Showing papers by "Zhigang Suo published in 2010"

Foldable Printed Circuit Boards on Paper Substrates

Abstract: This paper describes several low-cost methods for fabricating flexible electronic circuits on paper. The circuits comprise i) metallic wires (e.g., tin or zinc) that are deposited on the substrate by evaporation, sputtering, or airbrushing, and ii) discrete surface-mountable electronic components that are fastened with conductive adhesive directly to the wires. These electronic circuits—like conventional printed circuit boards—can be produced with electronic components that connect on both sides of the substrate. Unlike printed circuit boards made from fiberglass, ceramics, or polyimides, however, paper can be folded and creased (repeatedly), shaped to form three-dimensional structures, trimmed using scissors, used to wick fluids (e.g., for microfluidic applications) and disposed of by incineration. Paper-based electronic circuits are thin and lightweight; they should be useful for applications in consumer electronics and packaging, for disposable systems for uses in the military and homeland security, for applications in medical sensing or low-cost portable diagnostics, for paper-based microelectromechanical systems, and for applications involving textiles.

Fracture of electrodes in lithium-ion batteries caused by fast charging

Abstract: During charging or discharging of a lithium-ion battery, lithium is extracted from one electrode and inserted into the other. This extraction-insertion reaction causes the electrodes to deform. An electrode is often composed of small active particles in a matrix. If the battery is charged at a rate faster than lithium can homogenize in an active particle by diffusion, the inhomogeneous distribution of lithium results in stresses that may cause the particle to fracture. The distributions of lithium and stress in a LiCoO2 particle are calculated. The energy release rates are then calculated for the particle containing preexisting cracks. These calculations predict the critical rate of charging and size of the particle, below which fracture is averted.

Theory of dielectric elastomers capable of giant deformation of actuation.

Abstract: The deformation of a dielectric induced by voltage is limited by electrical breakdown if the dielectric is stiff, and by electromechanical instability if the dielectric is compliant. The interplay of the two modes of instability is analyzed for a dielectric elastomer, which is compliant at a small stretch, but stiffens steeply. The theory is illustrated with recent experiments of interpenetrating networks, and with a model of swollen elastomers. The theory predicts that, for an elastomer with a stress-stretch curve of a desirable form, the voltage can induce giant deformation.

Stress-relaxation behavior in gels with ionic and covalent crosslinks.

Abstract: Long-chained polymers in alginate hydrogels can form networks by either ionic or covalent crosslinks. This paper shows that the type of crosslinks can markedly affect the stress-relaxation behavior of the gels. In gels with only ionic crosslinks, stress relaxes mainly through breaking and subsequent reforming of the ionic crosslinks, and the time scale of the relaxation is independent of the size of the sample. By contrast, in gels with only covalent crosslinks, stress relaxes mainly through migration of water, and the relaxation slows down as the size of the sample increases. Implications of these observations are discussed.

A theory of constrained swelling of a pH-sensitive hydrogel†‡

Abstract: Many engineering devices and natural phenomena involve gels that swell under the constraint of hard materials. The constraint causes a field of stress in a gel, and often makes the swelling inhomogeneous even when the gel reaches a state of equilibrium. This paper develops a theory of constrained swelling of a pH-sensitive hydrogel, a network of polymers bearing acidic groups, in equilibrium with an aqueous solution and mechanical forces. The condition of equilibrium is expressed as a variational statement of the inhomogeneous field. A free-energy function accounts for the stretching of the network, mixing of the network with the solution, and dissociation of the acidic groups. Within a Legendre transformation, the condition of equilibrium for the pH-sensitive hydrogel is equivalent to that for a hyperelastic solid. The theory is first used to compare several cases of homogenous swelling: a free gel, a gel attached to a rigid substrate, and a gel confined in three directions. To analyze inhomogeneous swelling, we implement a finite element method in the commercial software ABAQUS, and illustrate the method with a layer of the gel coated on a spherical rigid particle, and a pH-sensitive valve in microfluidics.

Using indentation to characterize the poroelasticity of gels

Abstract: When an indenter is pressed into a gel to a fixed depth, the solvent in the gel migrates, and the force on the indenter relaxes. Within the theory of poroelasticity, the force relaxation curves for indenters of several types are obtained in a simple form, enabling indentation to be used with ease as a method for determining the elastic constants and permeability of the gel. The method is demonstrated with a conical indenter on an alginate hydrogel.

