Investigations on the Adhesive Contact Behaviors between a Viscoelastic Stamp and a Transferred Element in Microtransfer Printing

Microtransfer printing is a sophisticated technique for the heterogeneous integration of separately fabricated micro/nano-elements into functional systems by virtue of an elastomeric stamp. One important factor influencing the capability of this technique depends on the adhesion between the viscoelastic stamp and the transferred element. To provide theoretical guidance for the control of adhesion in the transfer printing process, a finite element model for the viscoelastic adhesive contact between a polydimethylsiloxane (PDMS) stamp and a spherical transferred element was established, in which the adhesive interaction was modeled by the Lennard-Jones surface force law. Effects of the unloading velocity, preload, and thermodynamic work of adhesion on the adhesion strength, characterized by the pull-off force, were examined for a loading-dwelling-unloading history. Simulation results showed that the unloading path deviated from the loading path due to the viscoelastic property of the PDMS stamp. The pull-off force increased with the unloading velocity, and the increasing ratio was large at first and then became low. Furthermore, the influence of the preload on increasing the pull-off force was more significant under larger unloading velocity than that under smaller unloading velocity. In addition, the pull-off force increased remarkably with the thermodynamic work of adhesion at a fixed maximum approach.

Composite material based on the polyethylene terephthalate polymer and modified fly ash filler

The possibility of filling the recycled polyethylene terephthalate (rPET) with fly ash was studied to make a polymer composite material (PCM). It is shown that high adhesion between polymeric matrix and mineral filler is the key parameter to produce high performance PCM. For this purpose the acid-basic interaction as well as the thermodynamic work of adhesion between components of PCM were calculated. The technique of modifying fly ash filler with 5% concentration solution of sulfuric acid to increase acid-basic interaction has been elaborated. The resulting behavioral patterns are listed and compared to those of composites containing untreated fly ash particles.

Electrostatic and Capillary Effects on the Detachment of Particles With Surface Deformation in Turbulent Flows

Rolling detachment of micro particles in turbulent flows under the presence of electrostatic and capillary forces was studied. The maximum adhesion resistance model and the effective thermodynamic work of adhesion including the effects of electrostatic and capillary forces were used in the analysis. The JKR and DMT models for elastic interface deformations and the Maugis-Pollock model for the plastic deformation were extended to include the effect of electrostatic and capillary forces. The turbulence burst model was used to evaluate the airflow velocity near the substrate. The critical shear velocities for removal of particles of different sizes were evaluated and the results were compared with those without electrostatic and capillary forces. The relative critical shear velocities as well as the material dependence were also studied. The effect of the direction of the combined Coulomb force was also included. The predictions of the electric detachment fields for particles were compared with the available experimental data and good agreement was observed.

Micro Particle Detachment in Turbulent Flows With Electrostatic and Capillary Effects and Surface Deformation

Rolling detachment of micro particles in the presence of electrostatic and capillary forces based on the maximum adhesion resistance was studied. The effective thermodynamic work of adhesion including the effects of electrostatic and capillary forces was used in the analysis. The JKR and DMT models for elastic interface deformations and the Maugis-Pollock model for the plastic deformation were extended to include the effect of electrostatic and capillary forces. Under turbulent flow conditions, the turbulence burst model was used to evaluate the airflow velocity near the substrate. The critical shear velocities for removal of particles of different sizes were evaluated and the results were compared with those without electrostatic and capillary forces. It shows that the capillary forces significantly increases the critical shear velocities for particles of all sizes, while the electrostatic forces only have major effects on large particles. The model predictions were compared with the available experimental data and good agreement was observed.

Quantitative adhesion measures of multilayer films: Part II. Indentation of W/Cu, W/W, Cr/W

Sputtered copper and tungsten thin films both with and without tungsten and chromium superlayers were tested by using nanoindentation probing to initiate and drive delamination. The adhesion energies of the films were calculated from the induced delaminations using the analysis presented in “Quantitative adhesion measures of multilayer films: Part I. Indentation mechanics.” Copper films ranging in thickness from 150 to 1500 nm in the as-sputtered condition had measured adhesion energies ranging from 0.2 to 2 J/m2, commensurate with the thermodynamic work of adhesion. Tungsten films ranging in thickness from 500 to 1000 nm in the as-sputtered condition had measured adhesion energies ranging from 5 to 15 J/m2. The superlayer was shown to induce radial cracking when under residual tension, resulting in underestimation of the adhesion energy when the film was well adhered. Under conditions of weak adherence or residual compression, the superlayer provided an excellent means to induce a delamination and allowed an accurate and reasonably precise quantitative measure of thin film adhesion.

Contact-Mechanics-Based Studies of Adhesion between Polymers

Contact mechanics deals with the deformation of solid bodies in contact. In recent years, significant advances have been made both in the theoretical and experimental areas of contact mechanics, especially in the area of soft solids, in relating the contact deformation to interfacial adhesion. On the theoretical front, new theories of contact mechanics have been developed to relate the interfacial force induced deformation to the thermodynamic work of adhesion both for elastic and viscoelastic solids. On the experimental front, several new techniques have been developed to measure the interfacial forces and the interfacial-force-induced deformations. These techniques have been used, with the aid of the theories of contact mechanics, to measure directly the surface and interfacial energies of a variety of polymers and other model surfaces. These experimental and theoretical developments have also been exploited to measure quantitatively the effect of interfacial chain diffusion on the adhesion of polymer interfaces. We summarize the recent developments in the theories of contact mechanics, and their applications in the design and interpretation of experimental measurement of molecular level adhesion between elastomers, glassy and viscoelastic polymers. We also review the experimental and theoretical developments related to the role of chain diffusion on interfacial adhesion. Finally, we identify some potential new applications of contact-mechanics-based techniques in such emerging area of adhesion science as molecular level studies of adhesion of viscoelastic materials and biomaterials.