Hyunjun Cho, Cheong Bong Chang, Jin Ho Jung and Hyung Jin Sung
The behavior of a microdroplet in a uniform flow and subjected to a vertical optical force applied by a loosely focused Gaussian laser beam was studied numerically. The lattice Boltzmann method was applied to obtain the two-phase flow field, and the dynamic ray tracing method was adopted to calculate the optical force. The optical forces acting on the spherical droplets agreed well with the analytical values. The numerically predicted droplet migration distances agreed well with the experimentally obtained values. Simulations of the various flow and optical parameters showed that the droplet migration distance nondimensionalized by the droplet radius is proportional to the S number (zd /rp = 0.377S), which is the ratio of the optical force to the viscous drag. The effect of the surface tension was also examined. These results indicated that the surface tension influenced the droplet migration distance to a lesser degree than the flow and optical parameters. The results of the present work hold for the refractive indices of the mean fluid and the droplet being 1.33 and 1.59, respectively.
DOI
Showing posts with label Physics of Fluids. Show all posts
Showing posts with label Physics of Fluids. Show all posts
Saturday, November 29, 2014
Thursday, July 24, 2014
Probing interfacial dynamics and mechanics using submerged particle microrheology. II. Experiment
Thomas Boatwright, Michael Dennin, Roie Shlomovitz, Arthur A. Evans and Alex J. Levine
A non-contact microrheological technique to probe the mechanics of the air/water interface is explored. Polystyrene spheres dissolved in water are trapped with an optical tweezer near the free surface of water, allowing the response functions of the particles to be measured as a function of the distance from the air/water interface. These measurements show that at the surface, the imaginary part of the response function increases by approximately 30% from the Stokes value measured in the bulk. As the particle is moved away from the surface via an optical trap, the response function returns to the bulk value. The method is tested by comparing the response function of particles near a rigid wall to the theory developed by Faxèn. A newly developed hydrodynamic theory is used to explain the results at the free interface through a calculation of the linear response function as a function of depth. These results show a range of sensitivity that can be utilized to study the microrheology of a Langmuir monolayer without distorting its structure.
DOI
A non-contact microrheological technique to probe the mechanics of the air/water interface is explored. Polystyrene spheres dissolved in water are trapped with an optical tweezer near the free surface of water, allowing the response functions of the particles to be measured as a function of the distance from the air/water interface. These measurements show that at the surface, the imaginary part of the response function increases by approximately 30% from the Stokes value measured in the bulk. As the particle is moved away from the surface via an optical trap, the response function returns to the bulk value. The method is tested by comparing the response function of particles near a rigid wall to the theory developed by Faxèn. A newly developed hydrodynamic theory is used to explain the results at the free interface through a calculation of the linear response function as a function of depth. These results show a range of sensitivity that can be utilized to study the microrheology of a Langmuir monolayer without distorting its structure.
DOI
Monday, October 19, 2009
History force on coated microbubbles propelled by ultrasound
Valeria Garbin, Benjamin Dollet, Marlies Overvelde, Dan Cojoc, Enzo Di Fabrizio, Leen van Wijngaarden, Andrea Prosperetti, Nico de Jong, Detlef Lohse, and Michel Versluis
In this paper the unsteady translation of coated microbubbles propelled by acoustic radiation force is studied experimentally. A system of two pulsating microbubbles of the type used as contrast agent in ultrasound medical imaging is considered, which attract each other as a result of the secondary Bjerknes force. Optical tweezers are used to isolate the bubble pair from neighboring boundaries so that it can be regarded as if in an unbounded fluid and the hydrodynamic forces acting on the system can be identified unambiguously. The radial and translational dynamics, excited by a 2.25 MHz ultrasound wave, is recorded with an ultrahigh speed camera at 15×106 frames/s. The time-resolved measurements reveal a quasisteady component of the translational velocity, at an average translational Reynolds number 0.5, and an oscillatory component at the same frequency as the radial pulsations, as predicted by existing models. Since the coating enforces a no-slip boundary condition, an increased viscous dissipation is expected due to the oscillatory component, similar to the case of an oscillating rigid sphere that was firstdescribed by Stokes [“On the effect of the internal friction of fluids on the motion of pendulums,” Trans. Cambridge Philos. Soc. 9, 8 (1851)]. A history force term is thereforeincluded in the force balance, in the form originally proposed by Basset and extended to the case of time-dependent radius by Takemura and Magnaudet [“The history force on a rapidly shrinking bubble rising at finite Reynolds number,” Phys. Fluids 16, 3247 (2004)]. The instantaneous values of the hydrodynamic forces extracted from the experimental data confirm that the history force accounts for the largest part of the viscous force. The trajectories of the bubbles predicted by numerically solving the equations of motion are in very good agreement with the experiment.
DOI
In this paper the unsteady translation of coated microbubbles propelled by acoustic radiation force is studied experimentally. A system of two pulsating microbubbles of the type used as contrast agent in ultrasound medical imaging is considered, which attract each other as a result of the secondary Bjerknes force. Optical tweezers are used to isolate the bubble pair from neighboring boundaries so that it can be regarded as if in an unbounded fluid and the hydrodynamic forces acting on the system can be identified unambiguously. The radial and translational dynamics, excited by a 2.25 MHz ultrasound wave, is recorded with an ultrahigh speed camera at 15×106 frames/s. The time-resolved measurements reveal a quasisteady component of the translational velocity, at an average translational Reynolds number 0.5, and an oscillatory component at the same frequency as the radial pulsations, as predicted by existing models. Since the coating enforces a no-slip boundary condition, an increased viscous dissipation is expected due to the oscillatory component, similar to the case of an oscillating rigid sphere that was firstdescribed by Stokes [“On the effect of the internal friction of fluids on the motion of pendulums,” Trans. Cambridge Philos. Soc. 9, 8 (1851)]. A history force term is thereforeincluded in the force balance, in the form originally proposed by Basset and extended to the case of time-dependent radius by Takemura and Magnaudet [“The history force on a rapidly shrinking bubble rising at finite Reynolds number,” Phys. Fluids 16, 3247 (2004)]. The instantaneous values of the hydrodynamic forces extracted from the experimental data confirm that the history force accounts for the largest part of the viscous force. The trajectories of the bubbles predicted by numerically solving the equations of motion are in very good agreement with the experiment.
DOI
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