Innovative Ultrasonic Spray Methods for Indoor Disinfection

Abstract

This study explores the challenges associated with dispersing disinfectant liquids for sanitizing individuals, indoor spaces, vehicles, and outdoor areas. Among the various approaches, fine aerosol sprays with a high particle surface area emerge as a particularly promising solution. Ultrasonic spraying, which leverages diverse mechanisms of ultrasound interaction with liquids, offers several distinct advantages. Notably, it enables the production of fine aerosols from liquids with a broad range of physical and chemical properties, including variations in purity, viscosity, and surface tension. This capability is especially critical for disinfectant liquids and suspensions, which often exhibit low surface tension and/or high viscosity. The article provides a comprehensive review of ultrasonic spraying methods and technologies developed by the authors’ team in recent years. It highlights innovative ultrasonic sprayers, including the latest designs, which are capable of generating aerosols with precise dispersion characteristics and high productivity from disinfectant liquids.

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 underscored 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 monkeypox 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 µm) 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 can pose 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 µm at 25 kHz to 10 µm 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. The proposed technologies promise to enhance the precision, efficiency, and safety of disinfection practices in various contexts.

2. Development of Ultrasonic and Combined Spraying Devices for Disinfectant Liquids

An analysis of existing data on the disinfection of various objects allows for the categorization of disinfection approaches based on the characteristic scales, the distance from the sprayer to the object, and the required droplet size [15]:

• For disinfecting people in factories, offices, hospitals, public places (including land, underground, and air transportation), and restaurants, the effective droplet size is 10–35 µm;
• For disinfecting indoor spaces and vehicles, droplet sizes between 25–70 µm are ideal;
• For disinfecting open spaces (e.g., roads, buildings, pavilions, and recreational areas), droplet sizes of 150–300 µm are required.

Thus, the spraying capacity must vary significantly to ensure the correct number of droplets for even surface coverage in a single layer, where the layer thickness does not exceed the droplet size.

The versatility of ultrasonic atomization, combined with the wide range of disinfection tasks, requires the development and use of specialized equipment. Recent research and development efforts [16] demonstrate that the authors’ ultrasonic atomizers effectively address these challenges.

2.1. Ultrasonic Liquid Layer Sprayers

The ultrasonic atomizer consists of an ultrasonic oscillating system (ultrasonic nozzle) and an electronic generator for its power supply. The design of the oscillating system is shown in Figure 1.

The design scheme of the ultrasonic atomizer developed by the authors is based on the half-wave Langevin transform. It consists of a concentrator (1), a spray surface (2), piezoceramic elements (3), and a reflective pad (4). The type of conversion of electrical energy into mechanical vibrations is piezoelectric. Inside, in the center of the Langevin converter, a through channel (5) is made for feeding the sprayed liquid to the spray surface (2). The atomizer is installed in a housing (6), in the rear flange (7) of which there are places for connecting the liquid supply systems (8) and for inputting an electric cable (9) (for exciting vibrations of the piezoelectric elements). When liquid is fed from the hole (5), it spreads on the surface (2) in a thin layer. Ultrasonic vibrations generate standing capillary waves on the liquid surface. Droplets detach from the crests of these waves and are ejected with an initial velocity perpendicular to the surface. Therefore, by changing the shape of the spray surface (for example, making it a cone with a different angle at the top), we can change the direction of droplet separation and, accordingly, the width of the spray torch, making it wider or narrower.

The number of capillary waves per unit area of the spray surface is finite, depending on the frequency of ultrasonic vibrations [15]. At a given frequency (which determines the size of the droplets), the spray performance is dictated by the area of the spray surface. The larger the surface area, the higher the spray performance.

Thus, the shape of the spray surface determines the geometric characteristics of the spray torch, and the area of the spray surface determines the spray performance. The shape and size of the spray surface may vary depending on the specific purpose of the sprayer.

The liquid flowing from the opening onto the spray surface spreads out into a circular layer with a radius R₀, as shown in Figure 2a. The value of this radius is determined by the viscosity and surface tension of the liquid and can be determined by the dependencies presented in the work [17]. If the radius of the spray surface exceeds the value of R₀, then it becomes necessary to make additional channels on the spray surface to ensure increased spray performance.

