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Ruobing Bai

Publications and source records attributed to Ruobing Bai.

22 records · Page 2Linked to original sources
arXiv2020

Collective behavior in the kinetics and equilibrium of solid-state photoreaction

There is current interest in developing photoactive materials that deform on illumination and can thus be used for photomechanical actuation. This is attractive since it can be affected at a distance, different frequencies can be used to actuate different modes and to sense, and corrosion-free lightweight fiber optic cables can deliver significant power over long distances. The strategy for developing new photomechanical materials is to first develop photoactive molecules in solution, and then to incorporate these in the solid-state either by crystallization or by inserting them into polymers. This letter shows that the kinetics and the nature of the photo-induced phase transitions are profoundly different in single molecules (solution) and in the solid state using a lattice spin model. In solution, where the molecules act independently, the photoreaction follows first-order kinetics. However, in the solid state where the photoactive molecules interact with each other and therefore behave collectively during reaction, photoreactions follow the sigmoidal kinetics of nucleation and growth as in a first-order phase transition. Further, we find that the exact nature of the photo-induced strain has a critical effect on the kinetics, equilibrium, and microstructure formation. These predictions agree qualitatively with experimental observations, and provide insights for the development of new photoactive materials.

cond-mat.mes-hall↗
arXiv2020

Photomechanical coupling in photoactive nematic elastomers

Photoactive nematic elastomers are soft rubbery solids that undergo deformation when illuminated. They are made by incorporating photoactive molecules like azobenzene into nematic liquid crystal elastomers. Since its initial demonstration in 2001, it has received increasing interest with many recent studies of periodic and buckling behavior. However, theoretical models developed have focused on describing specific deformation modes (e.g., beam bending and uniaxial contraction) in the absence of mechanical loads, with only limited attention to the interplay between mechanical stress and light-induced deformation. This paper explores photomechanical coupling in a photoactive nematic elastomer under both light illumination and mechanical stress. We begin with a continuum framework built on the free energy developed by Corbett and Warner (Phys. Rev. Lett. 2006). Mechanical stress leads to nematic alignment parallel to a uniaxial tensile stress. In the absence of mechanical stress, in the photo-stationary state where the system reaches equilibrium, the nematic director tends to align perpendicular to the polarization of a linearly polarized light. However, sufficient illumination can destroy nematic order through a first-order nematic-isotropic phase transition which is accompanied by a snap through deformation. Combined illumination and mechanical stress can lead to an exchange of stability accompanied by stripe domains. Finally, the stress-intensity phase diagram shows a critical point that may be of interest for energy conversion.

cond-mat.soft↗
arXiv2020

Optimal small data Scattering for the generalized derivative nonlinear Schrödinger equations

In this work, we consider the following generalized derivative nonlinear Schrödinger equation \begin{align*} i\partial_t u+\partial_{xx} u +i |u|^{2σ}\partial_x u=0, \quad (t,x)\in \mathbb R\times \mathbb R. \end{align*} We prove that when $σ\ge 2$, the solution is global and scattering when the initial data is small in $H^s(\mathbb R)$, $\frac 12\leq s\leq1$. Moreover, we show that when $0<σ<2$, there exist a class of solitary wave solutions $\{ϕ_c\}$ satisfying $$ \|ϕ_c\|_{H^1(\mathbb R)}\to 0, $$ when $c$ tends to some endpoint, which is against the small data scattering statement. Therefore, in this model, the exponent $σ\ge2$ is optimal for small data scattering. We remark that this exponent is larger than the short range exponent and the Strauss exponent.

math.AP↗