consume 98.9% of its initial volume and form a new droplet with a volume of 0.016 μL and pH of 12.2. In addition, to validate that this method is also suitable in the acid and alkaline solutions, 0.001 mol/L NaOH and H 2 SO 4 solutions were, respectively, operated using the same procedure. Both the volume and pH values could be controlled, which proved the potential application of our proposed method in analytical chemistry, precision engineering, and so on.

Nonequilibrium Spatiotemporal Sensing within Acoustically Patterned Two-Dimensional Protocell Arrays.

Acoustically trapped periodic arrays of horseradish peroxidase (HRP)-loaded poly(diallydimethylammonium chloride) / adenosine 5′-triphosphate coacervate microdroplet-based protocells exhibit a spatiotemporal biochemical response when exposed to a codiffusing mixture of substrate molecules (o-phenylenediamine (o-PD) and hydrogen peroxide (H2O2)) under nonequilibrium conditions. Unidirectional propagation of the chemical concentration gradients gives rise to time- and position-dependent fluorescence signal outputs from individual coacervate microdroplets, indicating that the organized protocell assembly can dynamically sense encoded information in the advancing reaction-diffusion front. The methodology is extended to arrays comprising spatially separated binary populations of HRP- or glucose oxidase-containing coacervate microdroplets to internally generate a H2O2 signal that chemically connects the two protocell communities via a concerted biochemical cascade reaction. Our results provide a step toward establishing a systematic approach to study dynamic interactions between organized protocell consortia and propagating reaction-diffusion gradients, and offer a new methodology for exploring the complexity of protocellular communication networks operating under nonequilibrium conditions.

A parameter to probe microdroplet dynamics and crystal nucleation

We present a simple and efficient digital-image processing method to simultaneously monitor the contraction of a statistically relevant number of microdroplets of the same size and the nucleation of single salt crystals inside. Each individual microdroplet image is reduced to a scalar, standard deviation σ of the grey-level of pixels inside a region of interest containing the microdroplet image, and overall microdroplet dynamics is monitored using standard-deviation time-evolution plots. It is shown that this approach makes it possible to measure the nucleation time and also that microdroplets interact via water diffusion dynamics. This effect actually decreases the nucleation rate, contrary to previous findings. This “σ approach” can be compared to recording the order parameter in phase transition, which makes it ideal for studying dynamics of systems where images are the primary outputs.

Optofluidic droplet dye laser generated by microfluidic nozzles.

We present an optofluidic droplet dye laser that is generated by an array of microfluidic nozzles fabricated on a polycarbonate chip. A droplet resonator forms upon pressurizing the nozzle backside microfluidic channel. Multimode low-threshold lasing is observed from individual microdroplets doped with dye. Additionally, droplets can be conveniently released from the nozzle by water rinsing from the top microfluidic channel and subsequently regenerated, and thus achieve optofluidic lasers on-demand. Our work demonstrates a new approach to generating on-chip laser source and laser arrays in a simple, reproducible, reconfigurable, and low-cost fashion.

Effect of the Duration of Liquid Nitrogen Storage on the Regrowth of Blackberry Cryopreserved by Droplet Vitrification

Summary In vitro shoot tips of the blackberry cultivar ‘Čačanska Bestrna’ were cryopreserved using the droplet vitrification technique. Upon loading (30 min) in a solution of 1.9 M glycerol and 0.5 M sucrose, the explants were dehydrated for 40 min on ice with the PVS A3 vitrification solution (glycerol 37.5%, dimethyl sulfoxide 15%, ethylene glycol 15% and sucrose 22.5%) and for 40 min at room temperature with the PVS3 solution (glycerol 50% and sucrose 50%). They were subsequently frozen in individual microdroplets of vitrification solution, by direct immersion in liquid nitrogen (LN), and kept therein for 2, 4, 8 and 24 h. The explant rewarming was performed in an unloading solution (0.8 M sucrose) for 30 min at room temperature. The duration of LN exposure did not exert significant effects on the survival and regrowth of explants in both types of vitrification solutions. The survival and regrowth of cryopreserved shoot tips dehydrated with PVS3 solution ranged between 90–95% and 80–90%, respectively. However, dehydration with PVS A3 resulted in a lower survival rate (80–90%) and a considerably lower regrowth rate (55–65%) of explants. Monitoring the shoots regenerated in the in vitro culture revealed their normal capacity for multiplication and rooting in comparison with the controls, which fully confirms the purpose of cryopreservation in the long-term preservation of plant material.

