Experimental investigation of the homogeneous freezing of aqueous ammonium sulfate droplets

We have measured the light scattering intensity and homogeneous ice nucleation temperatures from water droplets containing 0-33 wt % ammonium sulfate. In these laboratory experiments, we used a free-fall freezing tube technique to determine the fraction of frozen droplets at a particular droplet temperature by measuring the depolarized light scattering intensity from the droplets in free-fall. Previously reported freezing temperatures for solution concentrations greater than 5 wt % display a larger spread than can be accounted for by the reported experimental errors. We find freezing temperatures in good agreement with the lowest temperature freezing results reported by previous experiments. Our ammonium sulfate freezing temperature data set with water activity less than 0.98 is consistent with a curve that deviates in activity shift by about 5% from the best-fit ice nucleation temperature versus water activity curve found by Koop et al. in 2000, but the significance of this deviation will only be known with further high-precision ice nucleation temperature measurements for other aqueous solutions.     

A sorption balance-based method to study the initial drying of dispersion droplets

The initial drying of dispersion samples with varying geometries and surface areas was continuously recorded in a sorption balance at different temperatures and relative humidities. The samples were applied as small (5–20 mg) droplets on glass. We were able to show a linear dependence of the evaporation rate on the exposed surface area of the samples if the evaporative cooling was taken into account. The method outlined in the present paper facilitates scaling and comparison of results obtained from drying experiments with different-sized dispersion samples.     

Microemulsion droplets decorated by Brij700 block copolymer: phase behavior and structural investigation by SAXS and SANS

The phase behavior and structure of a four-component microemulsion system forming droplets with an oil core surrounded by the non-ionic C12E5 surfactant in water and "decorated" by long PEO chains using the block copolymer/surfactant Brij 700 has been studied. The surfactant-to-oil volume ratio, the coverage density of the droplets with decorating molecules, and the temperature were varied. For a surfactant-to-oil volume ratio of 2, the solutions form isotropic and clear solutions at room temperature, and the addition of Brij molecules stabilize the micelles: the transition to an opaque phase is shifted to higher temperatures as the surface coverage increases. At a surfactant-to-oil ratio of 1, the isotropic microemulsion phase is confined to a very narrow range of temperature, which location is shifted to increasing temperature, as the amount of Brij at the surface of the droplet is increased. For large surface coverages, the lower emulsification boundary varies roughly linearly with the surface coverage. The structure of the droplet phase was investigated by small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS). For a surfactant-to-oil ratio of 2, the SANS data revealed a transition from rodlike to spherical particles when Brij molecules are added to the system, which induces a larger curvature of the surfactant film. For a surfactant-to-oil ratio of 1, the droplets are nearly spherical at all surface coverages. The intermicellar interactions effects become increasingly more pronounced as Brij is added, due to the introduction of the highly swollen corona. A quantitative analysis of some of the SAXS data was done using an advanced model based on Monte Carlo simulations. It demonstrates the strong chain-chain interactions within the corona and confirms the increased interparticle interactions, as the coverage density is increased.     

Spectroscopic investigation of the solvation of organic molecules in superfluid helium droplets

The spectroscopy of molecules doped into superfluid helium nanodroplets provides valuable information on the process of solvation in superfluid helium. In continuation of an earlier report on emission spectra of various phthalocyanines showing a splitting of all molecular transitions in the range of about 5-12 cm(-1), the emission spectra of tetracene, pentacene, and perylene in superfluid helium droplets are presented. The new spectra and the results obtained for the phthalocyanines are explained by an empirical model which accounts for the existence of different metastable configurations of a nonsuperfluid solvation layer around the guest molecule.     

A high-resolution infrared spectroscopic investigation of the halogen atom-HCN entrance channel complexes solvated in superfluid helium droplets

Rotationally resolved infrared spectra are reported for the X-HCN (X = Cl, Br, I) binary complexes solvated in helium nanodroplets. These results are directly compared with those obtained previously for the corresponding X-HF complexes [J. M. Merritt, J. Küpper and R. E. Miller, Phys. Chem. Chem. Phys., 2005, 7, 67]. For bromine and iodine atoms complexed with HCN, two linear structures are observed and assigned to the (2)Sigma(1/2) and (2)Pi(3/2) ground electronic states of the nitrogen and hydrogen bound geometries, respectively. Experiments for HCN + chlorine atoms give rise to only a single band which is attributed to the nitrogen bound isomer. That the hydrogen bound isomer is not stabilized is rationalized in terms of a lowering of the isomerization barrier by spin-orbit coupling. Theoretical calculations with and without spin-orbit coupling have also been performed and are compared with our experimental results. The possibility of stabilizing high-energy structures containing multiple radicals is discussed, motivated by preliminary spectroscopic evidence for the di-radical Br-HCCCN-Br complex. Spectra for the corresponding molecular halogen HCN-X(2) complexes are also presented.     

