The spray bottle that holds our BOROHMA fragrance is designed to separate our minicell solution from the fragrances and effectively remix our minicell solution with our fragrance during the spraying process. We want to separate the minicell solution from our fragrances to prevent the dilution of our minicell broth so that the minicell can have an adequate amount of nutrients to prolong its survival. To achieve this separation, our minicell bottle will have a capsule containing the concentrated minicell broth attached to the bottom of the spray bottle.

During the spraying process, the tube that takes up the fragrance and the minicell solution will have three openings with the thin opening taking up the minicell solution and the two wider openings taking up the fragrance solution. The diameter of the openings will have a ratio that takes up approximately 20% of the minicell solution and 80% of the fragrance. Additionally, to avoid a pressure difference formed by having 3 different openings we will design the opening so that their added volume uptake is equivalent to the main opening. This will also make the spray easier to press thus enforcing our inclusivity goals. When the spray is pressed, the minicell solution and fragrance will be mixed to release our insect-repellent fragrance. To ensure that the fragrance solution does not flow backward into the minicell capsule and trigger the degradation process of minicells, we implemented a thin rubber one-way valve in the tube connecting to the minicell capsule that prevents backflow and only allows for upward minicell solution flow. This valve as depicted in Figure 2 is designed so that it does not affect the pressure that is required to press our trigger mechanism and block our already thin minicell tube.

In order to ensure that our dip tube design can manage effective fluid movement and maintain a normal pressure balance, the flow rates of the three small dip tubes have to be within the capacity of the main tube. In other words, the sum of the flow rates of two fragrance dip tubes and the minicell solution dip tube has to equal the flow rate of the main tube, as demonstrated by the equation below: \[ Q_{\text{main}} = 2 \cdot Q_{\text{fragrance}} + Q_{\text{minicell}} \]

After establishing this relationship, we will be able to calculate the pressure drop inside each of the tubes when the nozzle is pressed. By conducting experimental procedures, we can obtain accurate values for \(Q_{\text{main}}\), \(Q_{\text{fragrance}}\) and \(Q_{\text{minicell}}\). Then, using these \(Q\) values, the pressure drop inside each tube can be calculated using the Hagen-Poiseuille Equation. \[ \Delta P = \frac{8 \mu L Q}{\pi r^4} \]

Parameters:

• \(\Delta P\): pressure drop
• \(\mu\): viscosity
• \(L\): length of the tube
• \(Q\): flow rate
• \(r\): radius of the tube

Our future plan is to measure flow rates for each tube so that we can analyze the optimal values for the radius and length of each tube using assumed pressure differences. With radius(r) as the x-value and length(L) as the y-value, a regression model can be produced to identify an optimal combination of radius and length for each tube. The equation we will use is:

$$L = \frac{\Delta P \pi r^4}{8 \mu Q}$$

To validate our result, we can input the values obtained into the following equation to verify that the flow rate of the main tube is the additive result of the flow rates of the three small tubes. This verifies that the flow rate of the 3-way tube system is conserved, assuring the functionality of our unique dip tube design:

$$\frac{8 \mu L_{\text{main}} \Delta P_{\text{main}} r_{\text{main}}^4}{\pi r_{\text{main}}^4} = 2 * \frac{8 \mu L_{\text{f}} \Delta P_{\text{f}}}{\pi r_{\text{f}}^4} + \frac{8 \mu L_{\text{ms}} \Delta P_{\text{ms}}}{\pi r_{\text{ms}}^4}$$

The main goal we want to achieve with our product is to be eco-friendly and protect our environment. We plan to target this in two ways: the material used, and reusability.

First, we plan to design our bottle with biodegradable plastic that is also UV-protective so the minicells do not die. This will ensure that the product stays effective in its function of mosquito repellency. The material that will be used is a new synthetic bioplastic that has a bisfuran structure that does not allow UV light to pass through the film(University of Oulu, 2020).

In addition, we also designed our hardware so that when either the fragrance or minicell solution runs out, you can replace them directly. This will decrease the amount of plastic used in general and be more eco-friendly than replacing the entire bottle every time. We implemented a tight rubber ring that forms an airtight seal around both the small hole at the bottom of the fragrance bottle and the small hole at the top of the minicell solution. This makes it so that when you remove the dip tube, the rubber ring stops the liquid from flowing out. During fragrance replacement, the user can directly unscrew the top cap and pour the fragrance in from there due to this design. For minicell replacement, the user will have to buy a new minicell capsule that is prefilled and reattach it to the main bottle. For the fragrance, you can directly pour the replacement fragrance in from the top opening.

The actual spray nozzle we use will adopt a flat-fan shape, which allows for a wide distribution of minicell solutions without exerting too much stress on our minicells(FBN Network, 2024).

