Surface bistability

R. Barberi, M. Giocondo and G. Durand, Flexoelectrically controlled surface bistable switching in nematic hquid crystals, Appl. Phys. Lett. 60(9), 1085-1086, (1992). doi 10.1063/1.106452... 

Vacuum evaporation of SiO films onto glass substrates at a grazing angle can also result in multistable orientation. Usually, the evaporation provides either the planar ( to the evaporation plane) or tilted (in the evaporation plane) orientations. However, in a certain range of the incidence angles, and thickness of SiO, the bistable orientation is achieved [82]. The director is oriented at a certain polar angle to a substrate and takes one of the two azimuthal angles located symmetrically with respect to the evaporation plane. The electric field can switch the director from one stable position to the other, thus the electrically controlled surface bistability has been demonstrated [83]. 

FIGURE 4.32. Surface bistability in nematics [193]. Two degenerate director orientations Li and L2 are characterized by the same angle 0 with respect to the substrate normal z and the azimuthal orientations The electric field E is applied in the substrate plane. 

This surface bistability is at the basis of chiral smectic C surface stabilized ferroelectric liquid crystal (SSFLC) devices [92]. As their name indicates, these devices are made of thin cells in which the walls, imposing the orientation of the molecules at the surfaces, unwind the spontaneous smectic C helix and stabilize two uniform configurations of the director in the cell. Switching between these two states can be done by applying an electric field. 

The sensing methods summarized thus far are intended for absorption detection of molecules in the ambient, but molecules (or indeed thin films) on the microresonator surface can also be detected. In particular, if the surface is covered to such an extent that the optical energy absorbed heats the microresonator, the resulting thermal bistability in the frequency-scan response can be used to determine the absorption and/or thickness of the thin-film coating. This and surface characterization by measurement of the thermal accommodation coefficient were described in Sect. 5.5. These methods offer quite precise measurement, provided that certain reasonable and easily implemented assumptions are satisfied. 

We demonstrate by using ultrafast time resolved spectroscopy that the photoconversion from dihydroazulene (DHA) to vinylheptafulvene (VHF) is governed by two mechanisms The ring opening proceeds on the excited energy surface on the picosecond time scale. It is followed by an internal conversion to the VHF ground state that is accelerated by the presence of a conical intersection in the case of cyclopenta-DHA. This conical intersection hinders the photoinduced back reaction from the final VHF products. However, we successfully photo-converted the cyanophenyl-VHF-cis back to the DHA in an experiment with two delayed pulses. This opens the route to the development of bistable dihydroazulene switches. 

The effects of forced oscillations in the partial pressure of a reactant is studied in a simple isothermal, bimolecular surface reaction model in which two vacant sites are required for reaction. The forced oscillations are conducted in a region of parameter space where an autonomous limit cycle is observed, and the response of the system is characterized with the aid of the stroboscopic map where a two-parameter bifurcation diagram for the map is constructed by using the amplitude and frequency of the forcing as bifurcation parameters. The various responses include subharmonic, quasi-peri-odic, and chaotic solutions. In addition, bistability between one or more of these responses has been observed. Bifurcation features of the stroboscopic map for this system include folds in the sides of some resonance horns, period doubling, Hopf bifurcations including hard resonances, homoclinic tangles, and several different codimension-two bifurcations. 

Thermally or photochemically induced proton transfers represent bistable switching processes and are of interest for information storage. A lateral transfer of information on the surface of biological membranes is thought to occur by fast proton conduction through protonic networks [8.234]. 

We show that application of a constant force (bias held) results in shifting the position of the ordinary SR peak together with the anticipated reduction of its height and sharpness. For the quadratic SR the situation is more complicated. There, the joint action of the thermal noise and constant bias leads to formation of a mountain-like surface over the plane of those parameters. In other words, for each given value of the bias held there exists a unique value of the noise strength that maximizes SNR and vice versa. The discovered effect can be useful, for example, for evaluation of the parameters of bistable systems through susceptibility measurements. In addition, it has to be taken into account when designing any devices where the nonlinear SR is employed. 

