Atomization and Sprays, 31(8):1–8 (2021) FLASH BOILING ATOMIZATION UNDER NEGATIVE PRESSURE CONDITIONS Tali Bar-Kohany 1,2,* & Eran Sher 3 1 School of Mechanical Engineering, Tel-Aviv University, Tel Aviv, Israel 2 Department of Mechanical Engineering, nrcn, Beer-Sheva, Israel 3 Faculty of Aerospace Engineering, Technion–Israel Institute of Technology, Haifa, Israel *Address all correspondence to: Tali Bar-Kohany, School of Mechanical Engineering, Tel-Aviv University, Tel Aviv, 6997801,Israel, E-mail: talibk@tauex.tau.ac.il Original Manuscript Submitted: 11/26/2020; Final Draft Received: 5/18/2021 Flash boiling atomization is one of the most promising methods for generating high quality, uniform tiny droplets. Similar to other atomization methods, a higher pressure difference across the atom- ization element (higher vapor pressure or lower ambient pressure) would be expected to result in smaller droplets. In the present brief communication, we examine the theoretical effect of lowering the ambient pressure inside the atomizer well below absolute zero (negative pressures). We evaluate the bubbles’ growth rate and their size at burst time as a plausible indication of the droplets’ size. We also present how the efficiency of this process depends on the pressure negativity. Our results suggest that negative ambient pressures do not necessarily contribute to the process efficiency and hence to the droplets’ refinement. KEY WORDS: flash boiling, atomization and spray, negative pressure, tensioned liquid, homogeneous nucleation, efficiency 1. INTRODUCTION Flash boiling atomization is a promising method for generating small droplets that are of high quality and uniformity. A higher pressure difference across the atomization element (higher va- por pressure or lower ambient pressure) might result in smaller droplets. We have studied the theoretical effects of lowering ambient pressure inside the atomizer to well below absolute zero (negative pressure). Negative pressure can be achieved when a continuous liquid phase is sub- jected to outward forces, resulting in a metastable state in which tensile stresses within the liq- uid are developed. The use of negative pressures is an emerging technology that has already found various applications from drug and DNA delivery (Karshafian et al., 2009), gene therapy by sonoporation (Guo et al., 2017), tissue ablation (histotripsy) (Vlaisavljevich et al., 2015), nanoparticles generation by sonochemistry (Friedman et al., 2013), nuclear material detection and monitoring (Lapinskas et al., 2010), to synthetic trees to capture carbon-dioxide by using hydro-gels and reverse osmosis (Wheeler et al., 2008). 2. RATIONALE Consider an adiabatic container that contains compressed liquid at initial temperature and pres- sure (Point A on Fig. 1). The pressure is suddenly reduced to a pressure that corresponds to 1044–5110/21/$35.00 © 2021 by Begell House, Inc. www.begellhouse.com 1