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CORRELATION BETWEEN SURFACE FREE ENERGY AND SURFACE CONSTITUTION

Manoj K. Chaudhury and George M. Whitesides

Posted by Grant Gonzalez Fall 2012

Key Terms

Surface energy, Young's Equation, Contact Angles

Abstract

"Self-assembled monolayers (SAs) of alkylsiloxanes on elastomeric PONS (polydimethylsiloxane) were used as model systems to study interactions between surfaces. Surface free energies of these chemically modified surfaces were estimated by measui'ing the deformations that resulted from the contact between small semispherical lenses and flat sheets of the elastomer under controlled loads. The measured surface free energies correlated with the surface chemical compositions of the SAMs and were commensurate with the values estimated from the measurements of contact angles. This study provides, for the first time, direct experimental evidence for the validity of estimates of the surface free energies of low-energy solids obtained from contact angles."

Introduction

Spreading or wetting of a liquid on a surface is driven by lower interfacial energy. Therefore, a deformation occurs when a curved surface meets a flat one. This paper attempts to measure the surface tension of specific surface using contact angles.

Materials and Methods

PDMS is a smooth and compliant material ideal for testing. The experimental system is comprised of a semispherical lens and a flat sheet of PDMS. In a typical experiment (Figure 1), the lens was slowly brought into contact with the flat sheet, and an external load was then applied. A spontaneous deformation occurred at the contact zone as soon as the lens touched the flat sheet. This deformation increased in response to the external loads. Deformation was measured as a function of applied load. After compression, a decompression step occurred.

Results

Generally there was finite but minute hysteresis in contact deformations as obtained from the compression and decompression cycles. This hysteresis resulted in two different values of surface tension for a specific surface. In all cases, the average of the advancing and receding contact angles to estimate the surface tension. As the only interaction occur due to van der Waals intersactions between the surface, only the force due to van der Waals are calcualted. As PDMS is easlity fuctilized, other interactions may be measured.

Conclusions

This studies demonstrates that self-assembled alkylsiloxane monolayers on PDMS are excellent model systems for studying adhesion between surfaces. The method of deformations in a compliant polymeric lens and sheet is sensitive to small variations in the compositions of the surface. Contact angles were used to measure the surface energy. While the method focused on studying systems that interact primarily via van der Waals forces.