42 TechnologyNewsAdvances in spray technology

Advances in spray technology

For a number of years, 42 Technology has been investigating advances in spray technologies and the issues raised by the need to reduce hydrocarbon emissions from propellants in aerosol cans.

This interest has been driven by a number of factors; projects for clients to consider novel approaches; our own interest in Cleantech and the environment; our capability in fluid handling and control; and fortuitous relationships and inventions along the way.

This article presents an overview of the drivers for change and a description of one technology that 42 Technology has been investigating.

Advances in spray technology

Spray and drop generation is of importance in a wide variety of applications. Domestic applications range from paint to hair spray and polish to water repellent. Medical drug delivery devices provide a cloud of spray for deep inhalation into the lungs. Other applications include ink jet printing, agrochemical deposition onto plants and fire retardation. In many domestic and medical applications volatile organic compounds and hydrocarbons are used as the propellant. The volatile is present in the can as both a liquid mixed with the product and vapour in the head space. As the product is dispelled the volatile boils providing a nearly constant pressure and good spray performance over the life of the can. But the propellants are highly damaging to the environment and are being phased out. Products are available now that use compressed gas (nitrogen, air, or CO2 at up to 10 bar) as the energy source, but these suffer from a loss in pressure as the product is used. This is due to two factors; the head space volume increases and gas is often mixed with the product to improve the spray characteristics.

A new, low-cost, low-pressure delivery method is needed that will cater for a wide range of applications and the different spray characteristics required. For example, drop sizes for medical products need to be ~ 5μm in diameter and have very little velocity so that they are readily inhaled into the deep lung. Many domestic sprays conversely need to have a larger drop size >15 μm, so that the user does not inhale the product, and also have to be directional with excellent surface coverage. Agricultural sprays need to cover and adhere to plant foliage. Other products need to have fine droplets that readily evaporate to impart chemicals, or fragrance, into the air.

42 Technology is investigating a number of potential solutions. One is the generation of sprays by impacting low pressure liquid jets, or drops, onto a super-hydrophobic, or super oleo-phobic surface. Howard Biddle first saw the material some time ago when it was being presented by Professor Ullrich Steiner in the University of Cambridge’s Cavendish Laboratory. Whilst Howard was demonstrating how drops readily bounce off the surface he discovered that a higher velocity jet resulted in the creation of a fine spray. When we started our work super-hydrophobic surfaces were very novel, but now the effects can even be seen on YouTube! (see YouTube ‘Water Droplet Falling onto Super-Hydrophobic Surface’). Most early investigations into super-hydrophobicity, such as ink jet printing, painting and agro-spraying, were concerned with how to ensure a liquid drop adheres to a surface. Howard’s approach was different and he showed that a higher velocity jet of water directed onto such a surface creates an unstable disk that immediately breaks up into droplets, much like the high speed photographs of a falling milk drop forming a corona of smaller drops when it impacts a surface. No gas is needed and the pressure source can be very simple.

The method works with many superhydrophobic surfaces and we have investigated a number of these, including those from suppliers such as P2i and very advanced materials from MIT. Hydrophobic surfaces, much like lotus leaves and bird feathers, derive their properties from a complex combination of surface structure and surface chemistry. The advantage of the latest Cavendish developed coating is that it is potentially much cheaper than some of the others. The surface of interest is first coated with primer and then a liquid mixture of PTFE and small polystyrene spheres is applied. The material is fired to fuse the PTFE and the polystyrene spheres burn off leaving a surface structure of depressed craters within the PTFE coating.

The key to spray formation is the creation of instability in the fluid film. The factor that governs this is the Weber number, calculated as:

The Weber number calculation

For a good aerosol spray the Weber number should be 200 or more. Other factors also influence the formation of sprays and their drop size and 42T is investigating these.

One aim is to design a small device to replace the complex nozzle of an aerosol can and develop the technology so that different spray geometries can be generated at low pressure. Theoretically 3 bar should produce an aerosol spray with droplets about 5μm across, whereas at a pressure of 0.7bar they will be around 20μm.

A patent has been applied for on the principle of impinging a jet of liquid on a superhydrophobic surface and its use in various applications.

If you have an application or query please contact Dave Wilson, Howard Biddle, or Chris Walters