Direct measurement of slip length is based on the measured fluid velocity near solid boundary. However, previous micro particle image velocimetry/particle tracking velocimetry （microPIV/PTV） measurements have report...Direct measurement of slip length is based on the measured fluid velocity near solid boundary. However, previous micro particle image velocimetry/particle tracking velocimetry （microPIV/PTV） measurements have reported surprisingly large measured near-wall velocities of pressure-driven flow in apparent contradiction with the no-slip hy-pothesis and experimental results from other techniques. To better interpret the measured results of the microPIV/PTV, we performed velocity profile measurements near a hy-drophilic wall （z = 0.25-1.5 μm） with two sizes of tracer particles （φ 50 nm and φ200 nm）. The experimental results indicate that, at less than 1 μm from the wall, the deviations between the measured velocities and no-slip theoretical values obviously decrease from 93% of φ200 nm particles to 48% of φ50 nm particles. The Boltzmann-like exponential measured particle concentrations near wall were found. Based on the non linear Boltzmann distribution of particle concentration and the effective focus plane thickness, we illustrated the reason of the apparent velocity increase near wall and proposed a method to correct the measured velocity profile. By this method, the deviations between the corrected measured velocities and the no-slip theoretical velocity decrease from 45.8% to 10%, and the measured slip length on hy-drophilic glass is revised from 75 nm to 16 nm. These results indicated that the particle size and the biased particle concentration distribution can significantly affect near wall velocity measurement via microPIV/PTV, and result in larger measured velocity and slip length close to wall.展开更多
The hydrodynamics and locomotion mechanism of Euglena Gracilis(E.Gracilis)is investigated using microscopic shadow imaging and micro particle image velocimetry(MicroPIV).Three distinct locomotion modes were observed:t...The hydrodynamics and locomotion mechanism of Euglena Gracilis(E.Gracilis)is investigated using microscopic shadow imaging and micro particle image velocimetry(MicroPIV).Three distinct locomotion modes were observed:translation,spin,and left/right turn.Since the flagellum was not possible to image,the strokes were identified by evaluating the flow field around the protist.The flow field information is obtained using a phase-separated PIV evaluation,which uses a histogram-thresholding based dynamic masking approach.The temporal resolution of the experiment was sufficient to identify the sequence of two modes,translation and spin,and the stroke-pulling frequency.The flow field result during a stroke is compared with existing Stokeslet dipole theories and flow disturbance decay with distance is investigated.The results indicate that the organism has a complex locomotion technique that allows the change of direction,axial orientation and propulsion.展开更多
基金supported by the National Natural Science Foundation of China (10872203)the National Basic Research Program(2007AC744701)the CAS Research and Development Program of China (KSCX2-YW-H18)
文摘Direct measurement of slip length is based on the measured fluid velocity near solid boundary. However, previous micro particle image velocimetry/particle tracking velocimetry （microPIV/PTV） measurements have reported surprisingly large measured near-wall velocities of pressure-driven flow in apparent contradiction with the no-slip hy-pothesis and experimental results from other techniques. To better interpret the measured results of the microPIV/PTV, we performed velocity profile measurements near a hy-drophilic wall （z = 0.25-1.5 μm） with two sizes of tracer particles （φ 50 nm and φ200 nm）. The experimental results indicate that, at less than 1 μm from the wall, the deviations between the measured velocities and no-slip theoretical values obviously decrease from 93% of φ200 nm particles to 48% of φ50 nm particles. The Boltzmann-like exponential measured particle concentrations near wall were found. Based on the non linear Boltzmann distribution of particle concentration and the effective focus plane thickness, we illustrated the reason of the apparent velocity increase near wall and proposed a method to correct the measured velocity profile. By this method, the deviations between the corrected measured velocities and the no-slip theoretical velocity decrease from 45.8% to 10%, and the measured slip length on hy-drophilic glass is revised from 75 nm to 16 nm. These results indicated that the particle size and the biased particle concentration distribution can significantly affect near wall velocity measurement via microPIV/PTV, and result in larger measured velocity and slip length close to wall.
文摘The hydrodynamics and locomotion mechanism of Euglena Gracilis(E.Gracilis)is investigated using microscopic shadow imaging and micro particle image velocimetry(MicroPIV).Three distinct locomotion modes were observed:translation,spin,and left/right turn.Since the flagellum was not possible to image,the strokes were identified by evaluating the flow field around the protist.The flow field information is obtained using a phase-separated PIV evaluation,which uses a histogram-thresholding based dynamic masking approach.The temporal resolution of the experiment was sufficient to identify the sequence of two modes,translation and spin,and the stroke-pulling frequency.The flow field result during a stroke is compared with existing Stokeslet dipole theories and flow disturbance decay with distance is investigated.The results indicate that the organism has a complex locomotion technique that allows the change of direction,axial orientation and propulsion.
