In recent years, the control of ionic currents has come to be recognized as one of the most important issues related to the efficient transport of single molecules and microparticles in aqueous solutions. However, the complicated liquid flows that are usually induced by applying electric potentials have made it difficult to address a number of unsolved problems in this area. In particular, the nonequilibrium phenomena that occur in electrically non-neutral fields must be more thoroughly understood. Herein, we report on the development of a theoretical model of liquid flows resulting from ion interactions while focusing on the so-called electrohydrodynamic (EHD) flow. We also discuss the development of an experimental system to optically and electrically observe EHD flows using a 1 mm 2 cross-section pore placed in an ion-exchange membrane where cation and anion flows can be separated without the use of a charged environment. Although micro/ nanosized flow channels are usually applied to induce electric double layer overlaps to utilize strong electroosmotic effects, our system does not require such laborious fabrication processes. Instead, we visualize EHD flows by using a millimeter size pore immersed in an alkaline aqueous solution. In this setup, liquid flows passing through the pore along the direction of ion flow, whose velocity reaches on the order of 1 mm/s, can be clearly observed by applying a few volts of electric potential. Furthermore, the transient phenomena associated with ionic responses are theoretically elucidated.

Adhesive particles on polished wafers are detached by e.g. scrubbing, and removed by liquid flow from the wafer. However, the relationship between the characteristic of liquid flow and the removal of detached particles has not yet been examined in detail. Therefore, the re-adhesion of detached particles to wafer surfaces in liquid flow on rotating wafers was experimentally investigated. The number of residue particles on a wafer was counted using a defect inspection tool after de-ionized water (DIW) rinse for particle removal. To discuss the mechanism of particle removal, a model was constructed and compared with experimental results. The model is based on boundary layer theory of fluid dynamics and advection diffusion theory of transport phenomena for particle removal in liquid flow near the wafer surface. The model results confirmed that detached particles in liquid flow moved into the sublayer and re-adhered to the wafer surface by diffusion. Moreover, particles in the sublayer, where the liquid velocity is several hundred um/s, could not be moved from the sublayer and removed from the wafer. It was also found that the number of particles re-adhering to the wafer surface depends on the Sherwood number.

Electrohydrodynamic (EHD) flow is a type of liquid flow driven by an external electric force. In electrolyte solutions, anions and cations usually interact with each other to maintain electroneutrality. Under such a condition, it is difficult to drive a liquid flow by applying electric potentials on the order of 1 V; at least a few tens of volts is required to generate EHD flows, which may not be preferable for aqueous solutions. In this study, we propose a novel method of generating a liquid flow through a channel with cross-sectional dimensions of 1 × 1 mm2, which is placed in an ion exchange membrane to separate the cation and anion transport pathways. When the optimized design of the experimental apparatus was used, EHD flows were successfully generated in aqueous solutions by applying a relatively low electric potential of 2.2 V, and the flow velocity was measured over a wide range of electrolyte concentrations by particle image velocimetry. It was found that high concentration gradients caused the rapid discharge of ions passing through the channel and contributed to achieving a flow speed on the order of 1 mm/s. EHD flows were also theoretically explained using the Navier–Stokes equations to model an ion-drag flow driven by nonequilibrium ion transport in external electric fields. This flow generation method is practical only when ion transport pathways are well controlled and effectively rectified. The present findings will lead to the development of a promising technology to control liquid flows in multiscale fluidic channels.

Liquid flows driven by electric force is known as electrohydrodynamics (EHD). EHD flows are expected to be applied to micropumps, microactuators, and mixing devices. However, it is known that conventional EHD flows require at least tens of volts of the applied voltage. In this study, a novel device is developed to generate an EHD flow under a constant current condition with a few voltages. An ion-exchange membrane that has a small pore is set in a reservoir to separate the flow path of anion and cation. The reservoir is filled with NaOH aqueous solution and a constant electric current is applied across the membrane. When the cross sectional area of the ion-exchange membrane is 200 times larger than that of the pore, EHD flows observed in the pore become faster than those in previous studies. The maximum value of the flow velocity reaches 3 mm/s by applying a constant current of 0.8 mA.

A tangible augmented reality (AR) system was developed to visualize electromagnetic cascades through solid materials. 3-D cascade images calculated by the EGS4 simulation software are overlaid on a USB camera image almost in real time. Learners can observe electromagnetic cascades while handling blocks of real shielding materials. This system was demonstrated at a public science event. By way of a questionnaire survey, it appeared that it was the first time to hear about electromagnetic cascades for about the 90% of the child participants and that they felt the system made it easy for them to understand how cascades change with differences in both density of shielding materials and incident energies. Some teaching assistants, however, stated that some children had difficulty understanding the incident direction of photons. A new function called time evolution was added to show growing electromagnetic cascades. This system was improved to be suitable and effective tools for primary school educators to promote the radiation awareness by addition of the time-evolution function.

No abstract

To drive electrohydrodynamic (EHD) flows in aqueous solutions, the separation of cation and anion transport pathways is essential because a directed electric body force has to be induced by ionic motions in liquid. On the other hand, positive and negative charges attract each other, and electroneutrality is maintained everywhere in equilibrium conditions. Furthermore, an increase in an applied voltage has to be suppressed to avoid water electrolysis, which causes the solutions to become unstable. Usually, EHD flows can be induced in non-aqueous solutions by applying extremely high voltages, such as tens of kV, to inject electrical charges. In this study, two methods are introduced to generate EHD flows induced by electrical charge separations in aqueous solutions, where two liquid phases are separated by an ion-exchange membrane. Due to a difference in the ionic mobility in the membrane, ion concentration polarization is induced between both sides of the membrane. In this study, we demonstrate two methods. (i) The relaxation of ion concentration gradients occurs via a flow channel that penetrates an ion-exchange membrane, where the transport of the slower species in the membrane selectively becomes dominant in the flow channel. This is a driving force to generate an EHD flow in the liquid. (ii) A long waiting time for the diffusion of ions passing through the ion-exchange membrane enables the generation of an ion-dragged flow by externally applying an electric field. Ions concentrated in a flow channel of a 1 x 1 mm cross-section determine the direction of the liquid flow, corresponding to the electrophoretic transport pathways. In both methods, the electric voltage difference required for an EHD flow generation is drastically reduced to near 2 V by rectifying the ion transport pathways.

No abstract