A full voltage-controlled nanoelectrospray system and its steady characteristic analysed by empirically equivalent circuit method

Nanoelectrospray technique, or electrohydrodynamic spraying, has been utilised in wide applications including mass spectrometry, microcolloid thrusters and micro/nano-fabrication areas. In this article, a nanoelectrospray system was developed in a pure voltage-controlled fashion with low flow rates of nL/min. Electrospray occurs by applying a voltage to overcome surface tension of the liquid without auxiliary backpressure. Pronounced increase of current with applied voltage was revealed during the pressure-free spray. In the steady spray regime a linear correlation between current and voltage was clearly identified; this can be attributed directly to the unforced electrospray where the spray properties are uniquely dictated by the voltage. This behaviour may be captured by an equivalent circuit method based upon empirical results. The equivalent circuit resistance, derived from the best-fit model, represents an electrical equivalence of the gap between the nozzle and the collector; this resistance strongly depends on both the nozzle size and the spray distance. The suitability of an electrical equivalent is further confirmed by incorporating a series resistor. No obvious difference can be distinguished electrically between a resistor and the resistance of the gap, suggesting that unforced spray can be adjusted and stabilised externally by using the series resistor method.