Microdroplet Droplet

Beaded polymeric supports are produced by a two-phase suspension polymerization in which microdrops of a monomer solution are directly converted to the corresponding microbeads. The size of a microdroplet is usually determined by a number of interrelated manufacturing parameters, which include the reactor design, the rate of stirring, the ratio of the monomer phase to water, the viscosity of both phases, and the type and concentration of the droplet stabilizer. 

In the second technique, two streams of microdroplets (about 100 p.m diameter, 40 kHz generation frequency, 15 ms velocity) collide to form a single droplet stream, which is observed by Raman spectroscopy. The mixing time is 200 p.s. 

These microdroplets can act as a reaction medium, as do micelles or vesicles. They affect indicator equilibria and can change overall rates of chemical reactions, and the cosurfactant may react nucleophilically with substrate in a microemulsion droplet. Mixtures of surfactants and cosurfactants, e.g. medium chain length alcohols or amines, are similar to o/w microemulsions in that they have ionic head groups and cosurfactant at their surface in contact with water. They are probably best described as swollen micelles, but it is convenient to consider their effects upon reaction rates as being similar to those of microemulsions (Athanassakis et al., 1982). 

Abstract The self-organized and molecularly smooth surface on liquid microdroplets makes them attractive as optical cavities with very high quality factors. This chapter describes the basic theory of optical modes in spherical droplets. The mechanical properties including vibrational excitation are also described, and their implications for microdroplet resonator technology are discussed. Optofluidic implementations of microdroplet resonators are reviewed with emphasis on the basic optomechanical properties. 

Freely suspended liquid droplets are characterized by their shape determined by surface tension leading to ideally spherical shape and smooth surface at the subnanometer scale. These properties suggest liquid droplets as optical resonators with extremely high quality factors, limited by material absorption. Liquid microdroplets have found a wide range of applications for cavity-enhanced spectroscopy and in analytical chemistry, where small volumes and a container-free environment is required for example for protein crystallization investigations. This chapter reviews the basic physics and technical implementations of light-matter interactions in liquid-droplet optical cavities. 

Fig. 17.4 Steady state velocity of freely falling microdroplets as function of droplet radius calculated from the balance between gravitation and Stokes drag...

Here, r gas is the viscosity of the gas surrounding the liquid droplet and pliquid is the mass density of the liquid. Figure 17.4 shows the steady-state velocity of a water droplet in air as a function of the droplet radius. The quadratic dependence on the droplet radius gives rise to a dramatic slow down, thus making visualization of falling microdroplets practical. 

Fig. 17.6 Reproducible lasing spectra from dye doped microdroplets. Each spectrum is obtained in a fixed setup by a well controlled loading of a 750 nL EG droplet with a Rh6G dye concen tration of 2 x 10 2 mol L 1 which is subsequently pumped with an average pump power of 1.20 mW. Reprinted from Ref. 11 with permission. 2008 Optical Society of America...

Fig. 17.9 Sketch of a typical setup for ion trap experiments on lasing microdroplets. The oscillating field between the inner and outer ring electrodes forms the trapping potential, and gravitational forces can he opposed by static electrical fields to move the droplet to the trap center with no micromotion...

In optical tweezer experiments, the optical scattering force is used to trap particles, but the force can also be used to control the shape of liquid droplets26. An infrared laser with 43-mW power focused onto a microdroplet on a superhydrophobic surface enabled up to 40% reversible tuning of the equatorial diameter of the droplet26. Such effects must naturally also be taken into account when exciting laser modes in droplets in experiments with levitated drops. 

Significant charge loss also occurred when a nonradioactive microdroplet was injected in a C-contaminated balance. Figure 7 presents results obtained by Davis et al. for a droplet of dioctyl phthalate in a balance that had been contaminated by prior experiments. The voltage rapidly increased as the droplet charge was neutralized, and within two minutes the charge decreased from about 165,000 elementary charges to about 10,000. 

We note that since Q involves the scattering coefficients, the radiation pressure force has resonance or near-resonance behavior. This first was observed and analyzed by Ashkin and Dziedzic (1977) in their study of microparticle levitation by radiation pressure. They made additional measurements (Ashkin and Dziedzic, 1981) of the laser power required to levitate a microdroplet, and Fig. 19 presents their data for a silicone droplet. The morphological resonance spectrum for the 180° backscattered light shows well-defined peaks at wavelengths corresponding to frequencies close to natural frequencies of the sphere. The laser power shows the same resonance structures in reverse, that is, when the scattered intensity is high the laser power required to levitate the droplet is low. 

Phase functions can also be used to measure the size and refractive index of a microsphere, and they have been used by colloid scientists for many years to determine particle size. Ray et al. (1991a) showed that careful measurements of the phase function for an electrodynamically levitated microdroplet yield a fine structure that is nearly as sensitive to the optical parameters as are resonances. This is demonstrated in Fig. 21, which presents experimental and theoretical phase functions obtained by Ray and his coworkers for a droplet of dioctylphthalate. The experimental phase function is compared with two... 

Fig. 29. Evaporation rate data for several low-volatility species in Nj measured by Tallin et al. (1988) by levitating microdroplets in an electrodynamic balance. From Measurement of Droplet Interfacial Phenomena by Light-Scattering Techniques, by Daniel C. TafBin, S. FI. Zhang, Theresa Allen and E. James Davis, A/CIiE Journo/, 34, No. 8, pp. 1310-1320, reproduced by permission of the American Institute of Chemical Engineers 1988 AIChE.

A retention gap is used to improve peak shapes under certain conditions. If you introduce a large volume of sample (>2 pL) by splitless or on-column injection (described in the next section), microdroplets of liquid solvent can persist inside the column for the first few meters. Solutes dissolved in the droplets are carried along with them and give rise to a series of ragged bands. The retention gap allows solvent to evaporate prior to entering the chromatography column. Use at least 1 m of retention gap per microliter of solvent. Even small volumes of solvent that have a very different polarity from the stationary phase can cause irregular solute peak shapes. The retention gap helps separate solvent from solute to improve peak shapes. 

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