From Chaotic Cone Pulsation to Ion Evaporation in Electrosprays Open Access
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Spraying of electrified liquids, originally fascinating for its phenomenological richness, has recently offered a new arsenal of tools in diverse applications. As in numerous other cases, these applications emerged before a deep understanding of the involved phenomena. The investigations in this dissertation are primarily concerned with two fundamental aspects of electrospray ionization: the electrohydrodynamic pulsation of liquid menisci and the ion formation mechanism. Chapter 1 reviews the main milestones in electrospray research, including a brief historical background and notes on application developments. The presentation accounts for current theories and experimental results, but due to the large amount of literature in the field, it is by no means exhaustive. The pulsating Taylor cone regime is thoroughly characterized in Chapter 2. Fast imaging was used to capture an entire pulsation cycle and to prove that standing waves on the meniscus are responsible for the liquid pulsation. A rigorous demonstration is given that this pulsation is responsible for the oscillations observed in the spray current measurements. Electrospray is one of the main methods to obtain charged liquid droplets. Despite the interest in the amount of charge carried by these droplets, no method had been described to measure the net charge on a Taylor cone. Chapter 3 presents a simple method to estimate the charge on a pulsating liquid cone based on its pulsation frequency. The chaotic pulsation of the Taylor cone during its transition between dripping and pulsating cone regimes is described in Chapter 4. To my knowledge, this is the first report on extending the nonlinear dynamics of dripping faucets to electrosprays. Chapter 5 describes the morphology, structure and dynamics of charged water nanodroplets modeled by molecular dynamics simulations. Even a relatively short simulation time allowed us to observe the ion evaporation from nanodroplets charged close to the Rayleigh limit and the Coulomb explosion of excessively charged nanodroplets. Protrusions formed due to surface thermal fluctuations enhanced by the presence of ions were instrumental in the mechanism of ion evaporation. The evolution of charged nanojets hypothetically formed by the charge reduction of larger nanodroplets is explored in Chapter 6. Surface fluctuations are shown to affect the nanojet breakup, accelerating the charge reduction process compared to what is expected from macroscopic theories alone. Chapter 7 reviews the most significant applications of electrosprays that drive the research and establish the current trends in the related fields.