Large deformation and electrochemistry of polyelectrolyte gels

Abstract: Immersed in an ionic solution, a network of polyelectrolytes imbibes the solution and swells, resulting in a polyelectrolyte gel. The swelling is reversible, and the amount of swelling is regulated by ionic concentrations, mechanical forces, and electric potentials. This paper develops a field theory to couple large deformation and electrochemistry. A specific material model is described, including the effects of stretching the network, mixing the polymers with the solvent and ions, and polarizing the gel. We show that the notion of osmotic pressure in a gel has no experimental significance in general, but acquires a physical interpretation within the specific material model. The theory is used to analyze several phenomena: a gel swells freely in an ionic solution, a gel swells under a constraint of a substrate, electric double layer at the interface between the gel and the external solution, and swelling of a gel of a small size.

Poroelastic swelling kinetics of thin hydrogel layers: comparison of theory and experiment

Abstract: Thin poly(N-isopropylacrylamide) (PNIPAM) hydrogels were allowed to swell under two conditions: as freestanding layers and as substrate-attached layers. Through a combination of particle tracking and defocusing methods, the positions of beads embedded within the gels were monitored over time via fluorescence microscopy, providing a convenient method to track the kinetics of swelling for layers with thicknesses of the order 100 µm. These data are compared with the predictions of linear poroelastic theory, as specialized for polymer gels. This theory, along with a single set of material properties, accurately describes the observed swelling kinetics for both the freestanding and substrate-attached hydrogels. With the additional measurement of the substrate curvature induced by the swelling of the substrate-attached hydrogels, these experiments provide a simple route to completely characterize the material properties of the gel within the framework of linear poroelasticity, using only an optical microscope.

Averting cracks caused by insertion reaction in lithium–ion batteries

Abstract: In a lithium-ion battery, both electrodes are atomic frameworks that host mobile lithium ions. When the battery is being charged or discharged, lithium ions diffuse from one electrode to the other. Such an insertion reaction deforms the electrodes and may cause the electrodes to crack. This paper uses fracture mechanics to determine the critical conditions to avert insertion-induced cracking. The method is applied to cracks induced by the mismatch between phases in LiFePO4.

Nonlinear oscillation of a dielectric elastomer balloon

Abstract: Much of the existing literature on dielectric elastomers has focused on quasi-static deformation. However, in some potential applications, the elastomer deforms at high frequencies and undergoes nonlinear oscillation. While nonlinear oscillation has been studied in many areas of science and engineering, we are unaware of any theoretical analysis of nonlinear oscillation of dielectric elastomers. This paper reports a theoretical study of the dynamic behavior of a dielectric elastomer balloon subject to a combination of pressure and voltage. When the pressure and voltage are static, the balloon may reach a state of equilibrium. We determine the stability of the state of equilibrium, and calculate the natural frequency of the small-amplitude oscillationaroundthestateof equilibrium.Wefocus ontheparametricresponsesof thedielectric elastomerballoon.Whenthe voltage is sinusoidal, the balloon resonates at multiple frequencies of excitation, giving rise to superharmonic, harmonic and subharmonic responses. When the frequency of excitation varies continuously, the oscillating amplitude of the balloon may jump, exhibitinghysteresis. c � 2010 Society of Chemical Industry

Large deformation and electromechanical instability of a dielectric elastomer tube actuator

Abstract: This paper theoretically analyzes a dielectric elastomer tube actuator (DETA). Subject to a voltage difference between the inner and outer surfaces, the actuator reduces in thickness and expands in length, so that the same voltage will induce an even higher electric field. This positive feedback may cause the actuator to thin down drastically, resulting in electrical breakdown. We obtain an analytical solution of the actuator undergoing finite deformation when the elastomer obeys the neo-Hookean model. The critical strain of actuation is calculated in terms of various parameters of design. We also discuss the effect of the strain-stiffening on electromechanical behavior of DETAs by using the model of freely joined links.

