Study And Application Of Droplet Spreading Induced Wrinkling Of Nanofilms

As compared to the commonly adopted strategy for creating wrinkles by direct or indirect mechanical compression,the approach of relying elastocapillary effect between a liquid drop and soft materials possesses the advantages of ease of sample preparation,readiness of operation and applicability to different types of materials,which provides a unique solution for the cutting-edge research in wrinkling creation and its applications.However,the possibility of dynamic wrinkling caused by capillary elasticity has not been reported.Based on the above background,this thesis innovatively integrates the subjects of "droplet spreading on solid surface" with"elastocapillarity phenomenon",and proposes a novel idea that uses the elastocapillary effect due to the spreading of low surface energy liquid drop on a polymeric thin film to create and control dynamic wrinkling.Moreover,with quantitative measurement of the length development of the dynamic wrinkles,a new wrinkling-based method is developed to enable the mechanical property characterization of polymeric nanofilms.The new mechanism of dynamic wrinkling proposed in this work using the capillary elasticity of droplets in the spreading process not only provides a new technology with convenient operation and low price for the characterization of mechanical properties of nanofilms,a challenging task in the field of materials research.More importantly,it establishes a platform and paves a new way for the theoretical and applied research in dynamic wrinkling.The specific achievements include:(1)This thesis takes the polymer nanofilm floating on the water surface as the research object.With deposition of a silicone oil drop on a polymeric nanofilm floating on water surface,this thesis demonstrates that the elastocapillary effect associated with the drop spreading is able to deform the nanofilm and generate highly regular structured dynamic wrinkling patterns.On this basis,combined with image processing,a quantitative measurement method for the change of dynamic wrinkle length and number with droplet spreading radius was established,and its formation rule was further studied,and finally the mechanical properties of polymer nanofilms were characterized and tested.Specifically,we relied on the widely studied polystyrene(PS)nanofilms as the model system,and utilized the spin-coating and capillary transfer method to float a large-sized circular-shaped PS nanofilms on water surface.With the home-built setup consisting of a microinjection pump,silicone oil conveying pipeline,micro needle,stepper motor for droplet positioning,and lighting device and CCD camera,we were able to in-situ observe the spreading of silicone oil drop of different viscosity on the PS film and quantify the correspondingly generated dynamic wrinkling patterns.It has been found that the wrinkle length caused by the dynamic spreading of silicone oil drop increases first and then decreases with the contact radius of droplets.This nonlinear wrinkling length development behavioris significantly different from the supposed linear relationship as reported previously for the static wrinkling of polymeric thin film induced by water drop.(2)Furthermore,on the basis of the theories previously developed for the static wrinkling induced by a liquid drop,we established an analytical framework that allows for modeling the dynamic development of the liquid drop spreading induced wrinkles.This framework lays a foundation for an in-depth understanding of the unique evolution behavior of the dynamic wrinkling induced by droplet spreading.With assistance of the theoretical framework established in this thesis,we can determine the tensile modulus of a nanofilm by numerically fitting the experimentally observed nonlinear relationship between the wrinkle length and the droplet contact radius,which inherently offers the required statistical repeatability as desired in a mechanical testing method.This new method was successfully applied to characterize the elastic modulus of PS nanofilms.The results were consistent with those measured by DMA tensile testing and the wrinkling-based method by water drop induced static wrinkling analysis,which validated the dynamic-wrinkling approach developed in this thesis.(3)In order to demonstrate the flexibility of the new method developed herein,we further utilized the dynamic wrinkling-based mechanical property characterization technique to measure the elastic modulus for the polydopamine(PDA)nanofilms insitu polymerized at the water/air interface.This is the first demonstration for the insitu mechanical characterization of PDA films,which cannot be otherwise readily achieved through the conventional methods,such as nanoindentation and bulging test.In conclusion,this thesis innovatively combines the physics of droplet spreading with thin film wrinkling to develop an experimental method and establish a theoretical framework for quantitatively investigating the dynamic wrinkling phenomenon induced by drop spreading on a polymeric thin film.This leads to a novel dynamicwrinkling based mechanical characterization technique to enable the tensile modulus measurement of nanofilms with ease of operation,sample preparation and low-cost characteristics.The work presented here not only provides a basis for the study of mechanical properties of nanofilms,but also offers a means for the experimental measurement and theoretical analysis of the wrinkling behavior caused by capillary force,which will further promote the theoretical and application research of electrocapillarity in the fields of soft materials,nanotechnology and biology.