A numerical study on the coalescence of emulsion droplets in a constricted capillary tube

Using a new computational model, we have studied the dynamics and coalescence of a pair of two-dimensional droplets in pressure-driven flow through a constricted capillary tube, which is a prototype problem for the analysis of the interaction of emulsion droplets in porous media. We present simulations that quantify the effects of various system parameters on the droplet stability. These include the capillary number, the interfacial tension, the suspended-to-suspending-phase viscosity ratio, the valence and concentration of added electrolytes, the droplet-to-pore-size ratio, the pore-body-to-throat-size ratio, and the type of pore geometry. Our simulations show that the capillary number Ca plays an important role in determining whether the drops coalesce. At low Ca, drops deform only slightly and coalescence occurs at the entrance of the pore throat, whereas significant deformation enables the drops move through the pore without coalescence at high Ca. Coalescence is favored at intermediate values of the viscosity ratio. The destabilizing effect of added electrolytes is found to be insignificant for 10-mum drops, but significant for micron-size drops. Among the geometric-related parameters, the drop-to-pore-size ratio is the most significant.     

A homogeneous assay for protein analysis in droplets by fluorescence polarization

We present a novel homogeneous (“mix‐incubate‐read”) droplet microfluidic assay for specific protein detection in picoliter volumes by fluorescence polarization (FP), for the first time demonstrating the use of FP in a droplet microfluidic assay. Using an FP‐based assay we detect streptavidin concentrations as low as 500 nM and demonstrate that an FP assay allows us to distinguish droplets containing 5 μM rabbit IgG from droplets without IgG with an accuracy of 95%, levels relevant for hybridoma screening. This adds to the repertoire of droplet assay techniques a direct protein detection method which can be performed entirely inside droplets without the need for labeling of the analyte molecules.     

Radiation detection by using superheated droplets

The superheated emulsion, a radiation detector consisting of superheated droplets dispersed in a gel or a solid matrix, has been in use for some time now. It was initially designed to detect neutrons but also has been modified to detect energetic photons and heavy ions. It has been primarily used as a neutron dosimeter, but has also been used to obtain the energy spectrum for neutrons from Am–Be. The general features of the superheated emulsion detector along with newer analytic methods to obtain more precise results using it as an active device will be presented.     

Encapsulation of emulsion droplets by organo-silica shells

Surfactant-stabilized emulsion droplets were used as templates for the synthesis of hollow colloidal particles. Monodisperse silicone oil droplets were prepared by hydrolysis and polymerization of dimethyldiethoxysiloxane monomer, in the presence of surfactant: sodium dodecyl sulphate (SDS, anionic) or Triton X-100 (non-ionic). A sharp decrease in the average droplet radius with increasing surfactant concentration was found, with a linear dependence of the droplet radius on the logarithm of the surfactant concentration. The surfactant-stabilized oil droplets were then encapsulated with a solid shell using tetraethoxysilane, and hollow particles were obtained by exchange of the liquid core. The size and polydispersity of the oil droplets and the thickness of the shell were determined using static light scattering, and hollow particles were characterized by electron microscopy. Details on the composition of the shell material were obtained from energy-dispersive X-ray analysis. In the case of sodium dodecyl sulphate, the resulting shells were relatively thin and rough, while when Triton X-100 was used, smooth shells were obtained which could be varied in thickness from very thick ( approximately 150 nm) to very thin shells ( approximately 17 nm). Finally, hexane droplets were encapsulated using the same procedure, showing that our method can in principle be extended to a wide range of emulsions.     

Asymmetric electrowetting--moving droplets by a square wave

Here droplet oscillation and continuous pumping are demonstrated by asymmetric electrowetting on an open surface with embedded electrodes powered by a square wave electrical signal without control circuits. The polarity effect of electrowetting on an SU-8 and Teflon coated electrode is investigated, and it is found that the theta-V (contact angle-applied voltage) curve is asymmetric along the V = 0 axis by sessile drop and coplanar electrode experiments. A systematic deviation of measured contact angles from the theoretical ones is observed when the electrode beneath the droplet is negatively biased. In the sessile drop experiment, up to a 10 degrees increment of contact angle is measured on a negatively biased electrode. In addition, a coplanar electrode experiment is designed to examine the contact angles at the same applied potential but opposite polarities on two sides of one droplet at the same time. The design of the coplanar electrodes is then expanded to oscillate and transport droplets on square-wave-powered symmetric (square) and asymmetric (polygon) electrodes to demonstrate manipulation capability on an open surface. The frequency of oscillation and the speed of transportation are determined by the frequency of the applied square wave and the pitch of the electrodes. Droplets with different volumes are tested by square waves of varied frequencies and amplitudes. The 1.0 microl droplet is successfully transported on a device with a loop of 24 electrodes continuously at a speed up to 23.6 mm s(-1) when a 9 Hz square wave is applied.     

