From a practical viewpoint, the two superhydrophobic states thusappear to be extremely different: although the apparent (advancing)contact angles remain comparable, the adhesion is dramaticallyincreased in the Wenzel state.In a second series of experiments, we tried to induce directtransitions between both states.We started from a drop deposited on themicrotextured surface, and increased the pressure exerted by this dropon its substrate. Two different methods can be used for this purpose.(1) We varied the drop size:the larger the drop,the smaller the pressure.Large drops are flattened by gravity gto a thickness hfirst described byTaylor13:h= 2asin(θ*/2), where ais the capillary length (a= (γ/ρg)1/2,with γthe liquid surface tension and ρits density;ais 2.7mm for water).Such a flattened drop exerts a hydrostatic pressure ρghon its substrate—ofthe order of50Pa in our case.For drops small enough that the effect ofgravity is negligible (that is, radius Rsmaller than a), the internalpressure,∆P, in the superhydrophobic limit is given by the Laplace law(∆P= 2γ/R), which is also the pressure exerted by the drop on itssubstrate, hence the smaller the drop, the larger the pressure. We let Rvary between 4 and 0.9mm,which allowed us to increase the pressure upto 150Pa.(2) To reach higher pressures,we placed the drop between twoidentical substrates, and compressed it by using a micrometric screw,which also allowed us to measure the gap xbetween the plates.The pressure was simply deduced using the Laplace equation(∆P= 2γ|cosθ*|/x, for x<< R). Figure 2 shows a sequence of theseexperiments (note that because of the texture, the surface is iridescentand reflects in the drop, giving the colours). For each pressure ∆P, wetook numerical micrographs of the edge of the drop, from which wecould deduce the contact angle θ* with a precision of 5°; its value isplotted as a function of∆Pin the same figure.It is observed that the contact angle first has a plateau value,whichcorresponds to the air-pocket regime described above. The contactangle then decreases, which can be interpreted as a progressivesinking of the drop inside the texture (as seen in equation 2,exploringthe textures, that is, increasing φs, leads to a decrease ofθ*). For highpressures, the contact angle tends towards θ* = 145±3°, in closeagreement with the value obtained by condensing a water drop.We thus interpret this limit as a Wenzel state.We then monitored what happens when relaxing the pressure.In Fig. 3 is a series of snapshots showing the separation between theplates after imposing a pressure of about 250Pa.Although the contactangle hysteresis was very small in the Cassie regime, a huge hysteresisis observed here, which reveals the irreversibility of the transition.
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