Microdroplet

Microemulsions or reverse micelles are composed of enzyme-containing, surfactant-stabiHzed aqueous microdroplets in a continuous organic phase. Such systems may be considered as a kind of immobilization in enzymatic synthesis reactions. 

Beaded polymeric support, whether polystyrene-divinylbenzene, polymethacrylate, or polyvinyl alcohol, is conventionally produced by different variations of a two-phase suspension polymerization process, in which liquid microdroplets are converted to the corresponding solid microbeads (1). 

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. 

The pore size, the pore-size distribution, and the surface area of organic polymeric supports can be controlled easily during production by precipitation processes that take place during the conversion of liquid microdroplets to solid microbeads. For example, polystyrene beads produced without cross-linked agents or diluent are nonporous or contain very small pores. However, by using bigb divinylbenzene (DVB) concentrations and monomer diluents, polymer beads with wide porosities and pore sizes can be produced, depending on the proportion of DVB and monomer diluent. Control of porosity by means of monomer diluent has been extensively studied for polystyrene (3-6) and polymethacrylate (7-10). 

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. 

The development of an aerosol of microdroplets of BW into steam at the water/steam interface, caused by the sudden release of pressure under bubbles as the steam they contain is released. Typically, improved steam separation equipment is needed if misting persists in a boiler. 

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. 17.5 Ultrasonically levitated microdroplet dye laser. Left. Photograph of a lasing levitated microdroplet. Right. Schematics of ultrasonic field with the microdroplet being trapped at a node in the ultrasonic field. 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...

Ion traps have been used in a number of studies of optical properties of microdroplet lasers14,15 and the ion trap itself can be used as a useful mass spectrometer for fundamental studies of trapped particles and micrometer to nanoscale aerosols16. 

In order to achieve stationary and highly spherical microdroplets, the possibility to use superhydrophobic nanostructured surfaces has also been explored to make lasing22 and Raman lasing microdroplets23, where the high contact angle makes it possible to make long-term measurements on nearly spherical microdroplets at rest. 

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. 

Fig. 17.12 Top Outline of experimental setup for fiber taper coupling to liquid embedded microdroplet resonators. Bottom Measured optical transmission spectra form a fiber taper coupled 600 pm diameter liquid embedded microdroplet resonator. Reprinted from Ref. 35 with permission. 2008 Optical Society of America...

As demonstrated by the many implementations, microdroplet lasers provide a unique combination of features ... 

Tona, M. Kimura, M., Polarization effects in both emission spectra and microscopic images of lasing microdroplets levitated in an ion trap, J. Phys. Soc. Jpn 2002, 71,425 428... 

Sennaroglu, A. Kiraz, A. Dundar, M. A. Kurt, A. Demirel, A. L., Raman lasing near 630 nm from stationary glycerol water microdroplets on a superhydrophobic surface, Opt. Lett. 2007, 32,2197 2199... 

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