Time-Resolved Multifractal Analysis of Electron Beam Induced Piezoelectric Polymer Fiber Dynamics: Towards Multiscale Thread-Based Microfluidics or Acoustofludics

Abstract

Charge propagation through the films or fibers of ferroelectric and piezoelectric polymers leads to their (ultra)acoustic signal generation and micro(electro)mechanical dynamics. Surface acoustic waves can be generated in piezoelectric polymer materials under the electron beam. Therefore, SEM methods can provide not only metastable surface acoustic wave potential contrasts, but also excitation/induction of micromechanical local movements in piezosamples associated with the propagation of the surface acoustic waves. The dynamic phenomena of piezoelectric charging, charge traveling/wandering over their surface, charge gating and leakage, and micro(electro)mechanical movements under the electron beam can be studied by stroboscopic electron microscopy. However, it can provide only visualization, but not quantification of the structural effects, and does not reveal their reversibility/irreversibility. Therefore, we propose to combine time-resolved or stroboscopic SEM methods with multifractal analysis methods. This paper provides examples of using this approach to analyze the behavior of ferroelectric polymer networks under the electron beam. It has been shown that, when the effect is reversible, the multifractal spectra also change reversibly and soon return to the initial profiles. In the case of an incomplete relaxation or “tetanic contraction” of the polymer fiber, the multifractal spectra profiles do not completely return to their initial state. Also, it has been shown that the graphs of generalized fractal dimension D(q) as a function of q-order moment for the polymer fibers do not change significantly, so they cannot serve as an indicator of the equilibrium shifts in the fiber. At the same time, the shape of the multifractal spectrum f(α) as the function of Lipschitz-Holder exponent graph changes significantly, and hence can be used as a characteristic descriptor and predictor of the fiber state. Such techniques of analysis and prediction can be useful for the design of “artificial muscles” based on piezoelectric polymers, as well as for the design of thread microfluidics using capillarity and (electron beam-driven) electrocapillarity effects. In general, the above principles can be applied to create engineering/bioengineering systems with controlled electrowetting based on ferroelectric polymer fiber materials.