No abstract

Although many aspects of microchannel emulsification have been covered in literature, one major uncharted area is the effect of viscosity of both phases on droplet size in the stable droplet generation regime. It is expected that for droplet formation to take place, the inflow of the continuous phase should be sufficiently fast compared to the outflow of the liquid that is forming the droplet. The ratio of the viscosities was therefore varied by using a range of continuous and dispersed phases, both experimentally and computationally. At high viscosity ratio (g d /g c ), the droplet size is constant; the inflow of the continuous phase is fast compared to the outflow of the dispersed phase. At lower ratios, the droplet diameter increases, until a viscosity ratio is reached at which droplet formation is no longer possible (the minimal ratio). This was confirmed and elucidated through CFD simulations. The limiting value is shown to be a function of the microchannel design, and this should be adapted to the viscosity of the two fluids that need to be emulsified.

The authors recently proposed a promising technique for producing monodisperse emulsions using a straight-through microchannel (MC) device composed of an array of microfabricated oblong holes. This research developed new straight-through MC devices with tens of thousands of oblong channels of several microns in size on a silicon-on-insulator plate, and investigated the emulsification characteristics using the microfabricated straightthrough MC devices. Monodisperse oil-in-water (O/W) and W/O emulsions with average droplet diameters of 4.4-9.8 lm and coefficients of variation of less than 6% were stably produced using surface-treated straight-through MC devices that included uniformly sized oblong channels with equivalent diameters of 1.7-5.4 lm. The droplet size of the resultant emulsions depended greatly on the size of the preceding oblong channels. The emulsification process using the straight-through MC devices developed in this research had very high apparent energy efficiencies of 47-60%, defined as (actual energy input applied to droplet generation/theoretical minimum energy input necessary for making droplets) · 100. Straight-through MC devices with numerous oblong microfluidic channels also have great potential for increasing the productivity of monodisperse fine emulsions.

Summary Ohmic heating was applied to Japanese white radish, Rhaphanus sativus L. at 50 Hz‐10 kHz and 40 V cm−1 to examine the effects of frequency on the heat generation. Of the frequencies examined, 50 Hz gave the sharpest initial rise of temperature and the shortest time to raise the temperature at the mid‐part of radish to 80°C. The heating rates above 60°C were found to be almost the same and linear for all the frequencies. The pressurization (400 MPa, 25°C, for 10 min) of radish eliminated the sharp initial increase of temperature observed at 50 Hz, and gave an almost linear rise of temperature similar to those above 60°C. When radish was treated with a square wave (10 ms with 10 ms interval) of 45 V cm−1 for 30 s, its impedance decreased and never recovered even after storage at room temperature for 20 h. When radish was treated at 50 Hz and 40 V cm−1 until its mid‐part reached 30°C, or heated to 80°C in hot water and then cooled to 30°C, 1H‐NMR imaging analysis showed more free movement of liquid components than in untreated radish. These results suggest that the initial rapid heating up at low frequency is caused by the electroporation of radish tissue membrane, resulting in the reduction of its impedance.