Micropillar surface yields lower-temperature boiling, better heat shedding

System designers spend a lot their time, mental energy, and effort on heat, sources, intensity, and especially how to get it away from sensitive components (a mentor once told me that “away” is that wonderful place where the heat becomes someone else’s problem). Understanding the mechanisms by which excess heat can be channeled and conveyed are important parts of the design plan. Among the many options are heat sinks, pipes, and bridges to draw the heat away locally, as well as active and passive cooling with convection, conduction, fans, and air or liquid fluids.

Now, a multi-university team lead by researchers at Virginia Polytechnic Institute and State University (better known as Viiginia Tech or VPI) has leveraged a subtle thermal phenomenon called the Leidenfrost effect to lower the temperature at which water droplets can hover on a bed of their own vapor—around 230°C—and thus accelerate heat transfer. You may have observed this thermal-physics effect without realizing what it is when you sprinkle small drops of water on the surface of a hot pan.

Wait…everyone knows water boils at 100°C under standard conditions, so what’s going on? The Leidenfrost effect occurs because there are two different states of water coexisting. If you could see the water at the droplet level, you would observe that the entire droplet doesn’t boil at the surface, only part of it does. The heat vaporizes the bottom, but the energy doesn’t travel through the entire droplet. The liquid portion above the vapor is receiving less energy because much of it is used to boil the bottom.

That critical hot temperature is well above the 100°C boiling point of water because the heat must be high enough to instantly form a vapor layer. If it is too low, and the droplets don’t hover; if too high, the heat will vaporize the entire droplet.

That liquid portion remains intact, and this is seen as the levitation and hovering of liquid drops on hot solid surfaces on their own layer of vapor (no, this levitation is not some sort of anti-gravity effect). It is called Leidenfrost effect due to its formal discovery in the late 18th century by German physician Johann Gottlob Leidenfrost.

The Leidenfrost effect has been studied extensively for over 200 years, but the Virginia Tech team was able to use advanced instrumentation such as high-speed video camera operating at 10,000 frames per second for their project.

The traditional measurement of the Leidenfrost effect assumes that the heated surface is flat, which causes the heat to hit the water droplets uniformly. The team has found a way to lower the starting point of the effect by using a specially created surface covered with micropillars, thus giving the surface interface new properties.

Their micropillars were 0.08 millimeters tall, arranged in a regular pattern 0.12 millimeters apart, and fabricated on a silicon wafer by means of photolithography and deep reactive ion etching. A single droplet of water encompasses 100 or more of them, as these tiny pillars press into a water droplet, releasing heat into the interior of the droplet and making it boil more quickly, Figure 1.

Figure 1 Leidenfrost-like droplet jumping dynamics on a hot micropillared surface. a) Selected snapshots of Leidenfrost-like droplet jumping on the micropillared substrate ([D, L, H] = [20, 120, 80] μm) with surface temperature