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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.
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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 𝑇W = 130°C. The inset in (a) is the scanning electron micrography (SEM) of the micropillared substrate. a) Height variation of the center of mass of the droplet shown in (a). The time 𝑡 = 0 msec denotes the onset of the interfacial deformation. Source: Virginia Polytechnic Institute and State University
Compared to the traditional assessment that the Leidenfrost effect triggers at 230°C, their array of micropillars press more heat into the water than a flat surface. This causes microdroplets to levitate and jump off the surface within milliseconds at lower temperatures because the speed of boiling can be controlled by changing the height of the pillars. With the pillars, the temperature at which the floating effect started was down to 130°C significantly lower than that of a flat surface.
The Leidenfrost effect is more than an intriguing phenomenon to watch; it is also a critical point in heat transfer performance, Figure 2.
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Figure 2 Droplet jumping velocity and equivalent thermal boundary layer (TBL) thickness. a) Jumping velocity of droplets with different volumes during vibrational jumping (on substrate [D, L, H] = [20, 120, 20] μm ) and Leidenfrost-like jumping (on substrate [D, L, H] = [20, 120, 80 μm ). b) Simulated results of temperature distribution of quiescent TBL on the substrates with micropillar height H = 20 μm and H = 80 μm , respectively. c) Thickness of equivalent TBL on substrates with different micropillar heights (from 20 μm to 80 μm ) an different substrate temperatures (from 120 °C to 140 °C). d) Phase map of occurrence of droplet jumping behaviors on substrates with different micropillar heights placed on hot plate at different temperatures. Source: Virginia Polytechnic Institute and State University
Another benefit of micropillars is that the generation of vapor bubbles in their presence is able to dislodge microscopic foreign particles from surface roughness and suspend them in the droplet. This means that the boiling bubbles can physically move thermal-blocking impurities away from the surface while removing heat.
There’s a very rough heat-transfer analogy here with “solid state” cooling via standard heat sinks. With a heat sink, it is critical to minimize thermal impedance between the heat source and heat sink. Since even apparently flat surfaces have tiny surface imperfections, any mating between source and sink surfaces will have micro-voids and nearly invisible air pockets which act as micro-insulators and impede heat flow.
The standard solution is to interpose an extremely thin layer of thermal grease or a thermally conductive pad to fill those gaps and provide a thermally continuous, gap-free source to sink path, Figure 3. These micropillars have a similar role, using their intrusion into the cooling liquid to which they are transferring heat.
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Figure 3 The use of an interposed thermal grease layer or pad is essential to ensuring minimal thermal impedance between heat source and sink. Source: Taica Corporation/Japan
The team is not using overused words such as “revolutionary” or “breakthrough”; what they have done is look at this effect with a new perspective to see how and if it can be leveraged. If you want to read the full story including relevant intense thermal-physics equations and analysis, check their paper “Low-temperature Leidenfrost-like jumping of sessile droplets on microstructured surfaces” published in Nature Physics. (I had to look that word up, too: “sessile” is an adjective regularly in some technical disciplines, meaning “attached directly by the base, not raised upon a stalk”.) While that formal paper is behind a paywall, a pre-print version is here; both version also have links to some short but captivating videos of drops and their motions.
Their deeper insight of the potential modern-day thermal implications of the Leidenfrost effect may not result in any actual advances in cooling techniques and technologies; these sorts of project usually do not (but sometimes they certainly do have a huge impact). Either way, it’s interesting to see what modern solid-state material-fabrication tenancies, coupled with advanced instrumentation, can show us about fairly old physics.
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.
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