Fig. 1: Droplet generation induced by the Soret effect.

a Phase diagram of the ATPS of Dextran (M_w = 5.5 × 10⁵) and PEG (M_w = 3.5 × 10⁴). The black circles are phase boundary obtained by visual observation. A sample was prepared with an initial composition of DEX 6 wt.% and PEG 2.6 wt.% (open circle) in a two-phase region. The mixture was phase-separated, and the composition of each phase is indicated by a red circle and a triangle (see Methods). The upper PEG-rich phase of the phase-separated mixture was used in droplet generation experiments. The black curve represents the coexisting curve. b Experimental setup. The ITO coated on the cover glass absorbs laser light and causes local heating. c Droplet generation process induced by the Soret effect. (See text.) Side view of the laser focus region. The white particles denote the dextran molecules. (i) Before laser irradiation. (ii) A temperature gradient occurs at the ITO surface when the laser irradiation is initiated. (iii) Dextran molecules enter the hot region. (iv) A dextran-rich domain is formed. (v) The phase-separated droplets remain after the laser is switched off. d Droplet generation process (Sample: upper phase of DEX 6 wt.% and PEG 2.6 wt.% including fluorescently labelled DEX 0.2 wt.%). (top) Phase contrast images. (Bottom) Fluorescence images of dextran. When the laser irradiation was initiated, a dark dextran-rich domain gradually appeared, and the droplet interface became sharp when the laser irradiation was stopped (see text). Successful droplet generation was observed only for the sample at the phase boundary. Scale bar = 20 μm. e Cases in which no droplets were generated (DEX 0.7 wt.% and PEG 2.5 wt.% blue circle in the phase diagram). The sample composition was very close to that of (d) but only the behaviour caused by the Soret effect was observed. Dextran collected by the Soret effect diffuses rapidly after the laser is switched off (local heating is removed). (top) Phase contrast images. (Bottom) Fluorescence images of dextran. Scale bar = 20 μm.