Single cell epitaxy by acoustic picolitre droplets

The capability to encapsulate single to few cells with micrometre precision, high viability, and controlled directionality via a nozzleless ejection technology using a gentle acoustic field would have great impact on tissue engineering, high throughput screening, and clinical diagnostics. We demonstrate encapsulation of single cells (or a few cells) ejected from an open pool in acoustic picolitre droplets. We have developed this technology for the specific purpose of printing cells in various biological fluids, including PBS and agarose hydrogels used in tissue engineering. We ejected various cell types, including mouse embryonic stem cells, fibroblasts, AML-12 hepatocytes, human Raji cells, and HL-1 cardiomyocytes encapsulated in acoustic picolitre droplets of around 37 microm in diameter at rates varying from 1 to 10,000 droplets per second. At such high throughput levels, we demonstrated cell viabilities of over 89.8% across various cell types. Moreover, this ejection method is readily adaptable to other biological applications, such as extracting data from single cells and generating large cell populations from single cells. The technique described in the current study may also be applied to investigate stem cell differentiation at the single cell level, to direct tissue printing, and to isolating pure RNA or DNA from a single cell at the picolitre level. Overall, the techniques described have the potential for widespread impact on many high-throughput testing applications in the biological and health sciences.     

Wetting of structured hydrophobic surfaces by water droplets

Super-hydrophobic surfaces may arise due to an interplay between the intrinsic, relatively high, contact angle of the more or less hydrophobic solid surface employed and the geometric features of the solid surface. In the present work, this relationship was investigated for a range of different surface geometries, making use of surface free energy minimization. As a rule, the free energy minima (and maxima) occur when the Laplace and Young conditions are simultaneously fulfilled. Special effort has been devoted to investigating the free energy barriers present between the Cassie-Baxter (heterogeneous wetting) and Wenzel (homogeneous wetting) modes. The predictions made on the basis of the model calculations compare favorably with experimental results presented in the literature.     

Manipulation of droplets by dynamically controlled wetting gradients

The reversible transportation of droplets was realized by spatiotemporal control of the wetting gradient. The surface wetting was reversibly regulated by using electrochemical reactions of the ferrocenyl (Fc) alkanethiol monolayer, and the wetting gradient was generated by the application of the in-plane bias voltage to the substrate. The back-and-forth motion of the wetting boundary, where the surface changed from wetting to repulsive, sequentially caused a droplet unidirectional spreading and shrinking on the surface. These unidirectional deformations resulted in the net transport of the droplet in an inchwormlike manner. The droplet moved backward when the direction of the in-plane bias voltage was reversed.     

A microfluidic device for self-synchronised production of droplets

The primary requirement for a mixing operation in droplet-based microfluidic devices is an accurate pairing of droplets of reaction fluids over an extended period of time. In this paper, a novel device for self-synchronous production of droplets has been demonstrated. The device uses a change in impedance across a pair of electrodes introduced due to the passage of a pre-formed droplet to generate a second droplet at a second pair of electrodes. The device was characterised using image analysis. Droplets with a volume of ~23.5 ± 3.1 nl (i.e.~93% of the volume of pre-formed droplets) were produced on applying a voltage of 500 V. The synchronisation efficiency of the device was 83%. As the device enables self-synchronised production of droplets, it has a potential to increase the reliability and robustness of mixing operations in droplet-based microfluidic devices.     

A control method for steering individual particles inside liquid droplets actuated by electrowetting

An algorithm is developed that allows steering of individual particles inside electrowetting systems by control of actuators already present in these systems. Particles are steered by creating time varying flow fields that carry the particles along their desired trajectories. Results are demonstrated using an experimentally validated model developed in ref. . We show that the current UCLA electro-wetting-on-dielectric (EWOD) system contains enough control authority to steer a single particle along arbitrary trajectories and to steer two particles, at once, along simple paths. Particle steering is limited by contact angle saturation and by the small number of actuators that are available to actuate the flow in practical electrowetting systems.