Adsorption-induced stress

The mechanism for cantilever bending is assumed to be adsorption-induced stress. The adsorption decreases the surface free energy and surface free energy is density is surface stress. It is speculated that the hydrogen bonding between the nitro groups of the explosives molecules and the hydroxyl group of 4-MBA is responsible for the easily reversible adsorption of explosive vapours on the SAM-coated top surface of the cantilever. This... 

Therefore, the deflection of the cantilever is directly proportional to the adsorption-induced differential surface stress. Surface stress has units of N/m or J/m2. Equation (12.3) shows a linear relation between cantilever bending and differential surface stress. 

Adsorption-induced brittle fracture. This model is based on the hypothesis that adsorption of environmental species lowers the interatomic bond strength and the stress required for cleavage. This model of chemical adsorption can explain the fact that a certain alloy is susceptible to specific ions. An important factor in support of this mechanism is the existence of a critical potential below which the SCC does not occur in some systems, and this model underlines the relation between the potential value and the capacity of adsorption of the aggressive ion. It also explains the preventive action of SCC for some systems by cathodic protection. This model may interpret the rupture of plastic materials or glass. It is referred to as the stress-sorption model, and similar mechanisms have been proposed for HE and LME. In this model, the crack should propagate in a continuous way at a rate determined by the arrival of the embrittling species at the crack tip. The model does not explain how the crack maintains a sharp tip in a normally ductile material.156... 

The two micromechanism of LMIE are (i) the dissolution-condensation model (DCM) and (ii) the adsorption-induced localized slip model. In the DCM mechanism the crack is filled with melt and grows by dissolution of atoms from the crack tip where the chemical potential is increased due to applied stress.1,2 In the second model,... 

G.Y. Chen, T. Thundat, E.A. Wachter and R.J. Warmack, Adsorption-induced surface stress and its effects on resonance frequency of microcantUevers, Journal of Applied Physics 77 (1995) 3618-3622. 

Among other external factors that influence the adsorption-induced decrease of strength in solids one should mention the nature of applied stresses. As a rule, a decrease in strength is observed under the action of hard stressed states, among which the tensile components prevail. The amount of adsorption-active medium and the means by which it is delivered into the system may also play an important role. 

Other factors affecting the signals in the dynamic mode are adsorption-induced effects, such as surface stress and position dependence, which can either stiffen or soften the cantilever, thereby varying the spring constant. The relationship between the surface stress and stiffness of a cantilever has been intensively discussed [20-22]. Lee et al. visually demonstrated the dependence of resonance frequency on a pattern of a gold layer on the surface of a cantilever [23]. In any case, we have to be careful about these effects when we analyze the signals obtained with the dynamic mode. 

FIGURE 7.4 (a) Deflection, Az, and change in surface stress, Ac, of a gold-coated AFM microcantilever are plotted as a function of time after exposure to vapors of alkanethiols with different chain lengths, (b) Adsorption-induced surface stress at saturation coverage (AOjai) is plotted as a function of alkyl chain length for = 4, 6, 8, 12, and 14. (From Berger, R., et al. 1997. Science 276 2021. Reprinted with permission from AAAS.)... 

To summarize, in order to determine the deformation of the gel, we have to solve the set of equations (2.1), (2.7), (2.9) and (2.10). We obtain 1) current density from the voltage of electrodes, 2) the adsorption rate from the current density, 3) surface stress and strain from the adsorption rate, respectively. Fig. 2.2 describes the deformation process of the beam of gel in uniform electric fields. Three arrows are current density vectors. Molecular density on the gel is large near the root and small near the tip of the gel. We can estimate that deformation speed near the root of the gel is larger than that near the tip due to the adsorption-induced deformation model. 

In the discussion of the reversibility of the Rehbinder effect, it was implied that there is a thermodynamically stable interface present between the mutually saturated solid phase and liquid medium and that the effect vanishes when the liquid medium is ranoved, for example, by evaporation. These two peculiarities make the Rehbinder effect principally different from the corrosion caused by the action of aggressive media. At the same time, one must realize that complete segregation is not possible various processes can cover a fairly broad spectrum from idealized cases involving purely mechanical failure to purely corrosive processes (or dissolution). The Rehbinder effect, which involves the adsorption-induced lowering of strength, stress-facilitated corrosion, and corrosive fatigue, often occupies intermediate positions in these series. In this type of phenomenon, the action of external forces and the action of chemically active media both contribute to the net result in certain proportions. 

In the case of intensive repetitive actions, the facilitation of plastic deformation in the surface layer may at some point result in the opposite effect, namely, an additional strength increase due to the accelerated accumulation of distortions in the metal structure. Direct observations by electron microscopy, conducted by Kostetskiy et al., indicated a significant increase in the dislocation density in the surface layer. Under the appropriate conditions (temperature, stress, velocity, etc.), such a peculiar sample training may be used in the improvement of the structure and the mechanical properties of the surface layer. However, this already corresponds to the adsorption-induced fatigue region, studied in detail by Karpenko et al. These studies showed that at a certain level of stress the adsorption-caused acceleration of defect accumulation within the surface layer may lead to the premature development of cracks and partial failure after a certain number of cycles (cyclic fatigue). 

S. Kim, K.D. Kihm, Effect of adsorption-induced surface stress change on the stiffness of a microcantilever used as a salinity detection sensor. Appl. Phys. Lett. 93(8), 081911 (2008)... 

An example is HKUST-1, which, although small, shows adsorption-induced distortions when water, methanol, or ethanol are reversibly adsorbed. Allendorf et al. showed this property when a thin film of HKUST-1 was integrated with a microcantiveler surface [94]. The time-dependent responses to H O are shown in Fig. 3.12. It was also demonstrated that the sensor responds to CO2 when the MOF layer is dehydrated. The stress in this case was due to the coordination of COj with imsaturated copper sites in HUKST-1. Because the framework distortions were quite small, the group concluded that higher sensitivities can be achieved using more flexible MOFs. 

First principles calculations of the effect of gas adsorption on the surface stress of Pt(lll) have been performed [97Fei], The adsorption of hydrogen and oxygen was found to relief the tensile stress of the clean Pt(l 11) surface partially. The results of this study are presented in section 4.4.7.11 in Table 11 and in Fig. 11 and Fig. 12. These calculations suggest that the modification, i.e. weakening, of the surface bonds of the substrate in the vicinity of the adsorbate is an important issue in the discussion of adsorbate-induced stress. 

For the experiments referred to in Fig. 4.25(a), McEnaney was able to show, on reasonable assumptions, that the stress induced by adsorption swelling should be sufficient to fracture the carbon over short distances. A memory effect in the carbon network would lead to trapping of some adsorbed carbon tetrachloride molecules during the desorption run. 

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