Nature Adores a Vacuum

The idea of horror vacui (Latin for ‘horror of the vacuum’) is widely attributed to Aristotle from the 4^th century BCE (though the internet tells me that Aristotle would not have been speaking Latin). Horror vacui is usually expressed in English as ‘nature abhors a vacuum’, the idea being that a vacuum cannot exist as nearby matter would immediately rush in to fill it. This is not an unreasonable idea, but some of us routinely rely on vacuum for manufacturing or research, going to considerable lengths to remove most of the air from a system to facilitate processes such as thin‑film coating. Human‑made vacuum systems are not what most people would consider as nature, so is it a truism that nature abhors a vacuum?

Around this time of year, Torr Scientific’s resident scientist and explorer is on the lookout for a particular aquatic flower in the watery ditches of St. Leonards‑on‑Sea on the south coast of England, UK. The yellow flower emerges above the water surface on a single stem and is said to resemble a snapdragon. It is the flower of common bladderwort (Utricularia vulgaris) and is the only easily visible part of this free‑floating plant. Below the water surface is the main body of the plant, consisting of several branched stems called stolons, each hosting some spindly leaves and a number of small bladders that are said to be among the most sophisticated structures in the plant world. These bladders, up to about 1 cm in size and originally thought to be flotation aids, are in fact lethal traps that ensnare small creatures to supplement the nutrition of the bladderwort in its nutrient‑poor ditch water. Potential victims include nematode worms, Daphnia (water fleas), mosquito larvae and even vertebrates such as tiny newts. The mechanism used to trap these unfortunate victims relies on a trapped volume held at low pressure within the submerged bladders.

This trapped volume at low pressure is a vacuum in nature, right? That depends on how we define a vacuum. A strict but unrealistic definition requires a region devoid of matter; a more common definition describes a region of space at a lower pressure than the ambient pressure of the atmosphere. The low­‑pressure bladder of the bladderwort is full of water and biological matter, so by these definitions this is not a vacuum, but if you relax the definition slightly to include a trapped volume of fluid held at lower pressure than its surroundings then we will have something to work with.

Structures in the wall of the bladders actively pump water out of the interior. Active pumping consumes energy. As the interior water pressure decreases, the thin bladder walls flex elastically inwards, storing energy as a spring does. An opening in the bladder is sealed by a convex, hinged trap door. The seal of the door to the rim of the opening is maintained with the help of excreted mucilage. Many of you will be wondering what mucilage is. If that includes you, just substitute ‘snot’ for ‘mucilage’ and you’ll be along the right lines. Time for a quick disclaimer here: Torr Scientific does not recommend the use of snot as vacuum grease, but if you have tried it, we’d be interested to learn the outcome.

The pumping out of water increases the concentration of the remaining fluid in the bladder, leading to a tendency for the surrounding water to diffuse inwards by osmosis through the wall of the bladder. This osmotic pressure limits the ultimate minimum pressure achievable, which has been measured at around 17 kPa (170 mbar) below ambient. To those of us used to ultra‑high vacuum this is a rather modest differential. A similar pressure drop could be achieved by ascending from sea level to an altitude of around 1600 m. Modest or not, the effect of this pressure drop on the bladderwort victim is not trivial, as we will learn.

The outer surface of the trap door has several bristles that initiate the firing of the trap when touched by passing prey. This is generally considered a purely passive process, with the bristle acting as a lever to deform the trap door and break the seal. This contrasts with Venus fly traps where the plant actively detects and responds to prey. With the seal broken, the trap door swings inwards and the bladder wall expands as water rushes in to balance the pressure, dragging the victim with it into the bladder. The door very quickly snaps back into place, trapping the prey inside.

The capture process is very fast, far too fast to be seen by the human eye. The trap door is sealed again within about 15 milliseconds (0.015 s) of opening. We can put this into context with that classic measure of a brief moment: the blinking of an eye. A typical blink is an order of magnitude slower at 0.1 to 0.4 seconds. The standard television or cinema frame rate of 24 frames per second displays each individual frame for around 42 milliseconds (0.042 s), around three times longer than the trapping process. Pumping of water from the trap resumes immediately and the trap is fully reset in about 20 minutes.

The speed achieved by the victim during ingestion is around 4 metres per second, no more than a brisk running pace, but it reaches this speed in less than 10 milliseconds. Acceleration is often described in terms of g, the acceleration on Earth due to gravity, with 1 g being around 9.8 metres per second per second (more manageably, 9.8 m/s²). That modest pressure deficit in the bladder can accelerate the poor victim at 2800 g followed by a deceleration of 1900 g. Water is highly incompressible: the deceleration occurs because the moving prey and water meet a body of static, more viscous fluid within the bladder. Generally in life, sudden deceleration is best avoided. Until 2003, the highest deceleration survived was estimated at almost 180 g and involved decelerating to a stop from 173 kph (107 mph) over a distance of just 66 cm (26 in) when the car hit a solid barrier. This was surpassed in 2003 when another driver survived a 354 kph (220 mph) crash that registered 214 g and still holds the Guinness world record for ‘Highest g force endured – non‑voluntary’ (and yes, there is a voluntary record). The most recent fatality in Formula 1 racing followed a 254 g impact. Amazingly, the g‑force does not usually kill the bladderwort prey which tends to die in the bladder due to lack of oxygen.

We have seen that bladderwort actively pumps water from a sealed bladder creating a sealed volume at low pressure. When a small creature such as a mosquito larva touches the trigger mechanism on the door of the bladder, the door opens and the bladder walls expand as water flows into the bladder, carrying the hapless victim with it. The creature is accelerated and decelerated at g‑forces that would kill a human before the door is closed, all within less than two hundredths of a second. You can argue among yourselves on the definition of a vacuum, but it is my assertion that nature does not abhor a vacuum, though certain water fleas, nematodes and mosquito larvae probably do.

Image credits: Photos sourced from Wikipedia.