Whispering gallery mode laser based on cholesteric liquid crystal microdroplets as temperature sensor

We developed a tunable whispering gallery mode (WGM) microlaser based on dye-doped cholesteric liquid crystal (CLC) microdroplets with controllable size in an aqueous environment. An individual dye-doped CLC microdroplet confined at the tip of a microcapillary was optically pumped via a tapered optical fiber tip positioned within its vicinity. Numerical simulations and various spectral characteristics verify the WGM resonance of the lasing in microdroplets. Thermal tuning of the lasing modes is realized due to the thermo-optic effect of CLC. The proposed CLC microdroplet-based WGM resonator was applied as a temperature sensor and exhibited maximum temperature sensitivity up to 0.96 nm/°C.

Tapered optical fiber waveguide coupling to whispering gallery modes of liquid crystal microdroplet for thermal sensing application.

We demonstrate efficient coupling to the optical whispering gallery modes (WGMs) of nematic liquid crystal (NLC) microdroplets immersed in an immiscible aqueous environment. An individual NLC microdroplet, confined at the tip of a microcapillary, was coupled via a tapered optical fiber waveguide positioned correctly within its vicinity. Critical coupling of the taper-microdroplet system was facilitated by adjusting the gap between the taper and the microdroplet to change the overlap of the evanescent electromagnetic fields; efficient and controlled power transfer from the taper waveguide to the NLC microdroplet is indeed possible via the proposed technique. We also found that NLC microdroplets can function as highly sensitive thermal sensors: A maximum temperature sensitivity of 267.6 pm/°C and resolution of 7.5 × 10-2 °C were achieved in a 78-μm-diameter NLC microdroplet.

Appendix C: Automated Vitrification of Mammalian Embryos on a Digital Microfluidic Device.

This chapter introduces a digital microfluidic device that automates sample preparation for mammalian embryo vitrification. Individual microdroplets manipulated on the microfluidic device were used as microvessels to transport a single mouse embryo through a complete vitrification procedure. Advantages of this approach, compared to manual operation and channel-based microfluidic vitrification, include automated operation, cryoprotectant concentration gradient generation, and feasibility of loading and retrieval of embryos.

Voltammetric and microscopical investigation of the properties and behaviors of individual mercury micro-droplets

Mercury film modified electrode based stripping voltammetry has become a rapid and low cost method in routine trace element environmental analysis. However, the understanding of the physical and chemical properties of the plated mercury film still needs to be enhanced till now. It has been recognized that the so-called mercury film actually consists of numerous micro mercury droplets, so it is of significant importance to directly investigate the properties and behaviors associated with the individual droplet. Thus, a platinum ultramicroelectrode was used as the substrate for mercury plating, and only one mercury droplet is formed under this condition. Herein, Fourier Transformed Sinusoidal Voltammetry (FTSV) was employed to investigate both the growth process of a mercury droplet and the behaviors of the droplet after plating. Meanwhile, an optical microscope was used to provide direct visual observation. Investigation suggests that: (a) Both the gradual growth process and the fast coalescence event of individual mercury droplet can be resolved with FTSV; (b) The plated mercury droplet can be partially oxidized immediately after the disconnection between the working electrode and the potentiostat in some circumstances, e.g. the presence of trace amount of chloride ion. The present study not only sheds new light on the properties of mercury film electrode but also demonstrates the feasibility of employing FTSV as a new tool for the characterization of micrometer or even nanometer sized particles.

