A nanoscale view of the origin of boiling and its dynamics

Boiling, despite being a well-known phenomenon still lacks an understanding of its multiscale and non-equilibrium nature. Using a stochastic mesoscale model based on fluctuating hydrodynamics and diffuse interface approach we shed new light on boiling from nucleation to macroscopic bubble dynamics.
A nanoscale view of the origin of boiling and its dynamics

Boiling has the peculiarity of being involved both in frontier technological applications, such as nano cooling and in everyday life activities, such as food cooking. For that reason, everyone is very familiar with this kind of phase transition. Associated with the empirical knowledge of the process, despite decades of years of scientific exploration, there is yet no adequate physical understanding of the phenomenon and its profound nanoscale origins. The main problem is to decipher what is happening at the nanoscale when nucleation occurs on a heated surface.  The lack of fundamental understanding of the process has a great impact on the technological development of thermal science, especially in terms of a quantitative prediction of the physical parameters necessary to predict when the boiling occurs and at which rate. The central obstacle lies in the multiscale and non-equilibrium nature of boiling itself, where nucleation and bubble dynamics must coexist in a single description far from thermodynamic equilibrium. Moreover, such description is linked to length and time scales unreachable by the present measurement tools. To date, theory, simulations, and experiments have described the different scales of boiling always compartmentalised and blurred by many uncontrolled parameters. Therefore, until now,  a holistic description encompassing the whole aforementioned range of spatial and temporal scales is still lacking, even though strongly desired. Using the new computational tool, we shed new light on the controversial issue of the low boiling onset temperature on putatively ultra-smooth surfaces. The approach covers, with a modest computational cost, the area from nano to micrometers, where most of the controversial observations related to the phenomenon originate. In-silico experiments elucidated several important aspects. 

  •  Surface wettability can drastically influence the onset temperature on ultra-smooth surfaces.
  • At the nanoscale, on an ideal surface for any value of contact angle, the nucleation of stable bubbles depends on the imposed heat flux. 

  • Despite the strong out-of-equilibrium nature of the phenomenon, the Classical Nucleation Theory, once corrected with an appropriate Tolman length, can reproduce numerical findings. 
  • Sparse hydrophobic, nanometric patches are found sufficient to trigger nucleation at lower superheat, significantly reducing the onset temperature, and explaining the origin of the observed very low onset boiling temperature. 

All these findings are in line with experimental results, considering an extrapolation to microscales.

We also believe that the suitability demonstrated by the mesoscale technique of fluctuating hydrodynamics coupled with diffuse interface models to describe multiscale phenomena such as boiling and cavitation is encouraging for the study of a plethora of thermally activated transitions in dynamic conditions. Along this line, the mesoscale approach may constitute the missing link between macroscopic hydrodynamics and atomistic simulations of phase change. Therefore, the methodology could open a breakthrough pathway towards accurate understanding and prediction of phase transition and it will constitute a pathway for future multi-scale modelling approaches.

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Physics and Astronomy
Physical Sciences > Physics and Astronomy

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