Influence of the Polarity of the Electric Field on Electrorheometry

Uniaxial extensional flow is a canonical flow typically used in rheological characterization to provide complementary information to that obtained by imposing simple shear flow. In spite of the importance of having a full rheological characterization of complex fluids, publications on the rheological characterization of mobile liquids under extensional flow have increased significantly only in the last 20 years. In the case of the rheological characterization of electrorheological fluids, the situation is even more dramatic, as the ERFs have been exclusively determined under simple shear flow, where an electrorheological cell is attached to the rotational rheometer generating an electric field perpendicular to the flow direction and that does not allow for inverting the polarity. The very recent work published by Sadek et al., who developed a new electrorheological cell to be used with the commercial Capillary Breakup Extensional Rheometer (CaBER), allows for the very first time performing electrorheometry under extensional flow. By means of the same experimental setup, this study investigates the influence of the polarity of the imposed electric field on the filament thinning process of a Newtonian and an electrorheological fluid. Results show that a polarity against the gravity results in filament thinning processes that live longer or reach a stable configuration at lower intensities of the applied electric field

Optimal control-based inverse determination of electrode distribution for electroosmotic micromixer

This paper presents an optimal control-based inverse method used to determine the distribution of the electrodes for the electroosmotic micromixers with external driven flow from the inlet. Based on the optimal control method, one Dirichlet boundary control problem is constructed to inversely find the optimal distribution of the electrodes on the sidewalls of electroosmotic micromixers and achieve the acceptable mixing performance. After solving the boundary control problem, the step-shaped distribution of the external electric potential imposed on the sidewalls can be obtained and the distribution of electrodes can be inversely determined according to the obtained external electric potential. Numerical results are also provided to demonstrate the effectivity of the proposed method

Electric field induced self-assembly of mesoscale structured materials and smart fluids

This dissertation aims to study the forces that drive self-assembly in binary mixtures of particles suspended in liquids and on fluid-liquid interfaces when they are subjected to a uniform electric or magnetic field. Three fluid-particle systems are investigated experimentally and theoretically : (i) Suspensions of dielectric particles in dielectric liquids; (ii) Suspensions of ferromagnetic and diamagnetic particles in ferrofluids; and (iii) Dielectric particles on dielectric fluid-liquid interfaces. The results of these studies are then used to estimate the parameter values needed to assemble materials with desired mesoscale microstructures. The first fluid-particle system studied is an electrorheological (ER) fluid formed using a mixture of positively and negatively polarizable particles. An important property of ER fluids is that their rheological properties can be modified on demand, within a few milliseconds, by applying an external electric field. Then, after the field is switched off, they go back to their original state. However, if only positively or negatively polarizable particles are used, the distribution of particles will fragment into chains and columns. Experimental results show that if a suitable mixture of positively and negatively polarized particles is used for making the ER fluid, the particle chains come closer, and the volume they occupy decreases. These results agree with the direct numerical simulations (DNS) based on the Maxwell Stress Tensor (MST) and point dipole methods. The application of the electric field results in the formation of a closely packed three-dimensionally connected structure. The influence of varying the electric field intensity, particle size, polarizabilities, and number ratio are characterized in terms of the extent of connected pattern formation which is obtained numerically and the experimentally measured yield stress. The yield stress for an ER fluid formed using a particle mixture is larger than that for an ER fluid containing only one type of particles and is maximum for a critical volume fraction. The second problem studied is the magnetorheological fluids (MR) formed using mixtures of micron-sized iron and glass particles in a liquid. The rheological behavior of MR and ER fluids is similar. For example, when an external magnetic field is applied to a MR fluid, the particles are magnetized and rearrange relative to one another, which modifies its rheological properties almost instantly. Also, when only one type of particles is used to prepare MR fluids, i.e., either positively or negatively magnetized particles, the particle distribution becomes fragmented into chains and columns. If a suitable mixture of positively and negatively magnetized particles is used, individual particle chains of one type attract the other type, creating a band with no gaps. This results in the formation of a closely packed connected structure. The MR fluids’ yield stress behavior is experimentally investigated, formed by suspending mixtures of ferromagnetic and diamagnetic particles in ferrofluids (FF), which show that the yield stress is maximum when the volume fraction of ferromagnetic particles is around sixty percent. The rheological response of MR fluids depends on parameters such as the particles’ concentration, magnetic susceptibilities of the suspending liquid, and the applied magnetic field intensity. The third problem investigated is that of making UV-cured thin films with embedded monolayers of gold particles on their surfaces. This is achieved by self-assembly of gold nanoparticles on a UV curable liquid’s surface by applying an electric field normal to the surface. The substrates are then used for Surface-Enhanced Raman Scattering (SERS) applications. The experimental results show that the substrates’ performance depends on the particle concentration and the inter-particle distance. The laboratory-built substrates are found to be more efficient than the commercial SERS substrates