Electrochemical Liquid-Liquid-Solid Deposition of Ge at Hg Microdroplet Ultramicroelectrodes

Individual Hg microdroplet electrodes were used to perform Ge electrochemical liquid-liquid-solid (ec-LLS) crystal growth in aqueous solutions containing dissolved GeO2. The Hg microdroplets were prepared by electrodeposition of Hg onto pre-existing Pt ultramicroelectrodes (r < 10 μm). These Hg microdroplet ultramicroelectrode platforms were then used for analyses of Ge ec-LLS by both voltammetry and amperometry. Voltammetric responses indicated that Ge was neither immediately nor quantitatively dissolved into Hg upon electroreduction of aqueous solutions of GeO2. Separately, chronoamperometry performed at large overpotentials for reduction of dissolved GeO2 yielded crystalline Ge but with notable differences as compared to ec-LLS at a bulk Hg pool electrode. One set of experiments featured the surface of the Hg microdroplets possessing a complete Ge shell. The data suggested this shell formed within the first 1000 s and essentially passivated the electrode against further heterogeneous GeO2 reduction but allowed limited H+ reduction. The other set of experiments exhibited a similar shell but with fissures or cracks that exposed fresh Hg to the electrolyte and allowed Ge electrodeposition to persist much longer. In these experiments, hollow, spiral crystalline Ge tubes were extruded from the interior of the Hg microdroplets.

Mass Transfer in the Dissolution of a Multicomponent Liquid Droplet in an Immiscible Liquid Environment

The Epstein-Plesset equation has recently been shown to predict accurately the dissolution of a pure liquid microdroplet into a second immiscible solvent, such as oil into water. Here, we present a series of new experiments and a modification to this equation to model the dissolution of a two-component oil-mixture microdroplet into a second immiscible solvent in which the two materials of the droplet have different solubilities. The model is based on a reduced surface area approximation and the assumption of ideal homogeneous mixing [mass flux d(m(i))/dt = A(frac(i))D(i)(c(i) - c(s)){(1/R) + (1/(πD(i)t)(1/2)}] where A(frac(i)) is the area fraction of component i, c(i) and c(s) are the initial and saturation concentrations of the droplet material in the surrounding medium, R is the radius of the droplet, t is time, and D(i) is the coefficient of diffusion of component i in the surrounding medium. This new model has been tested by the use of a two-chamber micropipet-based method, which measured the dissolution of single individual microdroplets of mutually miscible liquid mixtures (ethyl acetate/butyl acetate and butyl acetate/amyl acetate) in water. We additionally measured the diffusion coefficient of the pure materials-ethyl acetate, butyl acetate, and amyl acetate-in water at 22 °C. Diffusion coefficients for the pure acetates in water were 8.65 × 10(-6), 7.61 × 10(-6), and 9.14 × 10(-6) cm(2)/s, respectively. This model accurately predicts the dissolution of microdroplets for the ethyl acetate/butyl acetate and butyl acetate/amyl acetate systems given the solubility and diffusion coefficients of each of the individual components in water as well as the initial droplet radius. The average mean squared error was 8.96%. The dissolution of a spherical ideally mixed multicomponent droplet closely follows the modified Epstein-Plesset model presented here.

Charge of water droplets in non-polar oils

Recent advances in droplet manipulation methods by electric fields and signals require a deeper understanding of water droplet charge. In this paper, we have investigated the electrophoretic motion of individual water microdroplets injected into non-polar silicone and paraffin oil by video optical microscopy on an individual droplet basis to determine droplet charge. It was found that the initial surface charge density of surfactant free droplets directly after injection from a micropipette is positive and of the order of 10−6 C/m2, regardless of pH and ion concentration in the range from pH 4 to pH 10 and from 0.01 mmol/l to 1.5 mol/l, respectively. The experimental results together with molecular dynamics simulations show that the nature and polarity of the charge can be explained by anisotropic orientation of water molecules at the interface rather than selective adsorption of ions. Furthermore, we showed that slip at the liquid-liquid boundary must be taken into account when interpreting electrophoretic measurements of droplets.

Size stabilization of surface-supported liquid aerosols using tapered optical fiber coupling

We demonstrate long-term size stabilization of surface-supported liquid aerosols of salt-water. Single tapered optical fibers were used to couple the light from independent heating and probe lasers into individual microdroplets that were kept on a superhydrophobic surface in a high-humidity chamber. Size stabilization of microdroplets resulted from competition between resonant absorption of the infrared heating laser by a microdroplet whispering gallery mode and water condensation in the sample chamber. Microdroplet size was continuously monitored using the tunable red probe laser. Thanks to the narrow linewidth of the heating laser, stabilization of the 110 μm radius of a microdroplet with a precision down to 0.54 nm was achieved for a period of 410 s.

