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RESEARCH

 Contact line motion

 Gels can be jetted!

 Gel atomization

 Thermal conductivity

 Spreading of suspension

 Gel structure change

 Preparation of spheroids

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 • Contact line motion

 Studies on a contact line motion are important in many industrial processes such as inkjet printing, screen printing, slot coating and so on. As inks in printing processes are complex fluids containing particles and/or viscoelastic additives, we need to consider the elasticity of complex fluids in the contact line motion. In this study, we consider the contact line motion of polymer solutions and polymer melts theoretically and experimentally. The study on the motion of dilute polymer solutions showed that the spreading of polymer solution is qualitatively similar to that of Newtonian liquid in that it followed the Tanner-Voinov-Hoffman relation. However the contact line speed is strongly affected by the polymer molecules. This appears to be caused by the migration of polymers away from the wall due to the hydrodynamic interaction between the polymer and the wall at the contact line region. The hydrodynamic interaction is caused by the elasticity of polymer molecules in the shear flow. Experiments using polyisobutylene solutions in polybutene support the migration theory. The recent experimental investigation using DNA solutions confirmed that the migration occurs near the contact line.

 The study on the polymer melts without polymer migration shows that the first normal stress difference (N1) increases the contact line velocity while the second normal stress difference does not affect the contact line motion by using the second order fluid model. The increased contact line velocity is caused by the hoop stress acting on the curved stream lines near the contact line. Experiments with silicone oils of widely different viscosities confirmed the faster motion of the contact line. As the experimental result confirms the theoretical prediction, the effect of N1 is established. The present result can be used in the derivation of the boundary condition for solving free surface flows of viscoelastic fluid on a solid surface.

 

Lambda DNAs(Elastic, 48kbps) in glycerin show strong migration near the contact line

Fragmented DNAs do not show migration

 

 

 

 • Gels can be jetted!

    : First demonstration of generating inkjet droplets from particulate gels

 

 The generation of inkjet drops of colloidal gels is studies experimentally. Particle suspensions are prepared by dispersing spherical polystyrene particles of 620nm in the 1:1 mixture of deionized water and ethylene glycol. The gels are prepared by adding polyethylene oxide to the suspensions by inducing the depletion interaction between particles. It is demonstrated that inkjet drops can be generated by using the colloidal gels. It is found that the ligament extended from the inkjet nozzle is stabilized so that the drop can be generated without satellite droplets behind the main drop and the velocity of the gel drop is faster than that of the polymer solution at the same concentration. The gel drop generation characteristics are found to be sensitive to input voltage.

 The success of the generation of the inkjet droplets from particulate gels is a result of our group's continuing efforts on the development and fundamental understanding of inkjet technology.

 

                              

                         Viscosity of the gels showing yield stress      Inkjet printing of PS 6.75%           

in PEO500ppm solution                

 

 

 • Gel atomization

 There has been a growing interest on the development of rocket propellant using gelled fuels and oxidizers because the rocket propellant using a gelled fuel has been considered to complement some drawbacks of conventional fuels or oxidizers. However it has been recognized that the rheological properties of gel material make it difficult to atomize gel propellant into fine drops. To overcome the difficulty we are doing researches on the change of spray and atomization characteristics with rheological properties by performing experiments with various fluids including Carbopol gels with or without nanoparticles. We have shown that the instability characteristics are quite different depending on fluids especially at low velocities (Please see the figures in the cover page). The present study can shed light on this field and contribute to the development of better propellants.

