Figure 2: Feed speed impact on break-up periods and sphere sizes.

(a) Typical high feed speed (v_f=90 μm s⁻¹) break-up result in 4 μm Si-core fibre (scale bar, 200 μm). (b) Typical low feed speed (v_f=2 μm s⁻¹) break-up result in 4 μm Si-core fibre (scale bar, 200 μm). (c) Break-up periods and corresponding sphere diameters(squares) as a function of the feed speed for the 4 μm Si-core fibre shown together with the magnified graphs of s.d. values (bars) as they appear within each one of the samples. Fibre sections were fed through the hot zone of the hydrogen/oxygen flame at flows of 0.3 and 0.1 l min⁻¹, respectively. In each experiment only the feed speed of the fibre through the flame is varied. The s.d. values in the break-up periods are shown 10-fold magnified while in sphere diameters—1,000-fold magnified. The error in the sphere diameter is assumed to originate from both the break-up period and the fibre core thickness instabilities (see Supplementary Note 1). (d) Schematic of the step-by-step formation of a sphere and dimensional analysis: in the time it takes to go over one full cycle of sphere formation, in which a length λ_d is fed at speed v_f, the core of diameter D is pinched at a speed u_p.