Figure 17.8 A molecular muscle based on a self-assembled monolayer of the palindromic bistable rotaxane 9S+ (a), anchored on the gold surface of microcantilever beams (b).81 (Adapted with permission from V. Balzani et al., ChemPhysChem 2008, 9, 202-220. Copyright Wiley-VCH Verlag GmbH Co. KGaA.)...

More generally, the surface can (for certain sets of parameters) be in two states, with either predominant CO or O coverage. Upon variation of one of the partial pressures, for example, a more or less abrupt transition between these states occurs, whereby the system may also show pronounced hysteresis effects—it exhibits bistability. Figure 2 reproduces corresponding experimental data for Pt(110), where the stationary work function change A4> (oxygen coverage) is plotted as a function of pco at fixed p0, and for various temperatures (24). Kinetic oscillations are ob-... 

The existence of bistability in the //under conditions under which chemical variable, on which the current depends, exhibits bistability as a function of DL. Thus, in S-NDR systems we have to require that the dynamic equations contain a chemical autocatalysis. As set forth below, m takes the role of the negative feedback variable. The positive feedback might be due to chemical autocatalytic reaction steps as is the case in Zn deposition [157, 158] or CO bulk oxidation on Pt [159], S-shaped current-potential characteristics may also arise in systems with potential-dependent surface phase transitions between a disordered (dilute) and an ordered (condensed) adsorption state due to attractive interactions among the adsorbed molecules. 

Formulating appropriate rate laws for CO adsorption, OH adsorption and the reaction between these two surface species, a set of four coupled ordinary differential equations is obtained, whereby the dependent variables are the average coverages of CO and OH, the concentration of CO in the reaction plane and the electrode potential. In accordance with the experiments, the model describes the S-shaped I/U curve and thus also bistability under potentiostatic control. However, neither oscillatory behavior is found for realistic parameter values (see the discussion above) nor can the nearly current-independent, fluctuating potential be reproduced, which is observed for slow galvanodynamic sweeps (c.f. Fig. 30b). As we shall discuss in Section 4.2.2, this feature might again be the result of a spatial instability. 

A lattice-gas model of the NO + CO/Pt(l 00) reaction was found by Makeev and Kevrekidis (2004) to exhibit bistability and kinetic oscillations. In this simulation, the catalyst surface was represented by a square lattice with periodic boundary conditions. The model includes both chemical reactions and transitions of adsorbed reactants from the nearest-neighbor sites, which are in the... 

Would not it be amazing if the tiny nanomotions of molecular machines could be cooperatively scaled up to do work on macroscopic objects Toward that end, it has been demonstrated [226, 227] that a monolayer of molecular muscles - palindromic bistable [3]rotaxanes (Fig. 31a) immobilized on the surface of a thin... 

Fig. 31 Mechanical actuation of a gold-coated microcantilever by molecular muscles [227]. (a) Structural formula of a palindromic, bistable [3]rotaxane with gold-binding dithiolane groups attached to the cyclophanes. (b) Reversible bending up and down of a cantilever by actuation of a monolayer ( 8 billion molecules) of the rotaxanes on its surface. The gold surface bends when the rotaxanes contract under the influence of an electrochemical oxidation that causes the cyclophanes to shuttle inward from the periphery of the molecule, (c) Electrochemical cell (Ag/AgCl, Pt, and the cantilever are the reference, counter, and working electrodes, respectively) and combined AFM device used to measure the bending by detecting a laser beam reflected off of the cantilever s surface...

Fig. 32 Transport of a diiodomethane droplet up a 12° incline on a surface with physisorbed, photoswitchable bistable [2]rotaxanes [228]. (a) Cartoon and photograph of the surface in the ground state. The macrocycle preferentially encircles an unfluorinated recognition site, (b) The same surface in the photostationary state (after UV irradiation). Olefin isomerization in the preferred recognition site causes some macrocycles to shuttle to the fluorinated site, changing the polarophobicity of the surface and moving the droplet several millimeters uphill...

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