The location and number of additional channels for delivering liquids to the spray surface are determined from the condition of ensuring its uniform coverage with a layer of sprayed liquid. Figure 2 shows the optimal location of additional channels for supplying the sprayed liquid.

To ensure the complete coverage of the spray surface, additional holes must be positioned at intervals of 2R₀ along the cone generatrix (Figure 2b). In this case, the most rational is to place additional holes (in addition to the central hole made at the top of the cone) on the circles (Figure 2c). Figure 2d shows that the radius of these circles will increase by $2 R_0\sin {\displaystyle \frac{\gamma }{2}}$ with each new circle located further from the center of the cone. The number of circles is calculated so that they are located at a distance of 2R₀ from each other and at a distance of R₀ from the outer edge. With a known radius of the spray surface R, the length of the cone generatrix will be equal to $N={\displaystyle \frac{R}{\sin \frac{\gamma }{2}}}$. Then, the number of circles that can be placed along such a cone generatrix will be equal to

$$N={\displaystyle \frac{\frac{R}{\sin \frac{\gamma }{2}}-R_0}{2 R_0}},$$

(1)

or, moving to a common denominator,

$$N={\displaystyle \frac{R-R_0\sin \frac{\gamma }{2}}{2 R_0\sin \frac{\gamma }{2}}}.$$

(2)

If the obtained value is not an integer, then it is rounded to the nearest integer value and the R₀ value is specified, ${\displaystyle \frac{R}{\sin \frac{\gamma }{2}}}=N\cdot 2R_0+R_0$; hence:

$$R_0={\displaystyle \frac{R}{(2 N+1)\sin \frac{\gamma }{2}}}.$$

(3)

The radius of each circle will be equal to the following:

$$R_i=2i{R}_0\sin {\displaystyle \frac{\gamma }{2}},$$

(4)

where i = 0 … N is the circle number.

The centers of the holes of the channels for supplying liquid on each of the circles are also uniformly located at a distance of 2 R₀ from each other along the length of the circle. The number of channels on each circle is as follows:

$$K={\displaystyle \frac{2\pi i\cdot 2R_0\sin \frac{\gamma }{2}}{2R_0}}=2\pi i\sin {\displaystyle \frac{\gamma }{2}}.$$

(5)

Once additional channels have increased the productivity of ultrasonic spraying, the next step is to transport the generated aerosol to the disinfection targets. Since the liquid droplets formed by the ultrasonic sprayer have a low speed of separation from the liquid film, then, to form a spray torch with a direction different from the vertically directed downwards, it is necessary to use additional air flows, as shown in Figure 3.

In ultrasonic spraying, air flows are intended only for transporting already formed droplets. In ultrasonic spraying, airflows are used solely to transport the already formed droplets.

Several types of sprayers with varying operating frequencies (and corresponding droplet sizes) and productivity levels are outlined below. These can serve as a foundation for developing specialized devices for disinfection applications. The operational frequency range of the sprayers currently in use spans from 18 to 160 kHz [18,19,20,21]. Increasing the frequency in this range reduces the average size of the formed particles from 65 to 18 μm. However, as the spray frequency increases from 22 to 160 kHz, productivity decreases by a factor of 100. The developed sprayers can be categorized into several groups, as shown in Figure 4.

Below are several different types of sprayers, differing in operating frequency (size of formed droplets) and productivity, which can be used as a basis for creating specialized devices for disinfecting various objects. The currently implemented range of operating frequencies of sprayers is from 18 to 160 kHz [18,19,20,21]. Increasing the frequency in this range allows reducing the average size of formed particles from 65 to 18 μm. However, with an increase in the spray frequency from 22 to 160 kHz, productivity decreases by 100 times.

All developed sprayers should be conditionally divided into several groups, shown in Figure 4.

The technical characteristics of the sprayers are presented in

Table 1. Technical specifications of sprayers.

Sprayer Number (Figure 4)	4a	4b	4c	4d
Power, VA, not more than	150	150	150	100
Frequency of ultrasonic vibrations, kHz	22 ± 1.65	22 ± 1.65	35 ± 2.63	160 ± 10
Amplitude of oscillations of the working tool, µm	20–30	20–30	35	15
Oscillating system, mm	Ø100 × 250	Ø170 × 80	Ø100 × 250	65 × 70 × 90
Liquid viscosity, no more than, cPz	30	5	5	3
Sauter diameter of formed droplets, µm	65	65	55	18
Productivity (by water), mL/s, no more than	15	5	3	1

.

