بررسي رفتار سورفكتان ها تحت جريان برشي با روش DPD

چكيده انگليسي :

99Investigation of Surfactants Behavior under Shear Flow with Dissipative Particle Dynamics Faezeh Looki Anaraki f lanaraki @yahoo com February 25 2012 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language FarsiSupervisor Dr Ahmadreza Pishevar pishe@cc iut ac ir Abstract Amphiphilic surfactants are important components in many applications such as the detergents cleaningagents emulsifiers in food pharmaceuticals cosmetics and also drag reduction Whereas a shear flowapplied on a surfactant solution influences the self assembled structure of surfactant molecules understanding the shear effects on the surfactant molecular assemblies is of importance This great interest isbeing reflected in the large number of experimental and theoretical studies The study of these self assembling systems however is quite challenging because of the different length scales and the associateddifferent times scales involved in the problem which implies large system sizes and long simulations Therefore we need a multi scale scheme that spans the required length and time scales at the same timepreserving the important aspects of the atomistic model The goal of this thesis is to investigate the behaviorof surfactant under shear flow by dissipative particle dynamics Dissipative particle dynamics DPD is anemerging method for simulating problems at mesoscopic time and length scale Studying the DPDmethodology based on dimensionless numbers and expressing its scales in terms of physical units can openways to understand the physical behavior of the system and to model practical problems In the DPD methodthe surfactant is modeled using particles which connected by harmonic springs Self assembly of surfactantmolecules is investigated to form a wide range of structures such as spheres cylinders bilayers and doublebilayers Then Shear flow is applied on a surfactant solution by means of Lees Edwards boundaryconditions The rheological behavior of surfactant such as the morphology of micelles the shear ratedependence of shear viscosity of the micellar solution and viscoelasticity of solution in differentconcentrations solutions and different shear rates are examined The results has been shown that the first shear thickening at low shear rate is accrued which is caused by thegrowth of spherical micelles while the consequent shear thinning for higher shear rates is caused byorientation of micelles by increasing the shear rate Results showed the stretching of spherical micelles in theflow direction under shear flow and the occurring a microstructural transition in high shear rates fromspherical to rod like structure We also measured the first and second normal stress difference The resultsshow that the first normal stress difference increases as shear rate raises whereas the second normal stressdifference N2 is almost zero The first normal stress difference is increased as concentration increasedbecause of viscoelasticity effects As a next step dissipative particle dynamics method is used for exploiting the behavior of surfactants underpoiseuille flow Morphology of micelles velocity profile and density profile are examined and it will beshown that micelles migrate to the regions with maximum velocity and stretched in the flow direction Thisbehavior is shown in both reverse poiseuille flow and poiseuille flow with solid boundary The steady velocity profiles are calculated The results illustrate that when surfactants are added to the water the velocity profiles will be dampened and when driven force are raised the average velocity will beincreased In high concentration of surfactant velocity profile will be flatted because of viscoelasticity effects Key Words Surfactant Shear flow Dissipative particle Dynamics Method Viscosity Viscoelastic fluid Poiseuille flow

لوكي اناركي، فائزه

بررسي رفتار سورفكتان ها تحت جريان برشي با روش DPD

ضريب لزجت , سيال ويسكوالاستيك , جريان پويزويل