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Wetting of Surfaces

140314-ultrahydrophobic-wetting01.jpgThe behavior of water droplets on a surface is basically controlled by two parameters: topography and chemistry. The chemical nature of a material decides whether its surface is hydrophilic or hydrophobic, i.e. whether water likes or dislikes the surface. This general surface property is the ampliphied if roughness at the nano and microscale comes in, Hydrophilic surfaces become superhydrophilic and are quickly and totally wetted by the water and hydrophobic surfaces are then superhydrophobic and retain their drop shape when placed on a surface as shown in the picture to the right.

Two wetting states are possible for superhydrophobic surfaces: In the Wenzel state the surface features penetrate into the drop and the surface is still fully wetted, in the Cassie Baxter state the water drop rests only on the tips of the surface features much a like a fakir sits on his nail bed.

Our research on these phenomena tackles basic academic questions in this context but also extends into application oriented work. Some examples:

Controlled wetting of micro and nanostructures

 silicon nanograss - sem picture

We use MEMS technologies to generate model surfaces with predefined roughness features from the micro to the nanorange and manipulate the surface chemistry of these samples. This allows a careful study of the physics of wetting as function of crucial parameters.

 

 

Mechanically stable nanostructured surfaces

 140411-microcones.jpg

The nanostructuring of surfaces is powerful way to manipulate surface wetting properties. But nanostructures are intrinsically fragile. We develop strategies to improve the resistance of such surface to shear by combining nanostructures with more robust structures in the micrometer range.

Anti-icing surfaces

 icing

The wetting behavior of a material or coating also determines the condensation and freezing behavior of water on that surfaces. We are looking for the relevant parameters and study the physics of ice formation on model surfaces.

 

Responsive surfaces

 switchable surface

The combination of light responsive polymers and nanostructured surfaces allow us to generate surface architectures that switch between wetting states upon illumination.

Wetting meets MEMS

   

 humidity sensor

We use carefully designed surface architectures in MEMS fabricated devices to quickly measure humidity even under very dry conditions.

   

 

Selected publications:

  1. Kondrashov V., Rühe J.
    Microcones and Nanograss: towards mechanically robust superhydrophobic surfaces
    Langmuir, 2014,30 (Just accepted manuscript).
    DOI: 10.1021/la500395e

  2. Groten J., Rühe J.
    Surfaces with Combined Microscale and Nanoscale Structures: A Route to Mechanically Stable Superhydrophobic Surfaces?
    Langmuir, 2013, 29, 3765–3772
    DOI: 10.1021/la304641q

  3. Dorrer C., Rühe J.
    Superaerophobicity: repellence of air bubbles from submerged, surface-engineered silicon substrates
    Langmuir, 2012; 28: 14968-14973.
    DOI: 10.1021/la303231z
  4. Groten, J., Bunte, C., Rühe, J.
    Light induced switching of surfaces at wetting transitions through photoisomerization of polymer monolayers
    Langmuir, 2012; 28: 15038-15046.
    DOI: 10.1021/la302764k
  5. Dorrer, C. and Rühe, J.
    Micro to nano: Surface size scale and superhydrophobicity
    Beilstein J. Nanotechnol., Vol 2, 2011, 327-332-1
  6. Dorrer C. and Rühe J.
    Some thoughts on superhydrophobic wetting
    Soft Matter, Vol 5, 51-61, 2009, 51-61
  7. Dorrer, C., Rühe, J.
    Drops on Microstructured Surfaces Coated with Hydrophilic Polymers: Wenzel´s Model and Beyond
    Langmuir, Vol 24, 2008, 1959-1964
  8. Dorrer, C., Rühe, J.
    Wetting of Silicon Nanograss: From Superhydrophilic to Superhydrophobic Surfaces
    Advanced Materials, Vol 20, 2008, 159-163
  9. Dorrer, C., Rühe, J.
    Condensation and wetting transitions on microstructured ultrahydrophobic surfaces
    Langmuir, Vol. 23 (7), 2007, 3820-3824
  10. Dorrer, C., Rühe, J.
    Contact line shape on ultrahydrophobic post surfaces
    Langmuir, Vol. 23 (6), 2007, 3179-3183
  11. Dorrer, C., Rühe, J.
    Advancing and Receding Motion of Droplets on Ultrahydrophobic Post Surfaces
    Langmuir, Vol. 22, 2006, 7652-7657

 

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