Mechanical Characterization Of Ultrathin Film Based On Surface Tension

Thin films are widely used in flexible displays,solar cells,flexible sensors and other devices.Therefore,understanding the mechanical properties of thin films,such as elastic modulus,yield strength and fracture strength,is of great importance to device design and performance optimization.At present,thin films are stretched or indented by atomic force probes or nanoindentation instruments to access their mechanical properties.However,the above methods have certain limitations on studying the overall mechanical properties of ultrathin films.It is often difficult to test ultrathin films in a free-standing state.For ultrathin film on a substrate,the substrate effect on the testing result cannot be ignored.Indentation is limited to a local area,which is doubted to represent the overall mechanical property of the film.Therefore,a new method is needed to measure the mechanical properties of thin films.The surface tension of water can reach 72 m N/m.Therefore,objects of low density can remain afloat on the surface of water.In addition,the viscosity of water at room temperature is only 0.89 m Pa ? s.In this case,objects floating on the liquid surface can slide almost without friction.Studies have shown that ultrathin films can be stretched,bent,or crumpled under the surface tension of water.Therefore,the mechanical properties of thin films can be measured by stretching on water.This work studies a method to characterize the mechanical properties of thin films based on liquid surface tension.The elastic modulus of polydimethylsiloxane(PDMS)ring films with micron thickness was studied.The PDMS annulus floated freely on the surface of water.The surface tension of the inner liquid surface of the ring was reduced by dropping surfactant.In this case,the inner and outer boundaries of the PDMS ring film were subjected to differential surface tension,and the annulus was stretched.In this work,the main factors influencing the stretches of the PDMS annulus were studied,such as the ratio of PDMS,film thickness and the concentration of surfactant.The relation of stretches dependent of time was summarized.Combined with the measurement results of the surface tension of the ring,the process of the tension deformation of the annulus was further discussed.Finally,based on the stretches and surface tension measured in the experiment,the elastic modulus of PDMS was calculated by using a constitutive model of hyperelastic material.This work has proposed a novel characterization method to measure mechanical properties of thin films,which introduces a difference of the surface tension of water to deform the afloat thin film.Using the water surface as the underlying support avoids the substrate effect of the test results.The method proposed here is simple to operate,and does not need the assistance of large precision instruments.The method has the potential to measure the mechanical properties of various kinds of thin films.What's more,this method is convenient for the complete transfer of free-standing thin film materials,and provides a new method and idea for the fabrication of large-size flexible devices.Therefore,the method of mechanical properties characterization of thin films based on liquid surface tension has wide application value and important research significance.