Using the respiratory system as a route to deliver medication has many advantages, such as the large surface area of the respiratory system and the ease of absorption via the lung tissue. Nebulizer technology uses the respiratory system to administer a wide variety of medication. Nebulizer technology creates microscale droplets that can be inhaled by the patient. Different types of nebulizer technology exists, and the vibrating-mesh nebulizer is one of them. A mesh with over a thousand nozzles, each with a diameter of 4.3 micrometer, vibrates against the liquid reservoir with a frequency of 128 kHz. This technology has already been proven to work, except for the use of one particular liquid, pure water.
The relation between the concentration of salt in a solution and the performance of the vibrating-mesh nebulizer was researched by Beck-Broichsitter et al. (2014). They found that a significant drop in performance takes place when the salt concentration of the solution decreases from 1 mM sodium-chloride to 0.01 mM sodium-chloride.
This research first investigated the theory on the properties of these solutions, and how these properties change when the concentration decreases from 1 mM sodium-chloride to 0.01 mM sodium-chloride. It was found that the hydrodynamic properties of the fluid do not change significantly over this range, but the electrostatic and -kinetic properties do change significantly over this range. This led to the hypothesis that the electrostatic and -kinetic effects have an influence on the formation of droplets and therefore the performance of the nebulizer. Three different hypotheses were contrived on how these effects influence the formation of droplets. The first hypothesis was that charged induced wetting causes the liquid from the nozzle to spread out over the surface at the end of the nozzle, instead of forming a droplet. The second hypothesis was that due to electrostatic forces in the neck of the droplet formation process, the neck is prevented to decrease in diameter, no pinch-off takes place and therefore no droplet is formed. The third hypothesis was that a droplet was formed, taking with it an electrostatic charge. The next droplet is then prevented from exiting the nozzle, by the electrostatic force the first droplet exerts on it. These hypotheses where based on engineering intuition, rather than experimental data.
Two different types of experiments were conducted to test the hypotheses. First the mass flow rate of the nebulizer was measured using different electrolyte solutions. Second, images where recorded, with various imaging speeds and magnifications, of the nebulizer while it was in working.
The results of the first type of experiments indicated that there is indeed a relation between the electrostatic and -kinetic properties of the fluid and the performance of the nebulizer. The results from the second type of experiments showed that the first hypothesis, the charged induced wetting hypothesis, was false. The results did not prove the other hypotheses to be false or true, but rather led to a new hypothesis. The new hypothesis was that the increased viscosity or decreased diameter of the nozzle, due to the electric double layer, bring down the mass flow through the nozzles or causes a portion of the nozzles to be inactive. As a prove of concept, an experiment was conducted where the viscosity of the liquid was decreased by increasing the temperature. This experiment gave the indication that the new hypothesis can be true.