The receding contact angle θ r is found to be

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The receding contact angle θ * r is found to be 35–40°,and the hysteresis about 105°, again in good agreement with our data for a condensed drop. This confirms the hypothesis of a Wenzel state in this limit of high pressure. Then, the drop can pin on the surface textures, which makes it split into two identical droplets while the plates are separating. The value of the receding contact angle after such a relaxation is also plotted in Fig. 3, as a function of the pressure imposed on the drop. 0 50 100 150 200 250 300 P (Pa) θ r * ( ° ) 0 20 40 60 80 100 120 140 160 Figure 3 Separation of the two plates after having imposed a pressure P of about 250 Pa. The (receding) angle is much smaller (around 40 ° ),and the drop sticks on both plates, which eventually leads it to split in two similar pieces.The receding angle θ * r ,observed after imposing and relaxing a pressure P ,is plotted as a function of P (open points).The filled points correspond to drops deposited on a single surface,and P is then the pressure applied by the drop on the surface; its variation is obtained by taking different drop sizes.The upper and lower dotted lines respectively indicate the value of θ * r for a drop deposited on the substrate or obtained by condensing a vapour.In the Cassie regime (upper points) the contact-angle fluctuations were small,and the error bars are therefore of the size of the data points. © 2003 Nature Publishing Group
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LETTERS 460 nature materials | VOL 2 | JULY 2003 | The situation there appears to be clear-cut: up to a pressure P * = 200 Pa, although the advancing angle was found to decrease significantly,after relaxing the pressure the drop recovers its Cassie state, with a very low hysteresis (high value for θ * r ).But above P *,that is,for pressures for which the advancing angle was close to the Wenzel value, the receding angle becomes very small (around 40°), confirming the pinning ofthedropin the texture.The value of P * is low (much smaller than the Laplace pressure associated with invasion of a micrometric texture, which would be of the order of 10 5 Pa), which betrays the fragility of a Cassie state when it is metastable.A systematic study of P * as a function of the characteristics of the texture should help understanding what fixes its value. This discussion can finally be summarized by comparing the drop adhesion in both states. We measured the maximum volume of water able to stick on the material tilted by 20°, by using a calibrated micropipette to deposit drops of a controlled volume with a precision smaller than 1 µ l.This maximum volume was found to be less than 1 µ l if depositing the drop (that is, in the Cassie regime), and 205 ± 25 µ l if condensing it (that is,in a Wenzel situation):there is a factor larger than 200 between these values,which stresses how different the two states are.
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  • Fall '19
  • Surface tension, Contact angle, Surface energy, Hysteresis, Wenzel, Cassie regime

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