For the practical implementation of the disinfection process, devices of various designs can be used based on the developed ultrasonic sprayers, both for manual spraying and for use in mobile devices.

2.2. High-Capacity Wearable Atomizer Designs for Generating a Variety of Particle Sizes

Figure 5 shows a device for forming an aerosol with an average particle size of no more than 50 μm and a capacity of up to 20 mL/s for treating people’s clothing, vehicles, and small rooms. To provide an increased spray area, a flexural–oscillating disk (oscillations are carried out on the second mode) with a diameter of 40 mm is used. Using a disk enables a nearly unrestricted increase in the spray surface area by exciting bending oscillations in higher-order modes. In turn, the use of a disk allows for a virtually unlimited increase in the area of the spray surface due to the excitation of bending oscillations at higher modes. This also reduces the disk’s thickness, positively impacting the sprayer’s mass and size characteristics. In general, the operation of the disk is similar to the oscillations of a membrane with free edges with the central excitation of oscillations. The distribution of oscillations over the surface of such a disk is described by Bessel functions of the corresponding order (equal to the oscillation mode of the disk). The resonant frequency of the sprayer is 40 kHz. Liquid is delivered to the spray surface through 12 holes positioned at the disk’s zero-oscillation points. The principle of constructing the oscillating system of the atomizer is similar to that shown in Figure 1.

As shown in Figure 6, the sprayer includes a pistol-type handle (9). On the handle there is a button (10) for starting the spraying. In this case, electric oscillations of ultrasonic frequency are supplied from the generator to the piezoelectric elements of the sprayer and the sprayed liquid is simultaneously supplied to the spray surface (4), through the channels (6). The electric oscillation generator operates from a 12 V battery and can be placed in a portable backpack behind the operator (together with a volume with a disinfectant solution).

Figure 6 shows a high-performance sprayer for forming an aerosol with an average particle size of 65 microns and a capacity of more than 60 mL/s, designed for treating public transport, the metro, and premises. To ensure such a high performance (more than 60 mL/s), the area of the spray surface was significantly increased. Similar to the previous device, it employs a bending–oscillating disk. The diameter of the disk is increased to 250 mm; it oscillates in mode 3.

The sprayed liquid is in a backpack container, which is located behind the operator. A liquid supply system (pump and valve) is installed on the liquid container. An electronic generator for powering the ultrasonic sprayer is also installed on the backpack container. The electronic generator operates using high-capacity lithium-ion batteries.

Thus, a number of ultrasonic sprayer models have been developed that have good user characteristics—high dispersion, narrow particle-size distribution, and satisfactory performance for various tasks. Their operating principle is based on the excitation of capillary waves in a liquid film using ultrasonic vibrations.

In the presented devices, an increase in spray performance is achieved by significantly increasing the area of the spray surface. However, it is not possible to infinitely increase the area of the spray surface. This requires the development of new ultrasonic liquid spraying methods based on a combination of ultrasonic action and other factors.

The following section introduces two novel ultrasonic spraying methods, leveraging alternative principles. First of all, they use the phenomenon of ultrasonic cavitation.

2.3. Innovative Acoustic–Dynamic Methods and Devices for Spraying Disinfectant Liquids

2.3.1. Multi-Stage Spraying

In ultrasonic liquid spraying, droplet formation occurs in a single stage, where droplets detach from the liquid film. In this case, the higher the ultrasound frequency, the higher the dispersion of aerosols, which is preferable in the problem under consideration. However, the generation of high-frequency oscillations (more than 80 kHz) with an amplitude sufficient for spraying limits the area of the radiating surface. These oscillations are dampened in the sprayed liquid and absorbed by the ultrasonic emitter material, reducing spraying efficiency. To overcome this limitation, a method of multi-stage ultrasonic spraying with the supply of acoustic energy to the liquid through gas is proposed [22]. The concept of implementing the proposed method of ultrasonic spraying is shown in Figure 7.

