What does negative pressure mean in ultrasound waves

[DaveE]

This article (sorry, there's a paywall) describes ultrasound waves creating 20 Megapascals of negative pressure. What does that mean?

 

[Baluncore]

I can see how, following cavity collapse, there can be a negative pressure transient of 20MPa, but that would be measured relative to the positive impulse of 20MPa, delivered by the final closure of a cavity.

 

[DaveE]

So I guess pressure is just a measure of net force over a defined surface. The acceleration of particles in some local area?

I'm confused about the sign. Is the bubble created with positive pressure on the inside, or negative pressure from the outside? I guess that pressure is a scalar and the direction would be defined by the geometry of the surface used. So we could chose any polarity we like. Is there an obvious, or standard, polarity definition?

PS: Wikipedia says negative pressure is tension, positive is compression. That seems to make sense.

 

[A.T.]

Wikipedia says negative pressure is tension

Have not read the article, but fluids can have absolute negative pressure (tension) due to the cohesive forces between molecules (also responsible for surface tension).

 

[Steve4Physics]

A few thoughts...

20MPa is not exceptional. E.g. 20MPa is the tension in a 1mm diameter vertical string with 1.6kg hanging from it. Think of that as a ‘pressure’ of -20MPa.

The article refers to ‘extremely powerful ultrasound’ which sounds dramatic but may be misleading. The author probably means ‘high intensity ultrasound’ - intensity of not the same as power.

When ultrasound passes through a liquid, pressure oscillates between positive and negative values. If the pressure is sufficiently negative for long enough, then cavitation (bubble formation) occurs – followed by collapse of the bubble as the pressure increases. In tissue, this can cause localised cell-damage.

I think the article should be saying this:

- we can use a pattern of widely-spaced, high intensity, short bursts of ultrasound;

- the lowest pressure (-20MPa) and duration ($\sim \mu s$) of each burst is sufficient to cause controlled cavitation resulting in local cell-damage;

- with a suitable choice of burst-spacing there is relatively little unwanted heating of non-target cells.

So target cells are destroyed without collateral damage caused by excess heat.

This means cavitation is no longer a nuisance. It can be used as the main mechanism to destroy targetted (unhealthy) cells.

Minor edits.

 

[Baluncore]

One big advantage of using ultrasound is that, the same array of transducers that images the target, can be used to strike the target, since the two-way transit times will be accurately known.
The imaging system can also correct for cyclic patient movement due to pulse and respiration.

 

[A.T.]

the lowest pressure (-20MPa) and duration ($\sim \mu s$) of each burst is sufficient to cause controlled cavitation resulting in local cell-damage;

In the below linked study (also paywalled) they achieved -1 MPa in water within a synthetic hydrogel, presumably sustained without cavitation:
https://www.nature.com/articles/nature07226

So -20 MPa as a transient state, just before cavitation occurs seems plausible.