Reasons for Hysteresis

What are the possible reasons for hysteresis Various explanations for contact angle hysteresis have been discussed. The contribution of each factor depends on the particular situation encountered. Possible causes of hysteresis are ... 

Consider now adsorbed molecular or ionic species that are, practically speaking, immobilized in the soil. Unless the soil is extremely acid, metals such as Cu, Cr, and Pb fall into this category. Also, certain anions such as phosphate bond so strongly on minerals that they too behave as immobile elements. The property that all of these ions have in common is that their sorption isotherms are not reversible within a time scale relevant to soil processes the adsorption (forward) isotherm is usually approximated closely by a Langmuir function of the strong-affinity type, but the desorptioii (backward) isotherm deviates markedly from the adsorption isotherm. This kind of nonequilibrium behavior, depicted in Figure 9.6, is sometimes referred to as hysteresis. Possible reasons for hysteresis in chemisorption are discussed in Chapter 4. 

Fig. 107 is an illustration of the capillary filling and capillary emptying as envisioned by the BdB theory. The difference in the filling and emptying geometry is the postulated reason for hysteresis. The sequence from left to right ... 

Liabastre and Orr [52] examined graded series of controlled pore glasses and Nuclepore membranes by electron microscope and mercury porosimetry in o er to determine compressibilities and explore the reason for hysteresis. 

There is no doubt that heterogeneity affects the wetting process. However, heterogeneity of the surface is apparently not the sole reason for hysteresis of the contact angle. This follows from the fact that not all predictions made on the... 

Unstable branches on the P(Q) curve and the appearance of hysteresis loops can occur for various reasons usually connected with an increase in viscosity. Thus, a non-monotonic P(Q) curve was first encountered in an analysis of the flow of a hot inert (non-reactive) liquid in a cold tube when the viscosity of the liquid was strongly dependent on temperature.190 The intense dissipative heat output may have been the reason for the instability in the flow of an inert liquid.191 In both cases, the reason for the nonmonotonic in P(Q) dependence was the strong dependence of viscosity on temperature, which is equivalent here to time dependence for viscosity. Detailed investigations of the hysteresis transitions shown in Fig. 4.24 proved that they have a wave character 192 in this case, the transition occurs at a constant flow rate. 

The studies of Ertl and co-workers showed that the reason for self-oscillations [142, 145, 185-187] and hysteresis effects [143] in CO oxidation over Pt(100) in high vacuum ( 10 4 Torr) is the existence of spatio-temporal waves of the reversible surface phase transition hex - (1 x 1). The mathematical model [188] suggests that in each of the phases an adsorption mechanism with various parameters of CO and 02 adsorption/desorption and their interaction is realized, and the phase transition is modelled by a semi-empirical method via the introduction of discontinuous non-linearity. Later, an imitation model based on the stochastic automat was used [189] to study the qualitative characteristics for the dynamic behaviour of the surface. 

Figure 17.1 has shown the change in shape of the hysteresis loop at smaller scale. If we analyze the reason for this change a linear capacitor which is much larger than the ferroelectric one can be identified. Waser and Lohse have given a nice overview of different contributions to the shape of the hysteresis in [8],... 

In this connection, it is not surprising that different investigators report different extents of hysteresis for one and the same system. Small changes in pretreatment or mechanical stability may make a leirge difference for instance vibration of the solid may reduce the difference between a(adv) and a(rec) ). The reason for this last phenomenon may be that the additional mechanical energy helps to move the system from one metastable state into the next. Systems that are fully hysteresis-free are rare. 

The phenomenon of hysteresis is widespread in nature. Behavior of many systems in physics [1], chemistry [2], biology [3], social science [4, 5], and interdisciplinary sciences [6] exhibit hysteresis. The most general reason for existing of these phenomena is as follows if we reverse the path in the control variables space, we do not necessarily reverse the path in state variables space. Physically it means that there are two or more different local minima and only one corresponds to the thermodynamic equilibrium state, the others must be metastable. These persisting metastable states are responsible for the origin of hysteresis. Among these systems adsorption hysteresis stands out because of its direct and close connection with a number of other complicate phenomena and relevant... 

The stationary solution now presents a bistability [124]. This bistability is the reason for the existence of the instability region that was first described by Hoho [56] (see Section 2.1). Another consequence is a hysteresis in the stationary I—U characteristics (see Section 4.4.1). 

Desorption occurs when the pressure is decreased from the saturation pressure. Many mesoporous systems exibit distinct adsorption-desorption behaviors, which lead to a characteristic hysteresis loop. The curve shape is linked to different geometrical factors that rule the adsorption and desorption processes. The reason for this hysteresis is that capillary condensation occurs differently in adsorption and desorption. Because of the concave meniscus of the liquid in the pores, N2 evaporates at a lower relative pressure because the vapor pressure of the liquid is reduced. The lowering of the vapor pressure for a capillary of radius rk is given by the Kelvin relationship... 

Current theories to explain hysteresis of contact angles are primarily based on the concepts of surface roughness, surface heterogeneity, friction, and adsorption phenomena. Unintentional adsorption, or contamination—the result of inadequate experimental technique—is, however, the most frequent explanation. As all systems involving solids are subject to the reasons indicated above for hysteresis, we chose the system mercury-benzene-water, which should be affected only by adsorption phenomena, controllable under proper experimentation. An additional advantage is the fact that all interfacial tensions involved can be measured. 

Low-temperature nitrogen adsorption. By taking capillary condensation as the reason for the hysteresis loops of the sorption isotherms (Fig. 7.26), it is possible to calculate the pore size distribution in accordance with the well-known Kelvin equation. This equation relates the relative pressure of vapor above a concave meniscus ofliquid in a capilLary to the curvature radius... 

---