The initial aerosol is formed by any method of atomization with sufficient productivity. In the case of technologically simple methods (e.g., hydraulic, low-frequency ultrasound, etc.), the initial aerosol will have low dispersion (droplet size of about 1–10 mm). Primary droplets, produced at high rates, move with a velocity under the influence of the ultrasonic field. Ultrasonic action causes successive droplet fragmentation each time they interact with a power antinode in the standing wave.

The primary flow of large droplets moves either due to gravity or is carried away by the transport gas flow towards the next stages of atomization.

At the next stages, the droplets enter the ultrasonic field (standing ultrasonic wave) in the gas. The droplets are broken down into smaller fragments at the antinodes of the standing wave due to deformation and a loss of stability (Kelvin–Helmholtz instability). Ultrasonic energy and air interaction atomize droplets, following mechanisms categorized by morphological characteristics [23,24,25]. As a result of such an interaction of the droplets with the antinodes of the ultrasonic field, a gradual destruction of the droplets occurs up to a certain limit (minimum size).

The developed method of ultrasonic atomization is implemented by means of a bending–diametrically oscillating tubular ultrasonic emitter. It is a hollow cylinder with a stepwise changing external and constant internal diameter. The internal diameter of the tubular emitter is selected from the condition of ensuring the resonance of ultrasonic vibrations in the internal air cavity of the tubular emitter (the formation of a standing acoustic wave), at the operating frequency of the emitter. The shape of the external surface of the emitter is determined from the condition of ensuring a uniform distribution of the amplitude of ultrasonic vibrations along the length of the emitter. A drawing of the tubular emitter and the distribution of the oscillation amplitude are shown in Figure 8.

The results of the calculations of the distribution of sound pressure created by the emitter, as well as a photograph of the manufactured emitter, are shown in Figure 9. The calculation of the distribution of the sound pressure level inside the tubular emitter was carried out in the ANSYS program. The Harmonic Acoustics harmonic acoustic analysis module was used. The volume of the calculation area was limited by the inner surface of the tubular radiator. Boundary conditions were set at the ends of the calculation area: the radiation boundary. The following air parameters were used in the calculations: sound speed—343 m/s; and density—1.2 kg/m³.

During the calculations, an iterative change in the emitter dimensions was made to ensure the formation of a standing wave in the internal cavity of the emitter at the emitter oscillation frequency.

As a result of modeling, the maximum sound pressure level on the emitter axis was determined to be 190 dB, with the standing wave consisting of three maxima successively located along the emitter. This ensures five stages of liquid droplet spraying. The developed emitter operates at a frequency of 22.4 kHz. The acoustic power emitted into the air is 35 W; the amplitude (span) of surface oscillations’ max–min is 51–40 μm; and the electric power is 48 W. The experimental sample has the following dimensions: D₂ = 92 mm, D₁ = 52 mm, and L = 96 mm.

2.3.2. Cavitation Combined Method

To overcome the limitations of ultrasonic liquid spraying methods in terms of productivity, we propose to combine the advantages of the hydraulic spraying method (with high productivity) with the capabilities of ultrasonic cavitation (to ensure high droplet dispersion). The main idea is to create a cavitation region in a liquid under excess pressure. This involves creating a cavitation region at the liquid outflow point of the hydraulic nozzle. The ultrasonic emitter placed here affects the volume of liquid above the end surface of its working end with ultrasonic vibrations. An important condition is a sufficiently high vibration amplitude, exceeding the threshold for creating cavitation in the liquid. Schematically, the cavitation combined spraying method is shown in Figure 10.

Under the influence of excess pressure, the cavitating liquid flows out of the pressure hole in the form of large drops, inside which there are one or more cavitation bubbles. The higher the hydraulic pressure, the greater the flow rate and, accordingly, the spraying performance. The cavitation bubble collapse generates pressure pulses that expand the droplet until the surface tension limit is reached, causing the large droplet to break up into smaller ones. The size of these fragments (new drops) depends significantly on the size of the cavitation bubbles, which, in turn, is determined by the parameters of the ultrasonic action.

Thus, hydraulic pressure determines the flow rate (and spray performance). Ultrasound characteristics (amplitude and frequency) determine the parameters of the cavitation region and, ultimately, the dispersion of the droplets.

