The cover shows an optical image of co-electrospun nanofibers of poly(vinyl pyrrolidone) (outside) and hydrophobic oil (inside), irradiated by UV light. The resulting non-woven mats present monosized beads regularly distributed along the nanofibers in work reported by Loscarteles and co-workers on p. 2110. Only the beads fluoresce, due to special markers added to the oil, indicating that the oil is indeed wholly encapsulated inside the beads.
There are many technical situations, such as various biological or medical applications, in which a hydrophobic fluid must be encapsulated inside a hydrophilic polymer shell in the form of tiny microscopic pieces. A novel approach is presented, based on the co-electrospinning of the hydrophilic polymer melt (outside) and the hydrophobic fluid (inside), which results in beaded micro- and nanofibers, such that the hydrophobic fluid is efficiently encapsulated inside the beads. For the selected fluid couple, the low liquid–liquid surface tension and the high viscosity of the melt prevent the varicose break-up of inner fluid in the coaxial electrified jet until the very end of the co-electrospinning process. The resulting fibers present beads filled with the hydrophobic fluid, separated by a rather uniform distance whose length depends partially on the melt flow rate. The bead diameter grows with the inner flow rate, going from a monosized to a bisized distribution. In the case under study, the maximum relative (inner-to-outer) flow rate is one. The diameter of the solid fibers between beads scales well with existing theories for simple electrospinning.