Direct numerical simulations and experimental investigation of dielectrophoresis

This dissertation deals with the numerical and experimental studies of the phenomenon of dielectrophoresis, i.e., the motion of neutral particles in nonuniform electric fields. Dielectrophoresis is the translatory motion of neutral particles suspended in a dielectric medium when they are subjected to an external nonuniform electric field. The translatory motion occurs because a force called the dielectrophoretic force, which depends on the spatial variation of the electric field, acts on the particles. As the generation of force involves no moving parts and the particles can be moved without touching them, dielectrophoresis can be used in many applications, including manipulation and separation of biological particles, manipulation of nanoparticles, etc. In the present study, the numerical simulations of the fluid-particle system are performed using a direct numerical simulation scheme based on the distributed Lagrange multiplier method. In this scheme, the fluid flow equations are solved both inside and outside the particle boundaries and flow inside the particle boundary is forced to be a rigid body motion by using the distributed Lagrange multiplier. The electrostatic force acting on the particles is computed using the point dipole method. The scheme is used to study the behavior of particles in the suspension under the influence of a nonuniform electric field. The numerical scheme is used to study the influence of a dimensionless parameter, which is the ratio of electrostatic particle-particle interactions and dielectrophoretic force, in the dynamics of particle structure formation and the eventual particle collection. When this parameter is of order one or greater, which corresponds to the regime where particle-particle interactions are comparable in magnitude to the dielectrophoretic force, simulations reveal that the particles form interparticle chains and the chains then move to the electrode edges in the case of positive dielectrophoresis. When this parameter is of order ten the particles collect in the form of chains extending from one electrode to the opposite one clogging the entire domain. On the other hand, when this parameter is less than order one, particles move to the electrode edges individually and agglomerate at the edges of the electrodes. The results of numerical simulations are verified experimentally using a suspension of viable yeast cells subjected to dielectrophoresis using microelectrodes. The experiments show that at frequencies much smaller than the crossover frequency where the value of the above parameter is greater than order one, the yeast particles form chains and then move and collect at the electrode edges. Where as, at frequencies closer to the crossover frequency where the value of the parameter is less than order one, particles move individually without forming chains and agglomerate at the electrode edges. The numerical simulation scheme is also used to study the dielectrophoresis of nanoparticles. Simulations show that in a uniform electric field the Brownian force is dominant and results in the random scattering of the particles. In the case of nonuniform electric field, it is possible to overcome the Brownian force and collect the particles at pre-determined locations, even though the trajectories of the particles are influenced by Brownian motion. Finally, the method of images is used to improve the electric field solution when the particles are close to the domain walls. Simulations performed for uniform electric fields with the method of images shows that when the distance between the particle and domain boundary is of the order of particle diameter the influence of the particles on the electric field boundary conditions is significant

A Review of Smart Materials in Tactile Actuators for Information Delivery

As the largest organ in the human body, the skin provides the important sensory channel for humans to receive external stimulations based on touch. By the information perceived through touch, people can feel and guess the properties of objects, like weight, temperature, textures, and motion, etc. In fact, those properties are nerve stimuli to our brain received by different kinds of receptors in the skin. Mechanical, electrical, and thermal stimuli can stimulate these receptors and cause different information to be conveyed through the nerves. Technologies for actuators to provide mechanical, electrical or thermal stimuli have been developed. These include static or vibrational actuation, electrostatic stimulation, focused ultrasound, and more. Smart materials, such as piezoelectric materials, carbon nanotubes, and shape memory alloys, play important roles in providing actuation for tactile sensation. This paper aims to review the background biological knowledge of human tactile sensing, to give an understanding of how we sense and interact with the world through the sense of touch, as well as the conventional and state-of-the-art technologies of tactile actuators for tactile feedback delivery

Optimal Control-Based Inverse Determination of Electrode Distribution for Electroosmotic Micromixer

This paper presents an optimal control-based inverse method used to determine the distribution of the electrodes for the electroosmotic micromixers with external driven flow from the inlet. Based on the optimal control method, one Dirichlet boundary control problem is constructed to inversely find the optimal distribution of the electrodes on the sidewalls of electroosmotic micromixers and achieve the acceptable mixing performance. After solving the boundary control problem, results are also provided to demonstrate the effectiveness of the proposed method; the step-shaped distribution of the external electric potential imposed on the sidewalls is obtained, and the electrodes with an interlaced arrangement are inversely derived according to the obtained external electric potential

RESEARCH TOWARDS THE DESIGN OF A NOVEL SMART FLUID DAMPER USING A MCKIBBEN ACTUATOR

Vibration reducing performance of many mechanical systems, decreasing the quality of manufactured products, producing noise, generating fatigue in mechanical components, and producing an uncomfortable environment for human bodies. Vibration control is categorized as: active, passive, or semi-active, based on the power consumption of the control system and feedback or feed forward based on whether sensing is used to control vibration. Semi-active vibration control is the most attractive method; one method of semi-active vibration control could be designed by using smart fluid. Smart fluids are able to modify their effective viscosity in response to an external stimulus such as a magnetic field. This unique characteristic can be utilised to build semi-active dampers for a wide variety of vibration control systems. Previous work has studied the application of smart fluids in semi-active dampers, where the kinetic energy of a vibrating structure can be dissipated in a controllable fashion. A McKibben actuator is a device that consists of a rubber tube surrounded by braided fibre material. It has different advantages over a piston/cylinder actuator such as: a high power to weight ratio, low weight and less cost. Recently McKibben actuator has appeared in some semi-active vibration control devise. This report investigates the possibility of designing a Magnetorheological MR damper that seeks to reduce the friction in the device by integrating it with a McKibben actuator. In this thesis the concept of both smart fluid and McKibben actuator have been reviewed in depth, and methods of modelling and previous applications of devices made using these materials are also presented. The experimental part of the research includes: designing and modelling a McKibben actuator (using water) under static loads, and validating the model experimentally. The research ends by presenting conclusions and future work

NSF/ESF Workshop on Smart Structures and Advanced Sensors, Santorini Island, Greece, June 26-28, 2005: Structural Actuation and Adaptation Working Group

This document is a result of discussions that took place during the workshop. It describes current state of research and development (R&D) in the areas of structural actuation and adaptation in the context of smart structures and advanced sensors (SS&AS), and provides an outlook to guide future R&D efforts to develop technologies needed to build SS&AS. The discussions took place among the members of the Structural Actuation and Adaptation Working Group, as well as in general sessions including all four working groups. Participants included members of academia, industry, and government from the US and Europe, and representatives from China, Japan, and Korea