The proposed method offers several notable advantages. It provides increased spraying performance comparable to hydraulic and vortex atomizers, while significantly surpassing the performance of conventional ultrasonic atomizers with similar technical specifications. Additionally, it facilitates the production of small-diameter droplets through ultrasonic cavitation, resulting in droplets smaller than those produced by hydraulic or vortex atomizers under similar pressure and flow rate conditions. Another benefit is the ability to use low-frequency ultrasonic vibrations to generate small droplets, whereas conventional ultrasonic atomizers require significantly higher frequencies to achieve similar results.

The proposed cavitation combined spray method is implemented using a two-half-wave ultrasonic emitter. A sketch and a photo of the emitter with a resonant frequency of 22.1 kHz are shown in Figure 11.

After manufacturing and assembling the emitters and transducers, the oscillation amplitude of the emitting end was measured. The measurement results showed that the maximum oscillation amplitude of the end surface of the developed piezoelectric transducer was 30 μm. The obtained oscillation amplitude values are sufficient to create cavitation in the liquid.

3. Methods

To investigate the developed sprayers, an experimental setup and testing methodology were created.

3.1. Experimental Setup

To determine the droplet size characteristics produced using the developed spraying methods, an experimental setup was assembled, with its structural diagram shown in Figure 12.

The experimental setup allows for the measurement of droplet size characteristics based on the parameters of ultrasound. The setup includes the following components: the tested sprayer (1), an ultrasonic generator (2), a liquid supply system based on a peristaltic pump (3), a receiver to eliminate pressure pulsations (4), an air compressor for creating excess pressure (5), a liquid flow regulator (flow meter) (6), and a nozzle (optional) (7). Additionally, the setup includes a Malvern SprayTec (SprayTec 2000, Malvern Panalytical Ltd, Malvern, Worcestershire, UK) optical aerosol analyzer (8), a vacuum aspirator (optional) (9), and a sound pressure level meter (optional) (10).

All droplet size measurements were conducted using a Malvern SprayTec particle analyzer [26], which operates on the principle of laser diffraction. The analyzer measures particle dispersion at frequencies up to 10 kHz, in a size range from 0.1 to 2000 μm. During measurements, the analyzer’s laser beam was positioned 50 mm from the sprayer’s nozzle. Settled tap water at a temperature of 23 °C was used as the sprayed liquid.

Ultrasonic vibrations in the developed sprayers were excited using custom-designed electronic generators. These generators were constructed using a half-bridge circuit with independent excitation, phase-locked frequency control, amplitude stabilization of the sprayer’s oscillations, and output power adjustable in the range of 0–200 W. The generator is controlled by an STM32 family microcontroller.

3.2. Testing Methodology

For measuring the droplet size distribution produced during ultrasonic spraying of a thin liquid layer, the following methodology was employed. The electronic generator (2) was activated, applying voltage to the piezoelectric elements of the sprayer (1). Then, the liquid was supplied using the peristaltic pump (3) at a minimum flow rate of 0.5 mL/s. The flow rate was gradually increased to the nominal values listed in Table 1. The liquid flow rate was monitored using the flow meter (5). Once the required flow rate was stabilized (over 30 s), droplet size characteristics were measured using the Malvern SprayTec analyzer. Measurements were taken at a frequency of 2 Hz and averaged over 60 readings (30 s of measurement time). All statistical processing and determination of the main droplet size distribution metrics were performed using the SprayTec software (version 3.20).

For these measurements, the compressor (5), nozzle (7), vacuum aspirator (9), and sound pressure meter (10) were not used in the experimental setup.

The droplet size measurement methodology for the combined cavitation sprayer was similar to the one described above, except that the air compressor (5) was used to create excess pressure in the liquid. Before starting measurements, the pump (3) filled the receiver (4) to 90% of its volume (3 L) with liquid. The air compressor (5) was then used to establish and maintain the required pressure in the receiver and the liquid. In this case, the liquid flow rate was controlled using the flow meter (6). The rest of the measurement procedure was identical to the one described for ultrasonic spraying of a thin liquid layer.

