Stream vs. Spray: What a Garden Hose Teaches Us About Olfactory Delivery
Author: Tim Rees, Scientific Advisor, Rocket Science Health
Introduction
Today we'll take a look at a fun bit of physics related to nose-to-brain therapeutics. Precision olfactory delivery is mission-critical for Rocket Science Health as it lies at the foundation of this emerging route of administration, so it's important to understand the basics of how things actually work. To that end, we'll explore some of the key differences between a liquid stream and a spray, but we'll keep it casual and understandable for a broad audience. A water-fight is a great place to learn about physics, so let's start there.
Targeted Delivery
Take an incident that occurred in my backyard last summer. I stepped outside one hot day to find my wife tending to the carrots while my youngest son, who was supposed to be helping her, stole berries off a nearby bush. I instinctively reached for the garden hose, determined to steer the lad away from a life of crime. But before I could dole out some icy-cold justice, I had an important decision to make: spray or stream? My son was partially sheltered by the fence, so it would be easy to switch the nozzle to spray and drench him and the entire garden with a fine mist. But he was standing quite close to my wife, so I had to be mindful of off-target effects. Off-target effects in a water-fight can be almost as dire as off-target effects in therapeutic delivery, and I did not want to spend the night on the couch. I twisted the nozzle selector to stream and set him back on the soaking-wet path to virtue.
Now, to see how this relates to nose-to-brain therapeutics, we need to understand the nasal anatomy. While we're all familiar with the nostril and lower vestibule, most of us are utterly oblivious to what lies beyond the nasal valve. The valve redirects airflow but also serves as an effective barrier to fingers and foreign objects that might otherwise find their way into the internal cavity. Nestled up along the ceiling of that internal cavity, occupying a narrow slit with direct access to the CNS via neurons that protrude through the cribriform plate, lies the olfactory cleft. The cleft is very narrow, representing about 5% of the nasal cavity by volume, and, not coincidentally, the majority of sprays deliver less than 5% of their contents to the region. This means about 95% is delivered off-target, potentially resulting in the physiological equivalent of an angry spouse. The nasal valve and shape of the internal space ensure that there is no direct, line-of-sight path from the external vestibule to the internal target site, posing a serious hurdle to precision therapeutic delivery. However, if you can secure a discharge point above the nasal valve, the choice between a liquid stream and spray becomes a no-brainer.
A liquid stream ejection (left) via the Rocket Science Health device and a conventional nasal spray (right). The liquid stream achieves a focused delivery while the spray deposits formulation across a much broader area.
Accuracy is one thing, but the next time you are dousing a child (preferably your own) with a garden hose, take a moment to marvel at the difference in range that a stream achieves over a spray. Two main factors contribute to this: the uncontrolled direction of droplet release and the increase in aerodynamic drag. While the limited length scales reduce the impact of these factors within the nasal cavity, they remain present. The large displaced volume of a misty spray cloud illuminates the difficulty of depositing a significant amount of spray within the nook that forms the olfactory cleft.
Targeted delivery demands a concentrated liquid stream, but even if you could achieve greater deposition with a spray, are droplets really the way to go?
The Perils of Droplet Production
Let's think about what is actually required to produce droplets with a nozzle. Generally speaking, we need a process that disrupts the surface tension holding a liquid in cohesion. You may have noticed that as a thin sheet of liquid flies through the air, say from a bucket of water flung at you in retaliation, the sheet breaks up into droplets. The instability that causes this fragmentation first shreds the sheet into filaments that then evolve into tiny spherical droplets under the renewed action of surface tension. We could generate a thin sheet with a nozzle by forcing liquid through a tiny orifice, but most sprays use a more sophisticated swirl approach. The swirl is achieved by forcing liquid into a cylindrical chamber immediately prior to its discharge point, resulting in a tangential flow that exits the nozzle in a conical sheet.
If the droplet production procedure sounds violent, that's because it is. Physically speaking, it tends to be a very high shear event, with adjacent layers of fluid moving rapidly past each other. This can impart substantial forces on sensitive therapeutic molecules within the liquid as they rub against each other.
Droplet production in a conventional spray device involves a high-shear event and a dramatic increase in the total surface area of the formulation.
Another interesting consequence of droplet production in sprays is the increase in a liquid's surface area. If you take a blob of liquid and shred it into 1,000,000 equal-sized droplets, each will have a radius 1/100th the size of the original, and the new total surface area will be 100 times larger. Shred it into a billion droplets and the surface area goes up by a factor of 1,000. Why does this matter? Well, therapeutic molecules that encounter an air-liquid interface of a droplet expose themselves to tensile forces that may be more harmful than the shear required to create the droplets in the first place. That increased surface area means we can expect a lot more therapeutic molecules to find themselves in an interfacial tug-of-war. Oh, and the internal pressure within a droplet grows inversely with its radius, potentially creating a microscopic pressure cooker.
A liquid stream, given a long enough flight path, will eventually break up into droplets too. But the initial thickness of a well-designed stream and the mere centimeters of distance it must cover before reaching its target nullify the problem. It is possible and practical to discharge a single, contiguous bolus of liquid within the nasal cavity without having to worry about all of the destructive forces associated with droplet formation, and, as it turns out, we can do it for a much broader class of liquids.
