A compliant bio-membrane with a nominally fiat reference configuration is prone to random transverse deflections when placed in water, due primarily to the Brownian motion of the water molecules. On the average, these...A compliant bio-membrane with a nominally fiat reference configuration is prone to random transverse deflections when placed in water, due primarily to the Brownian motion of the water molecules. On the average, these fluctuations result in a state of thermodynamic equilibrium between the entropic energy of the water and the total free en- ergy of the membrane. When the membrane is in close proximity to a parallel surface, that surface restricts the fluctuations of the membrane which, in turn, results in an increase in its free energy. The amount of that increase depends on the degree of confinement, and the resulting gradient in free energy with degree of confinement implies the existence of a confining pressure. In the present study, we assume that the confinement is in the form of a continuous parabolic po- tential well resisting fluctuation. Analysis leads to a closed form expression for the mean pressure resulting from this confinement, and the results are discussed within the broader context of results in this area. In particular, the results provide insights into the roles of membrane stiffness, number of degrees of freedom in the model of the membrane and other system parameters.展开更多
Undulation force, an entropic force, stems from thermally excited fluctuations, and plays a key role in the essential interactions between neighboring surfaces of objects. Although the characteristics of the undulatio...Undulation force, an entropic force, stems from thermally excited fluctuations, and plays a key role in the essential interactions between neighboring surfaces of objects. Although the characteristics of the undulation force have been widely studied theoretically and experimentally,the distance dependence of the force, which constitutes its most fundamental characteristic, remains poorly understood.In this paper, first, we obtain a novel expression for the undulation force by employing elasticity and statistical mechanics and prove it to be in good agreement with existing experimental results. Second, we clearly demonstrate that the two representative forms of the undulation force proposed by Helfrich and Freund were respectively the upper and lower bounds of the present expression when the separation between membranes is sufficiently small, which was intrinsically different from the existing results where Helfrich＇s and Freund＇s forms of the undulation force were only suitable for the intermediate and small separations. The investigations show that only in a sufficiently small separation does Helfrich＇s result stand for the undulation force with a large wave number and Freund＇s result express the force with a small wave number. Finally, a critical acting distance of the undulation force, beyond which the entropic force will rapidly decay approaching zero, is presented.展开更多
文摘A compliant bio-membrane with a nominally fiat reference configuration is prone to random transverse deflections when placed in water, due primarily to the Brownian motion of the water molecules. On the average, these fluctuations result in a state of thermodynamic equilibrium between the entropic energy of the water and the total free en- ergy of the membrane. When the membrane is in close proximity to a parallel surface, that surface restricts the fluctuations of the membrane which, in turn, results in an increase in its free energy. The amount of that increase depends on the degree of confinement, and the resulting gradient in free energy with degree of confinement implies the existence of a confining pressure. In the present study, we assume that the confinement is in the form of a continuous parabolic po- tential well resisting fluctuation. Analysis leads to a closed form expression for the mean pressure resulting from this confinement, and the results are discussed within the broader context of results in this area. In particular, the results provide insights into the roles of membrane stiffness, number of degrees of freedom in the model of the membrane and other system parameters.
基金supported by the programs in the National Natural Science Foundation of China (Grants 11232013 and 11472285)
文摘Undulation force, an entropic force, stems from thermally excited fluctuations, and plays a key role in the essential interactions between neighboring surfaces of objects. Although the characteristics of the undulation force have been widely studied theoretically and experimentally,the distance dependence of the force, which constitutes its most fundamental characteristic, remains poorly understood.In this paper, first, we obtain a novel expression for the undulation force by employing elasticity and statistical mechanics and prove it to be in good agreement with existing experimental results. Second, we clearly demonstrate that the two representative forms of the undulation force proposed by Helfrich and Freund were respectively the upper and lower bounds of the present expression when the separation between membranes is sufficiently small, which was intrinsically different from the existing results where Helfrich＇s and Freund＇s forms of the undulation force were only suitable for the intermediate and small separations. The investigations show that only in a sufficiently small separation does Helfrich＇s result stand for the undulation force with a large wave number and Freund＇s result express the force with a small wave number. Finally, a critical acting distance of the undulation force, beyond which the entropic force will rapidly decay approaching zero, is presented.
