Design And Applications Of Smart Responsive Materials Based On Liquid Crystal Microdroplet Elements

Liquid crystals are a typical functional soft matter,possessing both the fluidity of liquids and the ordering of crystals.Liquid crystals show rich self-assembled microstructures and unique physical and chemical properties.Liquid crystal-based functional materials not only own the optical properties of liquid crystals,but also enable response to stimulations such as light,temperature,mechanical force,electric and magnetic fields.Despite of the broad applications of liquid crystal-based functional materials in displays,liquid crystal-based functional materials are generally limited by the shortcomings,such as impurities,static electricity and ununiform thickness,due to conventional mechanical rubbing method to orientate liquid crystal molecules,which cause the liquid crystal-based functional materials an undesirable control over product quality.These challenges can be circumvented by surfactantinduced ordering of liquid crystal microdroplets,where liquid crystal microdroplets are dispersed in polymer containing surfactant such as sodium dodecyl sulfate(SDS)and polyvinyl alcohol(PVA).Such a uniform surfactant layer on the microdroplet surface can effectively induce the ordering of liquid crystal molecules.Moreover,the self-assembled microstructures of liquid crystals are tunable according to the composition of the surfactant.Together with microfluidic-3D printing techniques that enable digital deposition of droplet,liquid crystal microdroplets can serve as functional elements for fabricating optically functional materials.In this thesis,we explore a series of liquid crystal microdroplets by precisely controlling their size,composition and three-dimensional arrangement.Next,we demonstrate the responsive properties of liquid crystal microdroplets under the stimuli including electric field,temperature,mechanical force,and light.Finally,we design and fabricate various liquid crystal microdroplet-based smart materials with great potential application in soft robots,smart actuators and biomimetic systems.The main content and the obtained innovative results are described as follows:(1)The electric-optic response of the oblate nematic liquid crystal microdroplets is investigated.In contrast to bipolar spherical liquid crystal microdroplets,the bipolar oblate liquid crystal microdroplets show unique electric-optic response properties under electric field.Their bipolar axes will not recover to low-energy horizontal configuration after removing threshold electric fields,the bipolar structures are trapped in a local energy minimum.The oblate nematic liquid crystal microdroplets own the stable electric response property under electric fields and the application of the oblate nematic microdroplets in designing dual-stable PDLC is expected to overcome the disadvantages of traditional PDLC’s energy consumption and instability.(2)The interactions between the liquid crystal molecules and the amphiphilic phospholipid molecules at the liquid crystal/ water interface are investigated.We found the local high density of the phospholipid molecules at the liquid crystal/water interface during the crystals and growths process of the smectic phase layers.Investigations on the correlation between phospholipid density,surface tension and smectic liquid crystal ordering suggest that when domains of smectic liquid crystal layers nucleate and grow from isotropic at the liquid crystal /water interface as chloroform slowly evaporates at room temperature,phospholipids transition from liquid-expanded to liquid-condensed phases in response to the smectic ordering,which induces a higher surface tension at the interface and thus there is local high density of phospholipid molecules at the liquid crystal/water interface.(3)Inspired by the camouflage of an octopus via the elongation or contraction of its pigment cells,oblate cholesteric liquid crystal microdroplets are dispersed in the polymer matrix,serving as the role of pigment cells and showing structural color due to selective Bragg reflection by their periodic helical structure.The color of 3D-printed biomimetic systems could be tuned by changing the helical pitches via the chiral dopant concentration or temperature.When oblate liquid crystal microdroplets are heated up to isotropic,opaque and colored biomimetic systems become transparent and colorless.Meanwhile,isotropic liquid crystal microdroplets tend to become spherical,causing volume contraction along the film plane and volume dilation in the perpendicular direction.The internal strain combined with the gradient distribution of oblate isotropic liquid crystal microdroplets result in the corresponding shape transformations.The camouflage of a biomimetic octopus and the blossom of a biomimetic flower,both of which show synergetic color and shape responses,are demonstrated to inspire the design of functional materials and intelligent devices.(4)We present a facile method to increase the selective Bragg reflection region of cholesteric liquid crystal microdroplets dispersed in carbomer film by bidirectional stretching,and reversibly switch the film between random-scattering-dominant and Bragg-reflectiondominant states.By dispersing the cholesteric liquid crystal microdroplets in calcium alginate three-dimensional hydrogel,we design the 3D network hydrogel dispersed liquid crystal microdroplets.The structural color between random-scattering-dominant and Bragg-reflectiondominant states is switched by the reversible hydrated and dehydrated process of calcium alginate hydrogel,and various smart responsive actuators are designed by 3D-printing platform.(5)We serve liquid crystal microdroplets as functional elements,and precisely control over size,composition and three-dimensional arrangement of the microdroplets by microfluidic3D-printing.Embedding the microdroplets in elastomer materials,a series of functional materials are designed.Making use of the characteristics of microdroplets,various deformations of the composite elastomers triggered by temperature,solvent and light are designed.The biomimetic properties of the microdroplets based composite elastomers are investigated,and various soft robots and functional materials are designed,which pave the way for the application of 3D microfluidic printing platform in designing functional materials.In this thesis,liquid crystal microdroplets serve as functional elements,combining with the platform of microfluidic 3D printing.A series of stimuli-responsive materials are designed based on liquid crystal microdroplets,which pave the way for the development and applications of multiple-functional smart devices in the future.