Micropillar floor yields lower-temperature boiling, higher warmth shedding

System designers spend lots their time, psychological vitality, and energy on warmth, sources, depth, and particularly the way to get it away from delicate elements (a mentor as soon as informed me that “away” is that fantastic place the place the warmth turns into another person’s downside). Understanding the mechanisms by which extra warmth could be channeled and conveyed are necessary elements of the design plan. Among the many many choices are warmth sinks, pipes, and bridges to attract the warmth away domestically, in addition to lively and passive cooling with convection, conduction, followers, and air or liquid fluids.

Now, a multi-university group lead by researchers at Virginia Polytechnic Institute and State College (higher generally known as Viiginia Tech or VPI) has leveraged a delicate thermal phenomenon known as the Leidenfrost impact to decrease the temperature at which water droplets can hover on a mattress of their very own vapor—round 230°C—and thus speed up warmth switch. You’ll have noticed this thermal-physics impact with out realizing what it’s once you sprinkle small drops of water on the floor of a sizzling pan.

Wait…everybody is aware of water boils at 100°C beneath normal circumstances, so what’s occurring? The Leidenfrost impact happens as a result of there are two completely different states of water coexisting. If you happen to might see the water on the droplet stage, you’d observe that the complete droplet doesn’t boil on the floor, solely a part of it does. The warmth vaporizes the underside, however the vitality doesn’t journey via the complete droplet. The liquid portion above the vapor is receiving much less vitality as a result of a lot of it’s used to boil the underside.

That vital high temperature is nicely above the 100°C boiling level of water as a result of the warmth have to be excessive sufficient to immediately type a vapor layer. Whether it is too low, and the droplets don’t hover; if too excessive, the warmth will vaporize the complete droplet.

That liquid portion stays intact, and that is seen because the levitation and hovering of liquid drops on sizzling strong surfaces on their very own layer of vapor (no, this levitation just isn’t some type of anti-gravity impact). It’s known as Leidenfrost impact because of its formal discovery within the late 18th century by German doctor Johann Gottlob Leidenfrost.

The Leidenfrost impact has been studied extensively for over 200 years, however the Virginia Tech group was in a position to make use of superior instrumentation equivalent to high-speed video digital camera working at 10,000 frames per second for his or her challenge.

The normal measurement of the Leidenfrost impact assumes that the heated floor is flat, which causes the warmth to hit the water droplets uniformly. The group has discovered a approach to decrease the place to begin of the impact through the use of a specifically created floor coated with micropillars, thus giving the floor interface new properties.

Their micropillars had been 0.08 millimeters tall, organized in a daily sample 0.12 millimeters aside, and fabricated on a silicon wafer by the use of photolithography and deep reactive ion etching. A single droplet of water encompasses 100 or extra of them, as these tiny pillars press right into a water droplet, releasing warmth into the inside of the droplet and making it boil extra shortly, Determine 1.

Determine 1 Leidenfrost-like droplet leaping dynamics on a sizzling micropillared floor. a) Chosen snapshots of Leidenfrost-like droplet leaping on the micropillared substrate ([D, L, H] = [20, 120, 80] μm) with floor temperature 𝑇_W = 130°C. The inset in (a) is the scanning electron micrography (SEM) of the micropillared substrate. a) Peak variation of the middle of mass of the droplet proven in (a). The time 𝑡 = 0 msec denotes the onset of the interfacial deformation. Supply: Virginia Polytechnic Institute and State College

In comparison with the normal evaluation that the Leidenfrost impact triggers at 230°C, their array of micropillars press extra warmth into the water than a flat floor. This causes microdroplets to levitate and soar off the floor inside milliseconds at decrease temperatures as a result of the velocity of boiling could be managed by altering the peak of the pillars. With the pillars, the temperature at which the floating impact began was right down to 130°C considerably decrease than that of a flat floor.

The Leidenfrost impact is greater than an intriguing phenomenon to look at; it’s also a vital level in warmth switch efficiency, Determine 2.

Determine 2 Droplet leaping velocity and equal thermal boundary layer (TBL) thickness. a) Leaping velocity of droplets with completely different volumes throughout vibrational leaping (on substrate [D, L, H] = [20, 120, 20] μm ) and Leidenfrost-like leaping (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.

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.

Related content

• Misconception revealed: Can a heat sink be too big?
• My long-running affection for heat sinks
• Put a diamond topping on your die to avoid heat stroke
• Goodbye, thermal grease; hello, integral heat sink?
• What’s a “thermal jumper” do, anyway?

<!–

–>

<!–

div-gpt-ad-inread

–>

The post Micropillar surface yields lower-temperature boiling, better heat shedding appeared first on EDN.