Homogenization of a phase-separated droplet in a polymer mixture caused by the dielectric effect of a laser

It is found that a micrometer-sized droplet in an aqueous solution of binary polymers [water/polyethylene-glycol (PEG)/dextran] disappears upon irradiation with a focused yttrium-aluminum-garnet laser. The interface of the dextran-rich droplet broadens and disappears, and it reappears upon turning the laser off, whereas the PEG-rich droplet shrinks and disappears. These phenomena are discussed in terms of the free energy by considering the laser-induced dielectric potential.

Figure 1

Schematic phase diagram of the PEG/dextran/water system. The horizontal axis denotes the dextran concentration as a fraction of the total concentration of polymers.

Figure 2

(a) Spatiotemporal image of a gradually disappearing dextran-rich droplet around the focus of a YAG laser $(\lambda =1064\mathrm{nm})$ in the PEG-rich phase at a laser power of $1.5\mathrm{W}$, reconstructed from time-successive video frames. Actual pictures of the droplets are shown at the bottom. (b) Spatiotemporal image of a reappearing dextran-rich droplet after laser irradiation was discontinued, reconstructed from successive video frames. Actual pictures of the droplets are also shown at the bottom. Micrometer-sized droplets are generated both inside and outside the original dextran-rich droplet. The origin of the time axis corresponds to when the laser was switched (a) on or (b) off.

Figure 3

Spatiotemporal image of a shrinking PEG-rich droplet around the laser focus at a laser power of $P=1.5\mathrm{W}$, reconstructed from time-successive video frames. The diameter of the droplet decreased linearly with time. The origin of the time axis corresponds to when the laser was switched on.

Figure 4

Numerical results based on Eqs. (6, 7). Spatiotemporal image and snapshots of the time development are shown. The numerical results are colored depending on $\mid \nabla \rho \mid$, which corresponds to the phase-contrast observation. (a) Numerical results for a dextran-rich droplet in the PEG-rich phase as shown in Fig. 2. (b) Numerical results for a PEG-rich droplet in a dextran-rich phase as shown in Fig. 3.