Poroelasticity of a covalently crosslinked alginate hydrogel under compression

Abstract: This paper studies the poroelastic behavior of an alginate hydrogel by a combination of theory and experiment. The gel—covalently crosslinked, submerged in water, and fully swollen—is suddenly compressed between two parallel plates. The gap between the plates is held constant subsequently, and the force on the plate relaxes while water in the gel migrates. This experiment is analyzed by using the theory of linear poroelasticity. A comparison of the relaxation curve recorded in the experiment and that derived from the theory determines the elastic constants and the permeability of the gel. The material constants so determined agree well with those determined by using a recently developed indentation method. Furthermore, during relaxation, the concentration of water in the gel is inhomogeneous, resulting in tensile hoop stresses near the edge of the gel, and possibly causing the gel to fracture.

Osmotic collapse of a void in an elastomer: breathing, buckling and creasing

Abstract: This paper studies the collapse of a void in an elastomer caused by osmosis. The void is filled with liquid water, while the elastomer is surrounded by unsaturated air. The difference in humidity motivates water molecules to permeate through the elastomer, from inside the void to outside the elastomer, leaving the liquid water inside the void in tension. When the tension is low, the void reduces size but retains the shape, a mode of deformation which we call breathing. When the tension is high, the void changes shape, possibly by two types of instability: buckling and creasing. The critical conditions for both types of instability are calculated. A tubular elastomer collapses by buckling if the wall is thin, but by creasing if the wall is thick. As the tension increases, a thin-walled tube undergoes a buckle-to-crease transition.

Force generated by a swelling elastomer subject to constraint

Abstract: When an elastomer imbibes a solvent and swells, a force is generated if the elastomer is constrained by a hard material. The magnitude of the force depends on the geometry of the constraint, as well as on the chemistry of the elastomer and solvent. This paper models an elastomer crosslinked on the exterior surface of a metallic tubing. The elastomer then imbibes a solvent and swells. After the swollen elastomer touches the wall of the borehole, a significant amount of time is needed for the solvent in the elastomer to redistribute, building up the sealing pressure to the state of equilibrium. The sealing pressure and the sealing time are calculated in terms of the geometric parameters (i.e., the thickness of the elastomer and the radii of the tubing and borehole), the number of monomers along each polymer chain of the elastomer, and the affinity between the elastomer and the solvent.

Nonlinear deformation analysis of a dielectric elastomer membrane-spring system

Abstract: Due to their large strain capability, dielectric elastomers are promising materials for application as transducers in cameras, robots, valves, pumps, energy harvesters, and so on. The dielectric elastomer transducers are based on the deformation of a soft polymer membrane contracting in thickness and expanding in area, induced by the application of a voltage across the two compliant electrodes coated on both sides of the membrane. This paper focuses on the static large deformation analysis of a dielectric elastomer membrane–spring system. The system is constructed from attaching a disk in the middle of a circular dielectric membrane and then connecting the disk with a spring. This configuration can be potentially used as a key part in valves. The basic governing equations describing the large out-of-plane deformations are formulated, and the obtained equations are solved numerically. The relations related to the displacement of the disk, the spring force, the applied voltage, and the parameters of spring including stiffness and initial length are illustrated. The results show that the anticipated displacement of the disk can be controlled by adjusting either or both of the parameters of the spring and the applied voltage. In addition, the parameters of the spring, that is, the stiffness and the initial length, play an important role in the performance of the membrane–spring system.

The Determination of the Location of Contact Electrification-Induced Discharge Events†

Abstract: This paper describes a method for determining the location of contact electrification-induced electrical discharges detected in a system comprising a steel sphere rolling in a circular path on an organic insulator. The electrode of the “rolling sphere tool” monitors, in real time, the separation of charge between the sphere and the organic insulator and the resultant electrostatic discharges. For every revolution of the sphere, the electrometer records a peak, the height of which represents the amount of charge on the sphere. As the charge on the sphere accumulates, the resulting electric field at the surface of the sphere eventually exceeds the breakdown limit of air and causes a discharge. The position of this discharge can be inferred from the relative amplitudes and positions of the peaks preceding and following the discharge event. We can localize each discharge event to one of several zones, each of which corresponds to a geometrically defined fraction of the circular path of the sphere. The fractio...

On-demand and reversible drug release by external cue

Abstract: The invention provides a method to release drugs from a polymer matrix upon demand without degrading the matrix. By applying ultrasound to a self-healable polymer matrix in physiological environment, compounds of both low-molecular and high-molecular weights encapsulated in the matrix are delivered at controlled rates, while the integrity and stiffness of the matrix are unaffected.