Viscosity: The Challenge and the Opportunity
For most of us, water dominates our everyday experience to such an extent that our intuition about more viscous fluids is fairly limited. Now, you could build up that intuition by loading a water-gun with canola oil or maple syrup and then chasing down your kids, but I don't recommend it. If you do try this, expect some serious off-target effects — and don't say I didn't warn you. Instead, let's reason our way through the subject to see what the problems and opportunities really are.
You might wonder why anyone would want to deliver anything intranasally that is more viscous than an aqueous solution. Two reasons are evident. First, the ability to discharge viscous liquids expands the universe of therapeutic concentrations and excipient concoctions for formulation developers, adding flexibility that can only open opportunities. This is especially important when dealing with large molecules or situations where viscosity is required to maintain stability. Second, viscosity offers a mechanism to alter residence time within different sub-regions of the nasal cavity. This can, in turn, influence the amount of drug that crosses the mucus layer in a fixed period of time, giving a degree of control to drug release.
At low viscosity, the Rocket Science Health (far left) and conventional spray (center left) ejections operate as expected. At higher viscosity, the Rocket Science Health device (center right) still achieves a clean ejection, while the conventional spray (far right) generates an irregular profile with very different characteristics from its low-viscosity counterpart.
So, why not discharge viscous fluids through a conventional spray device? The answer involves a combination of boundary layer theory with the spray production mechanisms explained above. The thing to know is that viscous fluids refuse to flow along regions of contact with solid surfaces, creating what is called a no-slip boundary condition. Viscous flow through a confined space thus results in a velocity gradient, with flow speeds increasing as you move away from a solid surface. Tying it all together, we see that within a narrow conduit, viscosity can severely limit the progress of a flow, creating a self-choking effect that demands high forces to squeeze even a little liquid out. Viscosity also opposes the formation of a swirl as it damps out the tangential flow component. All of this adds up to the fact that it is a tall order to discharge a thin sheet of viscous liquid in a reliable way, and if you do pull it off it is anyone's guess as to which direction the flow will go.
Devices for ejecting liquid streams, such as those pioneered by Rocket Science Health, are not completely immune to the effects of viscosity. Indeed, there are viscous limits above which our devices fail. But the simple fact is that the flow conduits within a liquid stream device are much, much larger than those of a spray, and with no need to generate a swirl, a consistent discharge can be produced. It takes careful configuration of a device's energy source and internal damping characteristics, but precision deposition is possible across a surprisingly broad range of viscosities. As a point of fact, we can reliably deliver liquids with viscosity like that of water right up through to canola oil or sugary syrup.
Summary
An illustration of a precision deposition within a nasal cast model using the Rocket Science Health device. Note the concentrated localization of liquid within the target olfactory region and absence of substantial deposition in other regions of the cast model.
If the goal is to uniformly coat the interior of the nasal cavity, a conventional nasal spray is a decent choice. But if you are seeking targeted delivery, minimal destructive forces, and the possibility of delivering a viscous fluid, liquid streams are the way to go. There are a lot of finer details to justify this argument, and if you're interested in diving deeper, I suggest Coleman et al., Colloids and Surfaces B: Biointerfaces (2024) — doi:10.1016/j.colsurfb.2023.113661 and D’Souza et al., Drug Delivery and Translational Research (2026) — doi:10.1007/s13346-026-02097-7. But I don't think a deep understanding of physics is needed to appreciate the merits of sprays and liquid streams. My own kids, despite their lack of formal training, have proven this after wrestling the garden hose out of my hands.
About Tim Rees:
Tim Rees is a Scientific Advisor at Rocket Science Health. He holds a Ph.D. in Applied Mathematics, specializing in Fluid Mechanics. When he is not busy ambushing his kids, he enjoys making chocolate and wandering through the forest.
About Rocket Science Health
Rocket Science Health, founded by Canadian business leader Kenneth Irving, develops precision drug delivery technologies focused on brain-targeted therapeutics and vaccines. Our needle-free intranasal device enables non-clinical self-administration for at-home care of brain related conditions from CNS to mental health; emergency rescue, and mass immunization. Its fluidics are specifically engineered for today's delicate biologics and dose-dependent drugs. As a team, we are driven to create innovations that expand access to care — especially for under-resourced communities and for individuals who face physical, cognitive, or challenging care environments where thoughtful design can improve health outcomes.
Contact: hello@rocketscience.health
Sources:
Coleman H, Perez JS, Schwartz DK, Kaar J, Garcea RL, Randolph TW. Effect of mechanical stresses on viral capsid disruption during droplet formation and drying. Colloids Surf B Biointerfaces. 2024;233:113661. doi:10.1016/j.colsurfb.2023.113661
D’Souza AA, Kahan M, Yang A, Padmakumar S, Bleier BS, Amiji MM. Formulation considerations in enhancing olfactory mucosal deposition for nose-to-brain drug delivery. Drug Deliv Transl Res. 2026;16:2552–2577. doi:10.1007/s13346-026-02097-7
