Study provides insights into thermal mechanisms of droplet type cooling for microchips

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Study provides insights into thermal mechanisms of
droplet type cooling for microchips

Bengaluru,
November 4 (India Science Wire): Micro cooling is an emerging
line of research in heat dissipation in miniaturised electronic chips.
In this method, a tiny bead of the coolant is dropped on to the microchip that
is heating up, which is carried away by evaporation of the droplet. Although
the process appears obvious, the heat transfer mechanism is still being
deciphered.

A
team of Indian researchers from the Indian Institutes of Technology (IIT) at Madras
and Roorkee, along with scientist from the Indian Institute of Scientific Education
and Research, (IISER), Trivandrum, have now provided preliminary insights that
water droplets transport heat absorbed from microchip surfaces by generating
nano-range hydrothermal waves.

This
research adds a crucial step to study nanoscale hydrothermal waves by using a
technique known as optical trapping. The study opens avenues for engineering
these nano waves by other materials which will enhance heat dissipation in integrated
electronic chips.

Hydrothermal
waves
are thermally induced travelling waves occurring on a gas-fluid
interface and are particularly relevant to the cases of heat and mass transfer
in the microscopic domain, like in cooling of miniature chips. They directly
influence the evaporation rate of the liquid coolant,” explained Dr Basudev Roy,
team lead and assistant professor at IIT–M, while speaking to India Science Wire.

With
high-performance electronic devices rapidly shrinking in size, the power
density in them shoots up, and hence, the heat dissipation in them assumes
importance. Liquid coolants are many times more efficient than traditional air
cooling methods, and simple water droplet cooling is increasingly being
researched.

The
generated heat from the device surface is transferred to the droplet by
convection and transforms it into a sessile droplet – a suspended particle –
before evaporation.

This
sessile droplet has two interfaces – first, at the device surface where it
absorbs the dissipated heat, and second, a cooler top surface at the liquid-gas
interface before evaporation.

During
the process of evaporative cooling, hot molecules with higher kinetic energy
than average leave the water droplet and vaporise. Whatever is left behind is
on an average cooler because the more heated molecules are removed.

Existing
studies have found that there is a temperature gradient in the sessile droplet
between these two interfaces which is caused by the evaporation at the top
interface. As the evaporation progresses, there is generation of hydrothermal
waves at sub-micron levels.

Infra-red
cameras have helped to detect hydrothermal waveforms in the range of
millimetres in volatile coolants such as ethanol which has a temperature gradient
of five degrees. However, in sessile water droplets, the temperature difference
is only one degree, and no such waves could be seen, indicating a possibility of
non-occurrence of thermal waves in them.

Contrary
to this belief, the present research has shown that the low-temperature
gradient of water droplets also generate hydrothermal waves, but, they are in
the nanoscale range. Due to the extremely low amplitude of these waveforms,
they escape detection by the micron level infra-red camera.

To
demonstrate their findings, the team employed polymer microparticles as tracers
to observe the waveforms.

In
their experiment, they introduced two tracer particles into the top interface
of the sessile water droplet with the help of ‘optical tweezers.’  Optical tweezers is a technique devised by
Nobel laureate Dr Arthur Ashkin wherein a focused laser beam is used to hold or
trap a tiny particle in three-dimensional space without any influence of
gravity.

They
then observed the vertical oscillations of the particles between the two
interfaces of the droplet and found that, the generated waves had amplitudes
less than 500 nanometres and propagated parallel to the edge of the droplet.

 “This effect only appears late in the process
of evaporation from a water droplet when the thickness of the water layer is
less than 50 micrometres, and the convection currents become very strong. This
has strong implications for heat dissipation as the formation of nano waves
happen when the evaporation rate is the highest,” clarified Dr Roy.

With these results in hand, the team now looks
to take the research forward by experimenting with other materials to
enhance the nano waves, to accelerate cooling.

Apart
from Basudev Roy, the IIT-M team included, Dhanush Bhatt, Rahul Vaippully, Bhavesh
Kharbanda, Anand Dev Ranjan, and Dillip Satapathy; while, Sulochana R was from
IISER, Trivandrum and Viraj Dharod was from IIT-Roorkee. The results have been
published in the journal Optics Express .

By Susheela Srinivas  

 (India Science Wire)

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