# Innovative Ultrasonic Spray Methods for Indoor Disinfection
## 1 Introduction  
Viruses and bacteria pose significant biological threats to humanity due to their ability to mutate rapidly, complicating vaccine development. The COVID-19 pandemic undersocored the devastating potential of infectious diseases, with surfaces contaminated by infected droplets identified as a key transmission pathway [1]. While strict hygiene measures were implemented globally, new pathogens, such as the monkepyox virus, continue to emerge [2], emphasizing the need for effective disinfection technologies. In May 2020, the World Health Organization (WHO) issued guidelines for disinfectant spraying systems, highlighting their potential to neutralize pathogens on surfaces [3]. These recommendations emphasized the importance of safe and efficient disinfection methods to prevent viral and bacterial transmission in diverse settings, from personal clothing to public spaces and city streets [4, 5]. Since used personal protective equipment and wastewater in hospitals pose environmental hazards and require the attention of the scientific community, the development of new disinfection methods and means in hospitals can reduce this environmental burden [6, 7]. Despite their widespread use, traditional spraying methods--hydraulic and pneumatic--have notable limitations that hinder their effectiveness. Hydraulic spraying produces large droplets (~1 mm in size) with low surface-area-to-volume ratios. These droplets settle quickly under gravity, leading to uneven surface coverage, excessive disinfectant use, and waste. Conversely, pneumatic spraying generates ultrafine droplets (2-4 um) that evaporate before reaching surfaces, resulting in a low process efficiency and inadequate disinfection. Both methods also suffer from broad droplet size distributions, causing technological challenges like rebound effects, evaporation, and uneven coverage. Furthermore, they canpose health risks, such as the inhalation of aerosolized disinfectants and irritation of the mucous membranes [8; 9; 10]. These limitations underscore the urgent need for innovative spraying methods capable of generating droplets of optimal size, ensuring uniform surface coverage, and reducing waste. One promising solution is ultrasonic atomization, a technique that uses ultrasonic vibrations to create fine aerosols with a nearly monodisperse size distribution [11]. Ultrasonic atomization offers several key advantages over traditional methods [12; 13; 14]. One of its key benefits is the ability to control droplet size by adjusting the frequency of ultrasonic vibrations, with droplet diameters ranging from 90 \(\upmu\)m at 25 kHz to 10 \(\upmu\)m at 200 kHz for water. This method is also highly energy-efficient, achieving fine atomization at lower energy costs compared to hydraulic and pneumatic systems. Additionally, ultrasonic atomization is compatible with a wide range of liquids, including those with varying viscosity, low surface tension, or complex compositions, such as disinfectants containing silver nanoparticles. This compatibility ensures a uniform nanoparticle distribution within droplets and prevents agglomeration. Finally, ultrasonic atomization minimizes health and environmental risks by avoiding the production of hazardous ultrafine aerosol clouds and reducing waste from excessively large droplets, thus enhancing both safety and efficiency. However, ultrasonic spraying does face challenges, primarily its relatively low aerosol cloud production rate (approximately 1-2 mL/s), which limits its scalability for large-scale applications. Addressing this limitation requires further innovation, including the integration of ultrasonic and combined spraying techniques. This study aims to bridge existing gaps in disinfection technology by presenting novel ultrasonic and hybrid atomization methods. Drawing on over 15 years of the authors' experience, this work describes advanced devices designed to handle disinfectant liquids with diverse physicochemical properties, including high-viscosity solutions and two-phase systems with dispersed nanoparticles.