When studying the characteristics of the multi-stage sprayer, large droplets were introduced into the input of its tubular emitter (at either end of the tubular emitter shown in Figure 9) using nozzle (7). The nozzle produced liquid droplets with a size of 1500–2000 μm (initial droplet size) at a flow rate of 5–30 mL/s. Droplet size characteristics were measured at two points [22]:

• Approximately one-third of the distance from the upper end of the tubular emitter, after droplets passed through the first antinode of the standing wave inside the emitter. A vacuum aspirator was used to collect aerosol droplets at this stage, corresponding to the first stage of atomization.
• 50 mm from the lower end of the tubular emitter, to determine the droplet size characteristics of the aerosol produced after the multi-stage atomization process.

When investigating the multi-stage sprayer’s characteristics, sound pressure levels were measured using an Ecofizika-110A sound-level meter with a VMK-401 microphone (Ecofizika-110A, OKTAVA-ElectronDesign LLC, Moscow, Russia). Measurements indicated that the sound pressure levels inside the tubular emitter ranged from 160 to 182 dB.

The measurement procedure involved adjusting the output power of the ultrasonic generator (2) to establish an initial sound pressure level of 160 dB inside the tubular emitter, monitored using the sound pressure meter (9). The liquid flow rate was set and maintained using the pump (3) and the flow meter (6). Initially, the flow rate was set at 1 mL/s. Droplet size distribution measurements were then conducted with the SprayTec analyzer as described for ultrasonic spraying of a thin liquid layer. Afterward, the sound pressure level inside the tubular emitter was increased by 5 dB, and the procedures were repeated. Similar measurements were performed for different liquid flow rates.

This methodology enabled tracking the reduction in droplet size at each stage of atomization and evaluating the resulting droplet size distribution [22].

4. Discussion of the Results of the Development of Methods and Devices for Spraying Disinfectant Liquids

4.1. Droplet Size in Ultrasonic Atomization of a Thin Liquid Layer

Droplet size measurements during ultrasonic spraying of a thin film at different frequencies showed that ultrasonic spraying allows for the formation of a narrow-spectrum and smaller-size droplet distribution (Figure 13). For comparison, the figure shows the particle sizes of an aerosol formed by a hydraulic nozzle. However, the productivity of ultrasonic spraying is about 10…20 mL/s, while, with hydraulic spraying, it depends on the liquid pressure and can significantly exceed 100 mL/s.

Ultrasonic spraying produces higher droplet dispersion, increasing the contact surface area with microorganisms, enhancing disinfection efficiency, and reducing disinfectant usage. A narrower droplet size distribution ensures uniform surface coverage, preventing areas with excessive or insufficient disinfectant application, thereby improving disinfection quality and minimizing risks such as the formation of hazardous aerosol clouds.

4.2. Droplet Size in Multi-Stage Ultrasonic Atomization

In multi-stage spraying, liquid is supplied to the input using any technically simple and inexpensive technology, such as a hydraulic nozzle or the ultrasonic spraying of a liquid layer. Spraying performance will depend on the performance of the initial liquid feed. Since three antinodes of ultrasonic vibrations are formed along the axis of the developed emitter, it will provide three stages of droplet fragmentation to a certain maximum size.

Droplets are destroyed in a standing ultrasonic wave when passing through a series of antinodes (stages). The size reduction is the maximum when passing through the first and second stages, and then the reduction is not so significant. At the same time, by the third stage, the particle size distribution becomes narrower. As noted above, high dispersion and narrow particle size distribution are favorable conditions for effective disinfection. On the other hand, this method is not limited in productivity—it is determined by the productivity of the aerosol generation method supplied to the device input.

To confirm the operability of the proposed spraying method, the dispersed composition of droplets formed during the spraying of water (settled, tap water, at a temperature of 23 °C) was preliminarily analyzed. The sound pressure level is 160–182 dB. The water feed performance is 5–30 mL/s. The initial size of droplets entering the tubular emitter was 1500–2000 μm. The histograms of the droplet distribution at the outlet of the tubular emitter are shown in Figure 14.

A significant decrease in droplet size compared to the initial value was obtained in the experiment over the entire range of sound pressure. The average droplet size of the resulting aerosol (the Sauter diameter) depends linearly on the sound pressure level (Figure 15a). With an increase in the liquid feed rate to the device input, the droplet size increases (Figure 15b)—initially slightly, then more strongly.

Changing the sound pressure level in the range of 160–182 dB allows you to adjust the average diameter of the formed droplets within 37–170 µm. Thus, multi-stage ultrasonic atomization allows you to significantly reduce the size of the droplets supplied to the input and control the dispersion of the final aerosol over a wide range. At the same time, the spraying performance is limited only by the performance of the device generating the incoming aerosol.

