The selective concentration of the contents in a microdroplet using spontaneous emulsification was proposed and demonstrated in a microfluidic channel. Aqueous microdroplets having a 40-μm diameter, in octane containing 100 mM of Span 80, shrank to 10 μm within 10 min with nanodroplet formation at the interface of the microdroplets. The microdroplets' contents either stayed in the microdroplet or partitioned into the nanodroplets, depending on their properties. The size and the hydrophobicity of the contents are two parameters that determine concentration/separation. In addition, this method was applied to a bound complex and free ligand (B/F) separation to demonstrate its applicability to biochemical analyses. Here we report the separation of water-soluble molecules in microdroplets for the first time. This method is expected to enhance the flexibility of the design of droplet analytical processes and widen their applicability.

No abstract

Investigations on the dynamic behavior of molecules in liquids at high spatial resolution are greatly desired because localized regions, such as solid-liquid interfaces or sites of reacting molecules, have assumed increasing importance with respect to improving material performance. In application to liquids, electron energy loss spectroscopy (EELS) observed with transmission electron microscopy (TEM) is a promising analytical technique with the appropriate resolutions. In this study, we obtained EELS spectra from an ionic liquid, 1-ethyl-3-methylimidazolium bis (trifluoromethyl-sulfonyl) imide (C2mim-TFSI), chosen as the sampled liquid, using monochromated scanning TEM (STEM). The molecular vibrational spectrum and the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of the liquid were investigated. The HOMO-LUMO gap measurement coincided with that obtained from the ultraviolet-visible spectrum. A shoulder in the spectrum observed ∼0.4 eV is believed to originate from the molecular vibration. From a separately performed infrared observation and first-principles calculations, we found that this shoulder coincided with the vibrational peak attributed to the C-H stretching vibration of the [C2mim(+)] cation. This study demonstrates that a vibrational peak for a liquid can be observed using monochromated STEM-EELS, and leads one to expect observations of chemical reactions or aids in the analysis of the dynamic behavior of molecules in liquid.

We report a contactless surface tension measurement method of micrometer-sized aerosol droplets. In this method, we assume spherical spontaneous resonance of a thermally induced capillary wave. First, an aerosol droplet with a radius ranging from 4.7 to 12.4 μm is trapped by means of a simple single-beam optical trapping configuration, and the frequency shift power spectrum of the light passing the droplet is measured. The spectrum in each case exhibits several peaks in a frequency range of several tens to several hundred kilohertz. The peak frequencies agree well with theoretical ones predicted by the spherical resonant modes. After validating the above-mentioned assumption, we measure the surface tension of aerosol droplets containing sodium dodecyl sulfate, and we successfully obtain the surface tension value. The present method utilizes just two phenomena, that is, the droplet surface light scattering and spontaneous resonance of the capillary wave. These can be easily observed in aerosol droplets, and they can be utilized to gain scientific insights. The present method based on the nature of droplets can be used in various applications in aerosol science.