Matériaux multifonctions : antipluie, antibuée, antireflets

Abstract : Water on a lotus leaf is known to be surprisingly mobile. This surprising property arises from the hydrophobic micrometric roughness of the leaf. Like a fakir that sits only on the nails’ tip, water drop on such surface contacts only the tops of the surface features. Water is then, as a hovercraft, on an air cushion that makes it extremely mobile. This property water repelling property is called superhydrophobicity. However, in humid atmospheres or when in contact with hot water, water condensate in the roughness, which may destroy the repellence. Other natural surfaces are superhydrophobic: cicada wings are covered with hydrophobic conical features of typical size 100 nm. On those wings, water condensing seems to stay really mobile: merging drops can be ejected from the surface.In this thesis, we study with model superhydrophobic surfaces the shape and size effect of roughness on the antifogging properties. In particular, we focus on the decrease of size to the nanometric scale. This work has two main parts.In the first part, we studied the resistance of nanostructured materials to breath figures. We demonstrate, with model surfaces, the key role played by the shape of the features on antifogging property. Conical pillars are close to a full efficiency for jumping droplets: 95% of the coalescing drops jump of the substrate, with cylindrical pillars this rate falls below 0.5%. Naturally, we then studied this jumping mechanism. We found out that a momentum transfer from horizontal to vertical governs the jumping velocity of merging drops. We then observed that viscosity dissipation limits the jumping velocity of droplets with a radius lower than 5 µm.In the second part of this work, we probed hot water repellency. To do so, we studied adhesion of hot water drops on model nanotextures of size ranging from 50 nm to 1 µm. Our study shows that the denser the textures are, the more the surface resists to hot water. This property comes from the subdivision of condensation: close pillars limit the propagation of liquid in the air layer under the drop responsible for water mobility. On the contrary, if pillars are more spaced than condensation nucleii, water will invade all the roughness and the solid will behave as a hydrophilic surface and sticks the drop. This study does not take into account the dynamic effect of condensation. To investigate this, we probed antifogging ability in hot water drops bouncing experiments. Surprisingly, in this case taller features (typically a few micrometers) are more efficient than their nanometric counterparts. The time needed for condensation to fill the gap between the surface and pillars top can be greater than the bouncing time of water drops. In that case condensation has no effect on adhesion.In this thesis, we probed the different kind of antifogging abilities that appear when varying the textures’ scale.
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Timothée Mouterde. Matériaux multifonctions : antipluie, antibuée, antireflets. Matière Molle [cond-mat.soft]. Université Paris-Saclay, 2017. Français. ⟨NNT : 2017SACLX015⟩. ⟨tel-01852325⟩

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