# An Overview on Atomization and Its Drug Delivery and Biomedical Applications
## 4 Drop Size and Distribution in Spray  
Sprays are generated by the disruption of liquid jet or sheet and it consists of a number of droplets. The characteristics of these droplets depends on various factors. Spraying is a technique used in different applications like drug delivery, agriculture, painting, etc., and for each application the size of the drops vary to provide efficiency [52]. For example, while spraying pesticides in an agricultural field, the size of the drops in the spray should be small for adequate deposition and safeguard [53]. Similarly for drug distribution, the drops should be small enough to easily inhale the spray and at the same time it should not be very small so that the drug evaporates before inhalation [54, 55, 56]. [PERSON] et al. [43] were the first group of scientists who studied the various factors influencing the formation of a liquid sheet and later the disruption of this sheet into droplets. These factors include the properties of the fluid used (surface tension and viscosity) and the physical factors applied in the system (turbulence and pressure). They explained that when the liquid sheet projected from the nozzle comes in contact with the gas, friction between the two medium causes a wave pattern on the exterior of the sheet, which is then broken down to droplets. In 1953 [PERSON] studied the instability of a moving liquid film and observed a wave pattern on the surface of the film. He found that these waves were extending in magnitude to vary the thickness of the film (thinning) leading to the rupture of the film into fragments of Squire wavelength. These fragments will eventually form ligaments and then split into droplets [57]. There are different methods for modelling the droplet size and its distribution in a spray. One is the supreme entropy method which examines the surface tension of the fluid and its mass in calculating the diameter of the drop, but the limitation is that there is a detachment between the data obtained and the observed parameters. The discrete probability function model utilizes the straight and lateral rupture models by considering the physical factors like instability and swirl effect in the fluids and the limitation is that it cannot be used for studying the secondary atomization process. Another model is the empirical model in which an empirical curve is utilized to study the fit of the experimental data to the curve but the drawback is that it cannot be considered with a different atomization apparatus design. Several computational methods like Direct Numerical Simulations and Large Eddy Simulations are also reviewed for studying the numerical simulation of the atomization process but these methods are costly, less accessible compared to other techniques. The most accepted model for understanding the atomization process is the integral form of the conservative equations which can be later compared with the experimental values procured [58, 59]. [PERSON] et al. studied the characteristics of a droplet and its size distribution using a collaborative atomization procedure. They designed a new two-fluid vent for characterising the size and distribution of the drop with two atomizing nuclei. The numerical measurements of these droplets were done using the phase Doppler particle analyser. The  
Figure 4: Schematic representation of atomizers used in: (**A**) rotary atomization, (**B**) hydraulic atomization and (**C**) pneumatic atomization; reproduced from [15]. results showed that the characteristics of the spray and its size distribution is related to the initial and incidental atomization which depended on the structure of the atomizing nucleus and the fraction of mass drift rate of gas to liquid (GLR). They also compared the newly designed vent with the standard vent and found a decrease in the diameter of droplet by 33.97% from the new vent and an increase in size distribution and velocity.