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We show here that its presence induces a significant delay in freezing, when depositing water on cold solids

Delayed Freezing On Water Repellent Materials

LANGMUIR, no. 13 (2009): 7214-7216

Cited by: 299|Views8
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Abstract

Water drops oil hydrophobic microtextured materials sit on a mixture of solid and air. In standard superhydrophobic situations, the drop contacts more air than solid, so that we can think of exploiting the insulating properties of this sublayer. We show here that its presence induces a significant delay in freezing, when depositing water ...More

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Introduction
  • A liquid deposited on a solid decorated with a hydrophobic microtexture has two main configurations.1,2 Either it conforms to the contours of the solid surface (Wenzel state) or it rests on the crests of the roughness, leaving the cavities below filled with air.
  • Water drops on hydrophobic microtextured materials sit on a mixture of solid and air.
  • The authors show here that its presence induces a significant delay in freezing, when depositing water on cold solids.
Highlights
  • A liquid deposited on a solid decorated with a hydrophobic microtexture has two main configurations.1,2 Either it conforms to the contours of the solid surface (Wenzel state) or it rests on the crests of the roughness, leaving the cavities below filled with air
  • Water drops on hydrophobic microtextured materials sit on a mixture of solid and air
  • We show here that its presence induces a significant delay in freezing, when depositing water on cold solids
  • Comparing the surface energies of the two states indicates which one should be preferred, even if metastable Cassie states are often observed on dilute textures, despite a higher surface energy
Results
  • The substantial presence of air below the drop may provide a thermal barrier between the solid and the liquid.
  • Leidenfrost where water drops are observed to stay very long in hot spoons, due to the presence of a vapor film between the solid and the liquid.12 This film prevents contact between both phases and the nucleation of bubbles.
  • As shown from contact angle measurements and from direct observations, the liquid/air surface area below a fakir drop is much larger than the solid/liquid surface area.8,14 it appears likely that these air sublayers can provide substantial thermal insulation, and the authors demonstrate here that freezing is significantly delayed when depositing water on cold superhydrophobic materials.
  • The experiments consist of depositing a volume Ω of distilled deionized water of initial temperature T0 = 25 °C on copper of temperature T = -8 °C, and measuring the time τ when this drop starts to solidify.
  • The authors solved this problem by using regular tap water, which contains enough impurities to trigger freezing at 0 °C, as checked by direct measurements of the temperature inside the drop.
  • In a first series of experiments, the authors measured the freezing time τ as a function of the drop volume Ω on both surfaces.
  • It is observed in Figure 2a that the presence of microtextures dramatically affects the freezing time of the drops, for all the considered volumes.
  • The authors expect the freezing time τ to vary as Ω/S and to be sensitive to the surface area S for a given volume of water.
  • The two series of data are observed to be distinct, which suggests that the difference in surface area alone cannot explain the delay in freezing observed with the fakir substrate.
Conclusion
  • The representation in Figure 2b is a useful step to decouple the geometrical effects from other possible causes of freezing delay, such as the presence of air below the fakir drop.
  • The authors measured the freezing time τ for different volumes Ω, keeping the surface area S = πR2 constant.
  • In the experiment of Figure 4, water drops of volume Ω = 100 μL are deposited on cold plates tilted by an angle R = 40°.
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