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Ultra-highspeed photographs of
microbubbles forming on a microheater show the effect of residual
nanobubbles between heating pulses. The first pulse of a two-pulse
sequence (a) produces nearly identical microbubbles time after
time, but the second pulse (b) produces a random assortment of
bubbles of varying sizes. Vertical bar shows a distance of 15
micrometers.
Image by NIST |
You might think that the science of boiling
had been worked out some time ago, but it still has some mysteries,
particularly at the nanometer scale. As water and other fluids change
from their liquid state to a vapor, bubbles of the vapor form. The
bubbles usually form at “nucleation sites,” which can be small surface
irregularities on the container or tiny suspended particles in the
fluid. The exact onset of boiling depends on the presence and nature
of these sites.
To observe the process, the NIST/Cornell team used
a unique ultrafast laser strobe microscopy technique with an effective
shutter speed of eight nanoseconds to photograph bubbles growing on a
microheater surface about 15 micrometers wide. At this scale, a
voltage pulse of only five microseconds superheats the water to nearly
300 °C, creating a microbubble tens of microns in diameter. When the
pulse ends, the microbubble collapses as the water cools. What the
team found was that if a second voltage pulse follows closely enough,
the second microbubble forms earlier during the pulse and at a lower
temperature apparently, as conjectured by the team, because
nanobubbles formed by the collapse of the first bubble become new
nucleation sites for the growth of later bubbles. The nanobubbles
themselves are too small to observe, but by changing the timing
between voltage pulses and observing how long it takes the second
microbubble to form, the researchers were able to estimate the
lifetime of the nanobubbles - roughly 100 microseconds.
These experiments are believed to be the first
evidence that nanoscale bubbles can form on hydrophilic surfaces (previous
evidence of nanobubbles was found only for hydrophobic surfaces like
oilcloth) and the method for measuring nanobubble lifetimes may
improve models for optimal heat transfer design in nanostructures. The
work has immediate implications for inkjet printing, in which a metal
film is heated with a voltage pulse to create a bubble that is used to
eject a droplet of ink through a nozzle. If inkjet printing is pushed
to higher speeds (repetition rates above about 10 kilohertz), the work
suggests, nanobubbles on the heater surface between pulses will make
it difficult or impossible to control bubble formation properly.
The findings also may impact proposed thermal
cancer therapies in which nanoscale objects are designed to accumulate
in tumors and are subsequently heated remotely by infrared radiation
or alternating magnetic fields. Each particle acts as a nanoscale
heater, with nanobubbles being created if the applied radiation is
sufficient. The bubbles may have a therapeutic effect through
additional heat delivered and mechanical stresses they may impart to
the surrounding tissue. |
