Focusing capillary jets close to the continuum limit

The demand for techniques that can reliably deliver and control nanometre-scale volumes of liquid is a growing priority in biotechnology and medicine. Capillary jets are capable of supplying a steady stream of monodisperse liquid droplets. But because of the increasing forces and pressures needed to counteract surface tension for droplets of decreasing size, reaching the nanoscale with such an approach is difficult. One way of overcoming such limitations is to electrostatically focus a jet as it emerges from a capillary. Another, which we report here, is to focus such a jet by hydrodynamic means, a double flow-focusing arrangement that involves a manifold capillary that delivers a second immiscible fluid jet that envelopes and guides the jet from an inner capillary. Under the appropriate working conditions, this enables the generation of continuous steady capillary fluid jets down to submicrometre diameter - approaching the ultimate continuum limit, which is supported by a proposed theory.