Surfaces With Hysteresis

Andrieu et al. have studied the dewetting of dirty surfaces, in particular, films of mylar (transparencies) and of Teflon, both of which exhibit a strong hysteresis. L. Bacri has studied the dewetting of porous, hydrophobic substrates with very rough surfaces.The first question we will address is what becomes of the critical dewetting thickness Cc on such surfaces. 

FIGURE 7.7. (a) Advancing and receding thicknesses of a puddle placed on a dirty surface, (b) Puddle on a poroub, hydrophobic membrane (courtesy L. Bacri). 

Suppose now that we deposit a thick film, the thickness of which we gradually reduce by suction with a pipette (or with a pump if we are dealing with a porous material). A hole created in the film will close up when e e - When c a hole will remain stable since the angle 9 associated with e via equation (7.1) is between On and 6a and the contact line can neither advance nor recede. When e en, a hole will open up spontaneously since 6 6n- It therefore follows that the critical thickness for dewetting on a dirty or rough surface is en- 

The dynamical laws of dewetting are roughly the same as in the ideal case as long as we replace 6e Y 6r The velocity with which a hole opens up is constant and given by 

FIGURE 7.8. Dewetting of a film of water deposited on a rough and porous surface (courtesy L. Bacri). 

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The instrumentation required to measure the hysteresis loop of a ferromagnetic surface with MOKE can be very simple. Figure 4a shows one such implementation of the experimental setup for analysis of ultrathin film samples maintained in a... 

An example of interaction stiffness and force curves for a Si surface with a native oxide at 60% relative humidity (RH) is shown in Fig. 12 [104]. The stiffness and force data show an adhesive interaction between the tip and substrate. The hysteresis on retraction is due to a real change in contact area from surface oxide deformation and is not an experimental artifact. The adhesive force observed during retraction was consistent with capillary condensation and the surface energy measured from the adhesive force was close to that of water. 

Figure 7.42 Types of gas sorption isotherm - microporous solids are characterised by a type I isotherm. Type II corresponds to macroporous materials with point B being the point at which monolayer coverage is complete. Type III is similar to type II but with adsorbate-adsorbate interactions playing an important role. Type IV corresponds to mesoporous industrial materials with the hysteresis arising from capillary condensation. The limiting adsorption at high P/P0 is a characteristic feature. Type V is uncommon. It is related to type III with weak adsorbent-adsorbate interactions. Type VI represents multilayer adsorption onto a uniform, non-porous surface with each step size representing the layer capacity (reproduced by permission of IUPAC).

Adam and Jessop1 attempted to formulate the hysteresis as an effect of a frictional force F, operating along the surface with equal intensity, when advancing and receding motions were just prevented. Equation (2) would be modified, for advancing motion, to... 

To sum up, the effects on static contact angles of the departures from ideality of solid surfaces are qualitatively well understood and some of these effects are used in practice to improve or reduce wettability. Moreover, for simple geometries, a semi-quantitative agreement is obtained between experimental results and theoretical predictions. For surfaces with random roughness, predictions of wetting hysteresis present a great difficulty because the relevant size of defects is not yet well-established. 

Intrinsic hysteresis is a direct result of surface configuration change, which occurs as a result of wetting the surface with water. The plates were purposely immersed to a deeper immersion depth in the second cycle to observe the extent of intrinsic hysteresis. Significant surface configuration change affects the calculated contact angles on immersion and emersion, which violates the assumption of... 

Figure 5 contains the nitrogen adsorption isotherms for the 20% Cu containing sample, whilst the surface properties of all samples are summarised in Table 1. The isotherms are of Type IV with hysteresis loops of the H4 type. There are no significant differences in the shape of the isotherm or the hysteresis loop, but as observed for other mixed oxides, there was a decrease in the Sbet values for the... 

In 1944, Cassie and Baxter derived an equation describing contact angle hysteresis for composite smooth solid surfaces with varying degrees of heterogeneity ... 

The saturation of all adsorption sites on the solid surface (6 = 1) is characterized by a plateau in the isotherm. The Langmuir adsorption isotherm is based on the following assumptions the surface is uniform and every adsorption site is equivalent to the others, the substrate surface is saturated when aU adsorption sites are occupied and monolayer formation has occurred, there are no interactions between the adsorbed particles. In general, physisorption isotherms show various shapes. These are, according to the lUPAC classification, types II and III which describe adsorption on nonporous or macroporous adsorbent with strong and weak gas-solid interaction, respectively. Type IV and V are adsorption isotherms which show typically capillary condensation with hysteresis loops and type VI isotherm shows stepwise multilayer adsorption. 

Ferey et al. measured hydrogen adsorption in nanoporous metal-benzenedi-carboxylates, where the metal is trivalent chromium or aluminum. Also in this case the material has a framework structure with high specific surface area (llOOm g ). The authors report for these samples type I adsorption isotherms with hysteresis. The maximum storage capacity obtained for the chromium compound is 3.1 and 3.8 wt% for the aluminum compound at 1.6 MPa and 77 K [54]. 

An example of this approach is represented by the growth of 3-D coordination polymers with SCO properties via stepwise adsorption reactions for multilayer films based entirely on intra- and interlayer coordination bonds Fe(pyrazine) [Pt(CN) ] [218, 219], Indeed, after functionalization of the surface with the appropriate anchoring layer the coordination polymer is built in a stepwise fashion, alternating the metal ion (Fe "), the platinum salt ([Pt(CN) ] ), and pyrazine. The polymer shows many interesting properties, with the SCO transition accompanied by a variation in the dielectric constant of the material accompanied by a room temperature hysteresis of the dielectric constants. This dielectric hysteretic property may be useful in building molecular memory devices that can store information by high- and low-capacitance states. What must be remarked here is that these appealing properties cannot be exploited in bulk materials, but only in thin films. 

Atomic force microscopy is considered one of the most perspective methods for stud5ring polymer blends is because this method allows to clearly define the phase boundary and its scale. Polymer identification was performed by controlling the interaction of the probe with the sample surface at different points. When approaching the strrface of the cantilever is deflected downward (to the sample) due to attractive forces imtil the probe comes into contact with the sample. When the probe is withdrawn from the studied surface, a hysteresis is observed, associated with the adhesive forces. Adhesion forces between the probe and the sample are forcing them to remain in contact, which causes the cantilever to bend. Phase of... 

Although the above-mentioned materials may not have a lamellar structure, they do show mesoporosity (see Figure 11.3). The calcined (600°C) ZSU-L material has a BET surface area of 276m g and exhibits a type-I isotherm with hysteresis loop of type H4, associated with narrow sHt-like pores. The average pore size estimated by the BJH method was 3.6nm. Little change was observed in the BET surface area and pore size of the ZSU-L powders calcined at 800 °C, which indicates that the materials have high thermal stability. 

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