Probing of ultrahigh optical Q-factors of individual liquid microdroplets on superhydrophobic surfaces using tapered optical fiber waveguides

We report measurements of ultrahigh quality factors (Q-factors) of the optical whispering-gallery modes excited via a tapered optical-fiber waveguide in single glycerol–water microdroplets standing on a superhydrophobic surface in air. Owing to the high contact angle of the glycerol–water mixture on the superhydrophobic surface (>155°), microdroplets with the geometry of a truncated sphere minimally distorted by gravity and contact line pinning effects could be generated. Q-factors up to 2.3×106 were observed for such droplets with radii of 100–200 μm exposed to the ambient atmosphere in a closed chamber with controlled relative humidity. Placement of microdroplets in a constant humidity environment permitted prolonged characterization of Q-factors for individual microdroplets. We found that the Q-factors in air were stable over more than 1 h and their measured values were limited mostly by the thermally induced droplet shape fluctuations.

Quantitative Characterization of Individual Microdroplets using Surface-Enhanced Resonance Raman Scattering Spectroscopy

Surface-enhanced resonance Raman scattering (SERRS) spectroscopy is a highly sensitive optical technique capable of detecting multiple analytes rapidly and simultaneously. There is significant interest in SERRS detection in micro- and nanotechnologies, as it can be used to detect extremely low analyte concentrations in small volumes of fluids, particularly in microfluidic systems. There is also rapidly growing interest in the field of microdroplets, which promises to offer the analyst many potential advantages over existing technologies for both design and control of microfluidic assays. While there have been rapid advances in both fields in recent years, the literature on SERRS-based detection of individual microdroplets remains lacking. In this paper, we demonstrate the ability to quantitatively detect multiple variable analyte concentrations from within individual microdroplets in real time using SERRS spectroscopy. We also demonstrate the use of a programmable pump control algorithm to generate concentration gradients across a chain of droplets.

How nanorough is rough enough to make a surface superhydrophobic during water condensation?

Nanostructured surfaces which manifest superhydrophobic properties during water condensation have a potential to dramatically enhance energy efficiency in power generation and desalination systems. Although various such surfaces have been reported, their development has been fortuitous, not driven by an understanding of the underlying physical processes. In this work, we perform a comprehensive study of microscale water condensation dynamics on nanostructured superhydrophobic surfaces made using a variety of synthetic methods. We demonstrate that the growth mechanism of individual water microdroplets on these surfaces is universal and independent of the surface architecture. The key role of the nanoscale topography is confinement of the base area of forming droplets, which allows droplets to grow only through contact angle increase. The nearly spherical droplets formed in this fashion become highly mobile after coalescence. By comparing experimentally observed drop growth with interface free energy calculations, we show that the minimum observed confined microdroplet base diameter depends directly on the nanoscale surface roughness and degree of interfacial wetting. Specifically, we show that the microscale condensation mechanism depends on the height of a liquid film with volume equal to the fill volume between the nanostructures. This introduced roughness length scale is a universal metric that allows for facile comparison of arbitrarily complex surface architectures. We use this new fundamental insight to develop quantitative design guidelines for superhydrophobic surfaces intended for condensation applications.

Preliminary Study on the Visualization and Quantification of Elemental Compositions in Individual Microdroplets using Solidification and Synchrotron Radiation Techniques

Quantifying the solute composition of a cloud droplet (or a whole droplet) is an important task for understanding formation processes and heating/cooling rates. In this study, a combination of droplet fixation and SR-XRF microprobe analysis was used to visualize and quantify elements in a micro-scale droplet. In this study, we report the preliminary outcome of this experiment. A spherical micro-scale droplet was successfully solidified through exposure to α-cyanoacrylate vapor without affecting its size or shape. An X-ray microprobe system equipped at the beam line 37XU of Super Photon ring 8 GeV (SPring-8) was applied to visualize and quantify the elemental composition in an individual micro-scale droplet. It was possible to reconstruct 2D elemental maps for the K and Cl contained in a microdroplet that was dispensed from the 10-ppm KCl standard solution. Multi-elemental peaks corresponding to X-ray energy were also successfully resolved. Further experiments to determine quantitative measures of elemental mass in individual droplets and high-resolution Xray microtomography (i.e., 3D elemental distribution) are planned for the future.