Dropwise Condensation on Multiscale Bioinspired Metallic Surfaces with Nanofeatures

In this work a model for the mechanism of dropwise condensation on ion implanted metallic surfaces is proposed. Considering experimental results for respective surfaces, the model is based on droplet nucleation and growth on preferably wetted, elevated precipitates, resulting in short-term steam entrapment after droplet coalescence. According to wetting theory this transition state yields increased macroscopic contact angles enabling dropwise condensation. Open condensation phenomena like enlarging dropwise condensation areas in spite of increasing condensation rate become comprehensible by our approach. Furthermore, the model points out that contact angles and surface free energies measured under ambient air conditions are not usable for predicting the condensation form of steam on the modified surfaces. Although the suggested microscopic mechanism cannot be directly proved by experiment, its capability of explaining experimental observations colliding with previous theoretical approaches supports its validity. The results also reveal that dropwise condensation of steam can originate from microscopically different mechanisms.

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On the Characteristics of Ion Implanted Metallic Surfaces Inducing Dropwise Condensation of Steam

Langmuir ◽

10.1021/la904293f ◽

2010 ◽

Vol 26 (8) ◽

pp. 5971-5975 ◽

Cited By ~ 21

Author(s):

Michael H. Rausch ◽

Alfred Leipertz ◽

Andreas P. Fröba

Keyword(s):

Dropwise Condensation ◽

Metallic Surfaces ◽

Ion Implanted

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On the Mechanism of Dropwise Condensation of Steam on Ion Implanted Metallic Surfaces

Journal of Heat Transfer ◽

10.1115/1.4001646 ◽

2010 ◽

Vol 132 (9) ◽

Cited By ~ 7

Author(s):

Michael H. Rausch ◽

Alfred Leipertz ◽

Andreas P. Fröba

Keyword(s):

Experimental Studies ◽

Ambient Air ◽

Contact Angles ◽

Dropwise Condensation ◽

Free Energies ◽

Metallic Surfaces ◽

Microscopic Mechanism ◽

Theoretical Approaches ◽

Type Contact ◽

Ion Implanted

Our recent experimental studies indicate that nanostructured, chemically inhomogeneous surfaces are the origin of dropwise condensation of steam on ion implanted metals. Yet, the underlying microscopic mechanism governing this special condensation form is still not clear. We suggest a condensation model based on droplet nucleation and growth on elevated precipitates, resulting in short-term steam entrapment after droplet coalescence. According to the wetting theory, this transition state yields increased macroscopic contact angles. Condensation phenomena such as enlarging dropwise condensation areas in spite of increasing condensation rate become comprehensible by our model. Furthermore, it points out that for this special surface type, contact angles and surface free energies measured under ambient air conditions are not usable for predicting the condensation form of steam. Although the suggested microscopic model cannot be directly proved by experiment, its validity is supported by its capability of explaining experimental observations colliding with previous theoretical approaches.

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Experimental Study on the Origin of Dropwise Condensation of Steam on Ion Implanted Metallic Surfaces

2010 14th International Heat Transfer Conference, Volume 2 ◽

10.1115/ihtc14-22147 ◽

2010 ◽

Author(s):

Michael H. Rausch ◽

Alfred Leipertz ◽

Andreas P. Fro¨ba

Keyword(s):

Experimental Study ◽

Free Energy ◽

Ion Implantation ◽

Surface Free Energy ◽

Nano Particles ◽

Saturated Steam ◽

Chemical Inhomogeneity ◽

Dropwise Condensation ◽

Metallic Surfaces ◽

Ion Implanted

The presented work provides new perspectives on the origin of dropwise condensation of steam on ion implanted metallic surfaces. For the experimental study metallic samples made of different materials implanted with various ion species were used. Condensation experiments with saturated steam and surface analysis via contact angle and surface free energy measurements, atomic force microscopy and scanning electron microscopy were performed. It could be shown that available explanation approaches for the appearance of dropwise condensation based on a reduction of the metal surface free energy by ion implantation have to be discarded. Our results rather suggest a nucleation mechanism proceeding on surfaces featuring nano-scale surface roughness and chemical inhomogeneity. These characteristics seem to be induced by nano-particles precipitated due to supersaturation of the metal substrate with the doping elements. On unimplanted high-alloyed materials like stainless steel, dropwise condensation was found to develop spontaneously because of strong oxidation effects. Obviously the latter naturally produce surface modifications which are similar to those obtained artificially by ion implantation.

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