Sticky Foam, Marker Foam and Gum-Guns

GURPs Ultra-tech 4e has a number of interesting foam-based tools, including blast foam, construction foam, survival foam and ablative foam. THS Cities of the Edge describes “Slidefoam”, which is used for both crowd control and leisure activities.
There is, however, a real world, less-lethal foam weapon that is not covered in GURPs.
Gum Gun in Action
Sticky foam came to public attention in 1995 when it was used by the USMC in Somalia. Whilst it was used successfully, there were reports of the delivery devices used clogging. It was also reported that some targets moved so fast they were able to prevent their feet sticking together.
THS-era technologies have improved the usefulness of sticky foam.
Microbots in the dispenser systems greatly reduce the likelihood of clogging.
Microbots in the foam itself aerate the foam, increasing its rate of expansion but also reducing the chance of a target being suffocated by foam.
Tangler technology has also improved sticky foam.
Sticky foam shares some features with tangler weapons, but is a distinct system with some different applications.
The distinction is often not appreciated by journalists, who due to ignorance or sensationalism often report the use of sticky foam as the employment of tanglers.
Compared to tanglers, sticky foam can fill a greater volume of space.
Sticky foam expands to about 40 times its stored volume.
A user can rapidly generate several cubic yards of foam that it would be inadvisable to enter. As a general rule of thumb, a pound of contained sticky foam will produce about five cubic yards of expanded sticky foam.
Sticky foam releases heat as it expands. This is not enough to cause injury but an inventive player may find an application for this property.
Sticky foam sets in a few seconds to form a lightweight, but hard material.
Sticky foam floats on water, even when set.
A target covered in sticky foam will be buoyant unless very dense or the quantity of foam is relatively small.
Solvents can soften set sticky foam but chipping it away is still a time consuming job, and can take several minutes or more.
Sticky foam dischargers range from small spray cans or tubes to backpack systems that resemble flame-throwers.
Grenade and satchel charge configurations are also available.
A tube weighing a ¼ of a pound would have a spray range of only 2 hexes and fill one cubic hex with foam. Such a small volume device would be rather specialized and consequently rare. Such a device might have medical applications and would therefore may also have aseptic properties.
A larger spray can with about ½ a pound of foam would be more common, and might be carried by individuals such as law enforcement or prison guards. Range 3 yards, three cubic yards of foam that can be released in up to three bursts.
A one-pound container would be a typical sticky foam grenade. This would resemble a typical 20th century “bean can” smoke/chemical grenade such as the AN-M8, rather than the tiny grenades described in the THS rulebook.
A grenade contains five cubic yards of sticky foam and weighs 1.5 lb. Some versions can be used either as a grenade or as a hand-held sprayer. (2 yards range as spray, producing one cubic hex of foam per burst). Sticky foam grenades and warheads are treated as biochemical liquids as according to [Ultra-tech 4e p.153]
Larger sticky foam dispensers (“Gum-guns”) resemble 8 lb or 20 lb fire extinguishers and should be treated as such for range and effect area. [HT p.29]
Bigger systems should be modelled on flame-throwers or similar devices such as the Slidefoam systems in Cities on the Edge p.44.
An interesting (real world) application of sticky foam is a proposal to incorporate it into high-security doors.
Attempting to force open or damage the door triggers the release of sticky foam, possibly immobilizing the would-be burglar or vandal.
The released foam may seal the door shut, which may or may not be desirable. Gunfire directed at the door may also trigger the release system. The occupant of a room may do this to deliberately release foam against marauders.
Pursuit denial is another possible application of sticky foam. Drop a grenade or satchel charge in a corridor to slow down the pursuing security guards.
As noted above, sticky foam had trouble adhering to fast-moving objects such as a runner’s legs. There may have been some improvements by 2100.
The GM may wish to reserve a bonus for escaping foam to targets that are moving at least 5 yards/sec.
Attempting to gum-up a fast spinning fan with sticky foam may end up sending foam everywhere but where you wanted it. 

Sticky Foam and Vehicles

Large volumes of foam can be used to immobilize vehicles. Sticky foam “mines” might be installed at a vehicle checkpoint and triggered on command.
How effective foam is against a vehicle will depend on a number of factors, including vehicle speed, type and the volume of foam.
A slow-moving or stationary vehicle will be easy to gum.
If the wheels of a vehicle are moving fast then sufficient foam may have trouble adhering. Perversely, what little does adhere may increase traction and road-handling!
Foam that does not stop the vehicle may still stick to it, making it obvious foam has been fired at it. Sticky foam may obscure the windscreen/cameras/vision blocks. Gun mounts or turrets may be unable to move if gummed. Hatches or doors may be sealed.
How effective sticky foam will be against a vehicle engine will depend on how easily the foam can enter the engine compartment.
For example, against a typical 20th century civilian vehicle with a water-cooled internal combustion engine, the foam will have trouble penetrating through the radiator grill and past the radiator. However, the underside of the engine compartment is usually open. Fast-moving parts of the engine will be difficult to gum, and the cooling fan will probably spray the foam all over the interior, ensuring everything is well coated with any foam that does enter.
Many vehicles in 2100 use other means of propulsion, however.
Often an electric “tractor” motor serves an individual wheel, and may even be built into the wheel hub. Such motors are little affected by sticky foam, even if they can be reached. Power for the motors comes from an engine or battery bank. Foam is an insulator so other than encasing them it has little effect on electronics. It will not cause shorts.
The GM will have to determine how accessible the engine of a hybrid-electric system is.
Modern military vehicles have countermeasures to reduce the chance of cyberswarms entering their internal spaces. The vehicles of wealthy, security-conscious or paranoid civilians may also have these features. Such systems may also counter the penetration of sticky foam.

Marker Foam

Marker foam is another useful tool of law enforcement. Marker foam is harmless, but contains a persistent dye. This dye stains clothing and skin and is very difficult to remove. Some varieties contain a malodorant, making a marked individual even more obvious. Some marker dyes contain a scent that is distinctive, but the victim might not notice. Dogs or electronic sniffers may be used to find an individual that has been scent-marked.
Marker foam can be used in traps. An attempt to break into a filing cabinet may result in the snooper covered in marker foam. Marker foam also sees applications in wildlife control.
The dyes and scents used in marker foam may also be incorporated into some sticky foams. Even when the chunks of foam are removed the skin and clothing beneath might be stained.