To design a flat fan spray nozzle that can deliver the minicell solution in a wide range without making the bottle harder to press we will take inspiration from Lechers Series 616/617.

We decided on this spray design because of its ability to distribute our minicell fragrance evenly, its non-clogging features, its compact design, and its low-pressure requirements. The non-clogging features of this nozzle design also ensure that there isn’t a buildup of minicells that could otherwise cause clogging. Typically flat fan spray nozzles are designed to be used with high pressure, but the series 616/617 is one of the nozzles that are designed for lower pressures being optimal at pressures as low as 0.5 bars(GmbH, n.d.). We will use a scaled-down plastic version of this nozzle design.

The factors that are involved in making a spray bottle difficult to press are primarily the pressure required to force the water out of the nozzle and the spring weight. Since the pressure of our spray bottle is adjustable, we must address the second factor of spring weight. While we want to make the spring as weak as possible to make the bottle easy to press, the spring has to still be strong enough to be able to return the trigger mechanism back to its original position after you press it. To solve this problem we can increase the pressing distance of the trigger mechanism. This distributes the force required over a larger distance making it so that at each instance in time the force required is smaller. This is shown by Hooke’s law. For a spring, Hooke’s Law states that the force required to compress or extend a spring is proportional to the distance it is compressed:

$$F_s = k|x|$$

Where:

• \(F\) is the force,
• \(k\) is the spring constant (a measure of the stiffness of the spring),
• \(x\) is the compression distance.

We can set the force required to compress the spring to be equivalent to the force that is required by the spring to bring the trigger mechanism back to its original position. This can be assumed if the spring retains 100% of the energy used to compress it:

$$k = \frac{F_s}{|x|}$$

Therefore if we are trying to set the spring constant (\(k\)) as low as possible while retaining the force (\(F\)) that is needed to bring the trigger mechanism back to its original position, we can instead increase the compression distance \(x\) which is inversely proportional to \(k\).

To prevent our trigger mechanism from being accidentally pressed when put in a bag we have also implemented a cap that can be put on after use. This solves the problem of accidental presses which would have otherwise been made worse by our easy-to-press trigger mechanism.

The adaptability of our spray nozzle presents a promising solution for customizable use, but ensuring that the system functions as intended requires extensive testing and validation process. Therefore we will conduct various tests to see if our hardware will actually perform the way it should in the real world.

1. Inclusivity testing

   While we have increased the height of the trigger to reduce the strength of the spring, we still have to make sure that it is easy enough to press for individuals with muscle deficiency. Our first phase of testing will focus on evaluating the strength required to press the trigger and determine if we have to increase the height of the trigger mechanism further. If necessary, we will fine-tune the system by adjusting the press distance for users with weaker finger strength. To verify these adjustments, we will implement a series of user-based trials. A diverse group of testers, including individuals with varying levels of hand strength and dexterity, will be selected to operate the bottle under different pressing distances and nozzle pressure settings. During these trials, we will gather quantitative data on the force required to press the spray and the evenness of distribution. This will help us determine if the current alteration provides sufficient ease of use while maintaining optimal spray performance.

2. Durability testing

   We will also conduct durability testing to ensure the long-term reliability of the bottle's components. To do this we will simulate repeated use to evaluate the wear on key parts such as the nozzle, one-way valve, springs, and seals. Our one-way valve, which prevents fragrance from entering the minicell capsule, will undergo extensive testing to confirm its effectiveness in different spraying conditions. We will also have to make sure that the valve is not disrupting the forward flow of minicells so that the ratio of minicells to fragrance will remain consistent. Additionally, since we are using a longer spring for a greater pressing distance we will have to test if the spring will be able to maintain its shape over many presses. If we find out that our current springs have low fatigue resistance(deform over time very easily) we can use materials that have better durability such as Beryllium Copper for our springs.

3. Dip tube testing

   To test the dip tube we will measure the product's efficiency in mixing the correct proportions of minicell solution and fragrance. This will involve precision testing of the dip tube and its three openings, ensuring the ratio of 20% minicell solution and 80% fragrance is consistently maintained during each spray. Testing will include both lab-based simulations and real-world scenarios to ensure the dip tube performs as expected. We can then adjust the tube’s opening radiuses based on the test results and values obtained from the Hagen-Poiseuille Equation as described earlier.

4. Reusability testing

   Finally, we will run reusability tests focused on the bottle’s refillability and the functionality of the self-sealing rubber rings. These tests will determine whether users can easily replace both the fragrance and minicell solution without leakage or contamination. By the end of this testing phase, we will set clear performance benchmarks, such as user ratings on the ease of replacement and the minimum number of spray cycles the product can endure without failure. This ensures that our product is actually reusable and thus eco-friendly.

Our thorough testing process will ensure that the bottle meets the standards of functionality, ease of use, and durability before being introduced to the market.