 

 

 • Shear reducing thermal conductivity of nanofluids

 Nanofluid is a colloidal suspension of nano-sized particles dispersed in conventional heat transfer fluids. Nanofluids show the increase of thermal conductivity compared with conventional heat transfer fluids such as water and ethylene glycol. The enhancement of thermal conductivity of nanofluid depends on particle size, particle shape, properties of particle and base fluid and volume fraction of particles. And rheological properties of nanofluid are also related to the above factors strongly.The thermal conductivity (k) of aqueous alumina nanofluid of various particle shapes (rods, bricks, blades) was measured at the dynamic state for the first time. The dynamic k was measured under torsional flows by using a homemade parallel-plate system. All the materials tested here showed decreasing k with increasing shear rate. This newly observed phenomenon was named ‘shear-reducing thermal conductivity.’ From the rheological properties of nanofluids it was inferred that the alumina nanofluids should have network structures and these microstructures should be destroyed or deformed by the shear. But not all the networks were destroyed by the shear. The effective medium theory cannot explain the shear reducing characteristics of nanofluids at the dynamic state. It is suggested that the Brownian motion of the primary particles cannot be excluded in heat conduction through nanofluids. The present results will shed light on the elucidation of the mechanism of the heat transfer enhancement of nanofluids.

 

 

 • The spreading of suspension drop on horizontal surface

 Experimental studies were performed on the contact line motion of the suspension of PS particles on a glass surface. The base liquids were silicone oil and glycerin. The particle size was in the range of 1 - 6μm and the particle loading was 0.5 - 5 volume %. The drop shape was determined by using a drop image and its reflection and the drop outline was traced to the subpixel level. The Tanner-Voinov-Hoffman relation was valid for suspensions as well as for pure liquids. Silicone oil suspensions showed almost no noticeable change compared with the pure fluid. Glycerin suspensions showed an increase in contact line speed at low particle loading. The difference was due to the microstructure change at the contact line region and the microstructure change was originated from the wetting characteristics of particles. Particle alignment occurred during the spreading stage for partially wetting particles. The contact line showed a stop-and-go fashioned motion due to surface irregularities. This result can be used as the boundary condition at the contact line in the numerical simulation of suspension spreading.

 

                        Particles dispersed in silicone oil     Particles dispersed in glycerin

 

 • Gel structure change by nanoparticles

 We explored the rheological characteristics of Carbopol C934 gel (polyacrylic acid) containing SUS 304 spherical nanoparticles of 100nm as a simulant of gel-propellants containing metal fuels. In comparision with the pure Carbopol gel, the SUS nanoparticle filled Carbopol gel exhibited stronger shear thinning and higher yield stress. As the concentration of nanoparticles increased yield stress increased, but viscosity and storage modulus increased first and then decreased abruptly beyond the critical limit. Also as the concentration of nanoparticles increased there was a transition in material characteristics from the ductile type to the brittle type, which means that highly filled Carbopol gels lost the structure almost instantaneously as the imposed stress was larger than the yield stress while Carbopol gels of low particle loading sustained the structure even after the imposed stress was larger than the yield stress. The cryo-SEM analysis revealed that the network structure changed abruptly when the rheological properties changed abruptly. The change in gel structure is attributed to the nanoparticles that compete with Carbopol chains in forming networks. The abrupt chage in gel structure with the addition of particles beyond the critical limit should be an exclusive phenomenon of nanoparticles.

 

 

• Preparation of spheroidal and ellipsoidal particles

 Generation of particles of various sizes and shapes can be of great interest to scientists and engineers. We have developed a new and robust apparatus which can generate oblate spheroids from spherical polymer particles. A sheet of film dispersed with spherical particles was held by an eight-jaw extensional apparatus. The jaws were positioned in the edge of a regular octagon and moved radially to induce biaxial extension in an oil bath above the glass transition temperatures of the film and particles. We have demonstrated that PS particles dispersed in a PVA film can be stretched to generate various shapes by this method. The microscopic studies show that the oblate spheroids obtained by this method are virtually exaxt spheroids without showing knife edges. Also depending on positions with regard to holders ellipsoids and even prolate spheroids can be obtained. The method has been found to robust in that the deformation is always reproducible regardless of film thickness and very small deformation can be applied for nearly spherical particles. We have confirmed that this method can be applied for particles of submicrons to 10 micrometers in diameter or even larger ones. It is expected that the spheroids and ellipsoids obtained by this method can be of help in many studies including colloids, suspension rheology, electrophoresis, printed electronics and pharmaceutical science.

 

 

 

 

 

 


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