An alternative explanation of back-relaxation in ionic polymer metal composites

ASME Letters in Dynamic Systems and Control

Rosen

Cha

et al. 2020

A recent experiment by Kim’s group from the University of Nevada, Las Vegas, has shown the possibility of actuating ionomer cilia in salt solution. When these actuators are placed between two external electrodes, across which a small voltage is applied, they move toward the cathode. This is in stark contrast with ionic polymer metal composites, where the same ionomers are plated by metal electrodes but bending occurs toward the anode. Here, we seek to unravel the factors underlying the motion of ionomer cilia in salt solution through a physically based model of actuation. In our model, electrochemistry is described through the Poisson–Nernst–Planck system in terms of concentrations of cations and anions and voltage. Through finite element analysis, we establish that Maxwell stress is the main driving force for the motion of the cilia. This study constitutes a first effort toward understanding the motion of ionomer cilia in salt solution, which, in turn, may help elucidate the physical underpinnings of actuation in ionic polymer metal composites.

Section: Resultssupporting

confidence: 92%

Modeling Actuation of Ionomer Cilia in Salt Solution Under an External Electric Field

ASME Letters in Dynamic Systems and Control

Rosen

Cha

et al. 2020

“…This surprising effect is commonly associated with the so-called added mass, whereby solvent molecules in the ionomer are first dragged toward the anode by counterions migration and then slowly diffuse back to drive the relaxation of the IPMC 24 . This explanation presents some inconsistencies when compared with experimental observations 16,25,26 , which could be partially resolved by embracing our thermodynamically-consistent continuum model as we have demonstrated in previous efforts 27,28 . While we are able to resolve some of the qualitative discrepancies of the added mass explanation with respect to existing experiments, it is difficult to confidently validate any theory of IPMC actuation and propose which approach should be preferred when designing IPMC actuators.…”

Section: Introductionmentioning

confidence: 81%

“…An alternative explanation might entail early back-relaxation of the samples 27 , in the absence of metal electrodes that could block the movement of the counterions and solvent across the ionomer-solution interface. The preliminary simulation results in our previous work 64 seem to support this explanation, whereby Maxwell stress generated by large electric fields at the ionomer-solution interface would lead to a consistent bending toward the cathode.…”

Section: Discussionmentioning

confidence: 99%

Contactless actuation of perfluorinated ionomer membranes in salt solution: an experimental investigation

Rosen

Cha

et al. 2019

Sci Rep

A variety of modeling frameworks have been proposed for ionic polymer metal composites (IPMCs), but the physical underpinnings of their actuation remain elusive. A critical step toward the validation of existing theories and transition to engineering practice entails the design of new experimental paradigms that could support hypothesis-driven research. While several factors exacerbate the complexity of experimenting with IPMCs, the presence of the electrodes plays a major role by hindering the repeatability of the results and bringing a number of difficult-to-measure parameters into the picture. Here, we seek to address these experimental confounds by investigating contactless actuation of perfluorinated ionomer membranes in salt solution. In contrast to IPMCs that bend toward the anode in response to an applied voltage, ionomer membranes display a consistent deflection toward the cathode. Through hypothesis-driven experiments where the membrane width, solution concentration, and voltage applied across the electrodes are systematically varied, we elucidate electrochemistry and mechanics of contactless actuation. The applied voltage and solution concentration have a dominant role on the electrochemistry, while mechanics is mainly affected by the applied voltage and membrane width. Our results depict a complex scenario, which is expected to inform future theoretical inquiries about IPMC actuation.

“…Since the normal fluxes of solvent and counterions at the electrodes are zero, J 0y and J þy must be zero as well throughout the thickness. For this condition to be identically fulfilled, we need the (electro)chemical potentials of the solvent μ 0 and counterions μ + to be constant, 25) 52 and substituting in Eq. ( 17), we find…”

Section: Equations For the Static Actuation Of Ionic Polymer Actuatorsmentioning

confidence: 99%

A non-ideal solution theory for the mechanics and electrochemistry of charged membranes

Porfiri

2022

npj Comput Mater

Understanding how ions and solvent molecules migrate within charged membranes is fundamental for advancing the analysis of biological membranes and the design of energy storage and production devices. Recent efforts highlighted a significant interplay between mechanics and electrochemistry in charged membranes, calling for the development of high-fidelity models to describe their interaction. Here, we propose a continuum theory of the chemoelectromechanics of charged membranes, accounting for potentially large deformations and non-idealities of the solution permeating the membrane. We demonstrate the potential applications of our theory within the study of ionic polymer actuators. Our theory predicts sizeable effects of non-idealities and mechanical deformations, enabling insight into the role of mechanics on solute and solvent transport within charged membranes.