— High-speed imaging at 13,600 fps reveals how three-dimensional surface morphology alters droplet boiling regimes and the Leidenfrost point, showing that negatively skewed micropit surfaces can delay stable vapor-film formation and improve high-temperature evaporative cooling.
The Leidenfrost effect is a critical limitation in high-temperature thermal management, spray cooling and metal machining. When a liquid droplet contacts a sufficiently hot surface, rapid vaporization can generate a stable vapor layer between the liquid and the solid. This vapor film suppresses direct solid-liquid contact and sharply reduces heat-transfer efficiency.
Researchers at Hefei University of Technology investigated how three-dimensional surface morphology influences this transition. Rather than focusing only on conventional roughness parameters, the study compared surfaces with similar roughness but fundamentally different peak-valley characteristics: positively skewed micropillar arrays and negatively skewed micropit arrays.
A Revealer high-speed camera was used as the primary diagnostic tool to capture droplet contact, spreading, vapor-bubble formation, splashing, breakup and rebound. These high-speed image sequences were then correlated with wall-temperature measurements to identify boiling regimes and determine both static and dynamic Leidenfrost points.
High-Speed Camera Captures Millisecond-Scale Droplet Boiling
The experimental system used a Revealer high-speed camera operating at 13,600 frames per second at 1280 × 1024 pixels. The camera viewed the heated surface from the side and recorded the transient evolution of cutting-fluid droplets after impact.
The imaging system was combined with a temperature-controlled heating platform capable of reaching 600 C, a K-type thermocouple, a microsyringe pump, an illumination system and thermal measurement instruments.
The test specimens were YT15 cemented-carbide tools. Picosecond laser processing was used to fabricate two types of microstructured surfaces: micropillar arrays with positive skewness, Ssk>0, and micropit arrays with negative skewness, Ssk<0. The surface structures were designed to maintain comparable roughness and a solid fraction of approximately ϕ=0.5.
By adjusting the droplet release height, the researchers controlled impact velocity and Weber number. High-speed imaging continuously tracked the droplet from approach and initial contact through spreading, vapor-bubble generation, liquid-film breakup, secondary droplet formation and eventual departure from the surface.
Static Leidenfrost Point Identified From Boiling-Regime Transitions
For nearly stationary droplets, the high-speed camera revealed four sequential regimes as wall temperature increased: single-phase evaporation, nucleate boiling, transition boiling and film boiling.
During single-phase evaporation, the droplet remained in direct contact with the surface. In nucleate boiling, vapor bubbles developed at the solid-liquid interface. At higher temperatures, these vapor bubbles coalesced and produced an unstable vapor layer, marking transition boiling.
Once a stable vapor film completely separated the droplet from the heated surface, the system entered film boiling.
The researchers therefore defined the static Leidenfrost point, TL, as the critical wall temperature separating transition boiling from film boiling.
This illustrates the measurement value of high-speed imaging. Temperature sensors provide the wall temperature, but they cannot independently identify whether a droplet is in direct contact with the surface or supported by an unstable or stable vapor layer. High-speed camera images provide the required physical-state evidence.
Dynamic Droplet Impact Reveals More Complex Boiling Regimes
Droplet impact introduces additional inertia and produces substantially more complex boiling dynamics.
On a smooth heated surface, the high-speed images showed transitions among single-phase evaporation, nucleate boiling, transition boiling, bounce atomization and gentle film boiling.
On microstructured surfaces, however, additional regimes appeared after transition boiling, including central splash, broken atomization and spraying film boiling.
A key finding was that secondary droplets could occur in several different regimes, but their physical origins were not the same.
During central splash and broken atomization, the liquid remained in direct contact with the wall. Secondary droplets were associated with rapid interfacial vaporization, vapor-bubble rupture and liquid-film breakup.
During spraying film boiling, by contrast, a vapor layer separated the droplet from the solid surface. Atomization was then more closely associated with vapor-bubble evolution inside the droplet and rupture of the free liquid surface.
The study therefore showed that the presence of secondary atomization alone is not sufficient to determine whether a microstructured surface has entered the dynamic Leidenfrost state.
Figure-Revealer high-speed camera at 13,600 fps captures droplet impact, nucleate boiling, transition boiling, atomization and spraying film boiling on heated microstructured surfaces for dynamic Leidenfrost point analysis.
High-Speed Imaging Establishes a Dynamic Leidenfrost Criterion
Based on high-speed image sequences acquired over a range of wall temperatures, the researchers established separate Leidenfrost criteria for stationary droplets, impact droplets on smooth surfaces and impact droplets on microstructured surfaces.
For stationary droplets:
transition boiling → film boiling
defines the static Leidenfrost point TL.
For impact droplets on smooth surfaces:
bounce atomization → gentle film boiling
defines the dynamic Leidenfrost point TDL.
For the microstructured surfaces investigated in this study:
broken atomization → spraying film boiling
provides the appropriate criterion for determining the dynamic Leidenfrost point.
This distinction is important because microstructures fundamentally alter the droplet-wall interaction. High-speed imaging converts millisecond-scale interfacial behavior into a sequence of identifiable physical states, allowing each boiling regime to be correlated with a corresponding wall temperature.
Micropit Surfaces Delay the Leidenfrost Transition
The experiments further showed that surfaces with similar average roughness can exhibit substantially different Leidenfrost behavior when their three-dimensional peak-valley characteristics differ.
Under comparable roughness and solid-fraction conditions, the negatively skewed micropit arrays exhibited higher Leidenfrost points than the positively skewed micropillar arrays.
The underlying mechanism was attributed to differences in vapor-flow resistance created by the two surface geometries. These differences modify vapor transport beneath the droplet and therefore influence how interfacial vapor pressure develops.
A force-balance model considering vapor pressure, gravitational pressure, capillary pressure, water-hammer pressure and dynamic pressure was used to interpret the experimental transition temperatures. Cooling experiments further confirmed that the micropit surface, which had the higher Leidenfrost point, maintained more effective high-temperature cooling.
The study demonstrates that average surface roughness alone is insufficient to describe Leidenfrost behavior. Three-dimensional parameters such as surface skewness and peak-valley morphology must also be considered when designing surfaces for boiling heat transfer and evaporative cooling.
More broadly, the study demonstrates how a high-speed camera combined with synchronized temperature measurement and three-dimensional surface characterization can connect droplet-impact dynamics with critical boiling transitions and macroscopic cooling performance. Such an approach is applicable to research in droplet impact, spray cooling, boiling heat transfer, high-temperature machining and functional microstructured surfaces.
Contact Info:
Name: Harrison Shawn
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Organization: HF Agile Device Co., Ltd.
Website: http://www.revealerhighspeed.com
Release ID: 89200955
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