Air Force

Self-clearing vents based on droplet expulsion

Applications include spacecraft fuel tanks and baby bottles

Materials Medical & Biotechnology Energy

U.S. Air Force scientists Wesley Hoffman and Phillip Wapner have invented new fluid flow technology by inventing novel surface and capillary geometries for wetted materials.

This illustration shows a capillary cross-sectional geometry embodiment with a non-wetting liquid.

On Dec. 10, 2019, the Air Force was issued U.S. Patent 10,502,448 for their work, making it available via license agreement to companies that would make, use, or sell it commercially.

In some applications, it is desirable to be able to increase or decrease the area of the surface in intimate contact with the liquid (wettability).

In the past, this has only been possible by changing the character of the liquid or of the solid in some manner, such as, by employing a liquid additive (for example, a surfactant), applying a surface coating, or changing the surface energy, for example.

The Air Force invention provides a way of controlling the area of the surface in intimate contact with the liquid (degree of contact) by controlling the surface geometry of the solid.

The surface geometry of the solid may comprise a plurality of surface discontinuities, such as pits, pores or trenches, having at least one solid included angle.

On the other hand, it may comprise a plurality of capillaries with each capillary having at least one cross-sectional and/or one axial geometry. The cross-sectional and/or one axial geometry may include at least one capillary included angle.

Alternatively, this invention is able to control the entrance of liquids into and the flow of liquids through free-standing capillaries by proper selection of the cross-sectional and/or one axial geometry of the capillary. This control applies to both wetting as well as non-wetting fluids.

The invention also works for a capillary device with a capillary path. The capillary path may carry different fluids in separate streams through the same capillary opening. These different fluids may consist of two or more immiscible non-wetting liquids or of one or more non-wetting liquids and gas. In the case of two liquids, the first liquid has a first contact angle and the second liquid has a second contact angle.

The invention provides a self-clearing vent, based on a calculated transitional included angle, that allows the passage of a gas or a vapor through the vent, excludes the passage of liquid through the vent, and automatically removes condensed vapor from the interior surfaces of the vent.

The vent comprises a metering section to which the two halves of the vent are attached axially along the major axis of the valve at their minimum dimension i.e. truncated vertex. To function, each half of the vent must possess at least one geometric feature with each geometric feature having at least one included angle and an ever-expanding region from the truncated vertex in the direction of droplet expulsion.

In certain applications, such as heat pipe and spacecraft fuel tanks during zero-gravity conditions, it is necessary for the liquid to spontaneously move from one location to another on the overall surface entirely because of capillary forces. This migrating behavior is commonly referred to as “wicking,” and only occurs on planar surfaces if the contact angle approaches zero.

If, however, the overall surface is covered with inverted V-shaped features, for example, that have an included angle less than the transitional included angle the wetting liquid will increase both its contact with the surface and the volume of liquid being wicked considerably enhancing the wicking action.

This is because the actual area of contact between the liquid and the solid surface has been increased. On the contrary, if the included angle .delta. is greater than the transitional included angle some decrease in wicking activity in comparison to the flat surface will occur. By varying the included angle, it is possible to control the location on the overall surface to which the liquid will migrate.

There are many applications where a microporous hydrophobic membrane material made by sintering PTFE (Teflon) or another polymeric material is used to allow air, vapor, or another gas into or out of a container to equalize the pressure or prevent a vacuum from forming while at the same time not allowing liquid to pass through the membrane.

Hundreds of millions of these vents have been manufactured for applications as diverse as storage and shipping containers that can breathe, medical device filtration and separation, amplifier and antenna vent filters, GPS/navigation micro filters, venting of automobile headlights and ABS brakes, marine lighting water repellent venting, instrument cluster protective micro filters, bag and tubing vents, as well as military equipment protective vents for harsh environments to name a few.

These packaging vents manufactured from PTFE as well as polypropylene, polyethylene, polyvinylidene fluoride, acrylic copolymer, and polyethersulfone polymer are supplied by numerous companies, such as Gore, Pall, Porex, Millipore, and Siemens. The vents work very well in low humidity or low vapor pressure environments while in high humidity and high vapor pressure environments their performance is degraded.

In many applications, such as in baby bottles and infant drinking cups as well as medical containers and devices, it is required to wash and sanitize the container for subsequent use. When a microporous hydrophobic material is used as a vent in one of these containers, it is flooded and thus is clogged by water during the dishwasher cycle.

In this condition, it is useless until the condensed water vapor in the micropores evaporates. This flooding of the membrane is due to the fact that a microporous hydrophobic material will not let water into its pores while vapor in the form of steam is free to enter the structure and condense inside the porous structure. Thus, there is a need for a non-porous vent that will not flood and subsequently cease to function when vapor condenses on the surface of the vent.

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