Difference between revisions of "Nonlinear oscillation of a dielectric elastomer balloon"

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(New page: Jian Zhu, Shengqiang Cai, and Zhigang Suo Polymer International 59 (2010) p. 378–383 wiki entry by Emily Russell, Fall 2010 The article can be found [http://www.seas.harvard.edu/suo...)
 
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==Overview==
 
==Overview==
(entry in progress)
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This paper gives a theoretical treatment of a balloon made of a dielectric elastomer, where the radius of the balloon is affected not only by internal pressure, but also by the voltage applied across the membrane.  Under appropriate conditions, such a balloon will oscillate around a stable equilibrium radius; these oscillations may be excited not only by a driving voltage at the resonant frequency, but super- and sub-harmonically by driving voltages at fractional and integer multiples of the resonant frequency.
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==Model==
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The balloon is considered as a spherical, elastic membrane which is incompressible (constant total volume), and has a fixed dielectric constant.  The system is then described in terms of the stretch of the balloon, <math> \lambda = \frac{r}{R} </math>, where r is the deformed radius of the balloon and R its undeformed radius, and the electric displacement, <math> D = \frac{Q}{4 \pi r^2} </math>, with Q the magnitude of the surface charges induced by the applied voltage.
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The free-energy function of the balloon is modeled as having a dielectric contribution <math> \frac{D^2}{2\epsilon} </math>, and a neo-Hookian elastic contribution, <math> \mu / 2 * (2\lambda^2 + \lambda^{-4} - 3) </math>.  The variation in free energy with <math>\lambda</math> and D is then equated to the virtual work done by the applied voltage, the internal pressure, and the inertial force as the radius changes, to give a nonlinear differential equation for <math>\lambda</math>, <math> \frac{d^2 \lambda}{d T^2} = 2\lambda - 2\lambda^{-5} - (\frac{pR}{\mu H})\lambda^2 - 2(\frac{\epsilon \Phi^2}{\mu H^2}) \lambda^3 </math>.
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The balloon is first treated as being under constant pressure and voltage; up to some critical pressure, the balloon can reach an equilibrium stretch, and the natural frequency of oscillations around this equilibrium is calculated; both <math> \lambda_{eq} </math> and <math> \omega_0 </math> are calculated algebraically.  An AC voltage component is then added to the model, and the response of the balloon to this driving force is computed numerically.
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An interesting note is that the final equations are all presented in terms of dimensionless quantities; the stretch itself is dimensionless, and the time, pressure, and voltage are all scaled by the appropriate combinations of the density, undeformed radius and thickness, shear modulus, and dielectric constant of the material.  Dimensionless quantities - as discussed at some length in the reading for this topic - allow the direct comparison of systems which have different parameters but the same intrinsic timescale, or determination of the result of the application of a given pressure or voltage to one system by comparison to the effect of the same scaled pressure or voltage to a very different system.
 +
 
 +
 
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==Results==
 +
For a given applied voltage, there is some critical pressure above which there is no equilibrium; the balloon will expand indefinitely, until in a real system it pops.  This critical pressure is lowered by increasing the applied voltage, which thins the membrane; at a given sub-critical pressure, the equilibrium stretch therefore also increases with applied voltage.  Conversely, at a given pressure, there is a critical voltage, above which the balloon is unstable.  For pressures and voltages which allow a stable equilibrium, a small perturbation from equilibrium will oscillate with some characteristic frequency.
 +
 
 +
The more interesting results are the response to a driving force, here applied through an oscillating component of the voltage.  The balloon can resonate strongly with
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 +
 
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==Comments==
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I am curious to what degree the effects seen in this model are determined by the details of the model, such as the inclusion of the <math> \lambda^{-4} </math> term in the elastic contribution to the free energy.

Revision as of 01:41, 30 November 2010

Jian Zhu, Shengqiang Cai, and Zhigang Suo

Polymer International 59 (2010) p. 378–383


wiki entry by Emily Russell, Fall 2010


The article can be found here.


Overview

This paper gives a theoretical treatment of a balloon made of a dielectric elastomer, where the radius of the balloon is affected not only by internal pressure, but also by the voltage applied across the membrane. Under appropriate conditions, such a balloon will oscillate around a stable equilibrium radius; these oscillations may be excited not only by a driving voltage at the resonant frequency, but super- and sub-harmonically by driving voltages at fractional and integer multiples of the resonant frequency.


Model

The balloon is considered as a spherical, elastic membrane which is incompressible (constant total volume), and has a fixed dielectric constant. The system is then described in terms of the stretch of the balloon, <math> \lambda = \frac{r}{R} </math>, where r is the deformed radius of the balloon and R its undeformed radius, and the electric displacement, <math> D = \frac{Q}{4 \pi r^2} </math>, with Q the magnitude of the surface charges induced by the applied voltage.

The free-energy function of the balloon is modeled as having a dielectric contribution <math> \frac{D^2}{2\epsilon} </math>, and a neo-Hookian elastic contribution, <math> \mu / 2 * (2\lambda^2 + \lambda^{-4} - 3) </math>. The variation in free energy with <math>\lambda</math> and D is then equated to the virtual work done by the applied voltage, the internal pressure, and the inertial force as the radius changes, to give a nonlinear differential equation for <math>\lambda</math>, <math> \frac{d^2 \lambda}{d T^2} = 2\lambda - 2\lambda^{-5} - (\frac{pR}{\mu H})\lambda^2 - 2(\frac{\epsilon \Phi^2}{\mu H^2}) \lambda^3 </math>.

The balloon is first treated as being under constant pressure and voltage; up to some critical pressure, the balloon can reach an equilibrium stretch, and the natural frequency of oscillations around this equilibrium is calculated; both <math> \lambda_{eq} </math> and <math> \omega_0 </math> are calculated algebraically. An AC voltage component is then added to the model, and the response of the balloon to this driving force is computed numerically.

An interesting note is that the final equations are all presented in terms of dimensionless quantities; the stretch itself is dimensionless, and the time, pressure, and voltage are all scaled by the appropriate combinations of the density, undeformed radius and thickness, shear modulus, and dielectric constant of the material. Dimensionless quantities - as discussed at some length in the reading for this topic - allow the direct comparison of systems which have different parameters but the same intrinsic timescale, or determination of the result of the application of a given pressure or voltage to one system by comparison to the effect of the same scaled pressure or voltage to a very different system.


Results

For a given applied voltage, there is some critical pressure above which there is no equilibrium; the balloon will expand indefinitely, until in a real system it pops. This critical pressure is lowered by increasing the applied voltage, which thins the membrane; at a given sub-critical pressure, the equilibrium stretch therefore also increases with applied voltage. Conversely, at a given pressure, there is a critical voltage, above which the balloon is unstable. For pressures and voltages which allow a stable equilibrium, a small perturbation from equilibrium will oscillate with some characteristic frequency.

The more interesting results are the response to a driving force, here applied through an oscillating component of the voltage. The balloon can resonate strongly with


Comments

I am curious to what degree the effects seen in this model are determined by the details of the model, such as the inclusion of the <math> \lambda^{-4} </math> term in the elastic contribution to the free energy.