4.3. Droplet Size in Combined Cavitation Spraying

When implementing this spraying method, it was found that there is an optimal mode corresponding to a pair of values: liquid hydraulic pressure–ultrasonic vibration amplitude. This pair corresponds to the optimal mode, in which the developed cavitation is established in the pre-nozzle volume. In this case, the aerosol dispersion is the highest. Figure 16 shows the dependences of the critical ultrasound amplitude and the Sauter diameter of the aerosol in the optimal mode on the hydraulic pressure.

To implement this method, it is important that the ultrasound amplitude be sufficient to create cavitation. For higher hydraulic pressure values, it turns out that such an amplitude must be higher. When providing the cavitation spray mode, we obtain a truly fine aerosol, which is ideal for disinfection tasks. The higher the hydraulic pressure, the higher the ultrasound amplitude required to switch to the cavitation mode, and the higher the aerosol dispersion, as shown in Figure 15.

With increasing liquid pressure, it is necessary to increase the ultrasound amplitude to achieve the optimal mode, in which the droplet size will be minimal. At the same time, with increasing pressure in the optimal mode, the droplet size decreases. Moreover, with increasing pressure, the liquid flow rate increases, as is typical for hydraulic nozzles. In our case, this increase was from 4–5 mL/s for 2 atm to 14–15 mL/s for 12 atm. Typical histograms of aerosol particle size distribution for 6 atm and 11 atm are shown in Figure 17.

Thus, the implementation of the optimal cavitation mode in the combined acoustic–dynamic spraying method allows us to obtain a highly dispersed aerosol with a sufficiently high productivity.

5. Conclusions

The results of this study highlight the potential of ultrasonic atomization as a transformative approach to the disinfection of individuals, surfaces, vehicles, and large spaces. Traditional spraying methods, such as hydraulic and pneumatic systems, exhibit significant limitations, including inefficient droplet size control, uneven surface coverage, and incompatibility with disinfectants of varying viscosities or those containing nanoparticles. These shortcomings contribute to excessive disinfectant usage, reduced efficacy, and potential health risks from aerosol inhalation.

In contrast, ultrasonic atomization offers precise control over droplet size, nearly monodisperse droplet distributions, and compatibility with diverse liquid compositions. By leveraging ultrasonic vibrations, this technique ensures a uniform nanoparticle dispersion within droplets, minimizes waste, and enhances safety. Additionally, innovative acoustodynamic methods, such as multi-stage atomization and cavitation-assisted spraying, further optimize droplet dispersion and increase productivity, addressing the constraints of conventional ultrasonic systems.

This work makes several key contributions. It advances droplet generation technology by enabling ultrasonic atomizers to precisely control droplet sizes, allowing customization for various disinfection needs, ranging from personal hygiene to large-scale applications. The developed systems also enhance compatibility, efficiently atomizing high-viscosity liquids and two-phase disinfectant solutions, including those with silver nanoparticles, while ensuring uniform application without agglomeration. Additionally, innovative atomizer designs were introduced, featuring enhanced performance, controlled droplet size, and improved efficiency. Finally, the approach offers significant environmental and health benefits by reducing aerosol exposure risks and optimizing disinfectant use, leading to safer and more sustainable disinfection practices.

The proposed ultrasonic spraying systems have the potential to significantly improve global disinfection efforts, particularly in the face of emerging infectious diseases. By overcoming the limitations of traditional methods, these technologies enhance the effectiveness and efficiency of disinfection in public health and industrial contexts. Applications extend to hospitals, public transportation, schools, and urban spaces, reducing pathogen transmission and improving environmental sustainability.

Future research should aim to enhance aerosol production rates by developing hybrid designs that combine ultrasonic and hydraulic mechanisms. It should also explore novel materials and geometries to improve the energy efficiency and overall performance of these systems. Another important direction is the creation of automated, adaptive systems that can adjust in real time to varying liquid properties and disinfection conditions. Addressing these challenges will enable ultrasonic spraying technologies to become a cornerstone of modern disinfection strategies, contributing to public health protection and supporting sustainable hygiene practices globally.