Multipoint Holographic Optical Velocimetry in Microfluidic Systems

We show how holographic optical trapping can be used for the multipoint measurement of fluid flow in microscopic geometries. An array of microprobes can be simultaneously trapped and used to map out the fluid flow in a microfluidic device. The optical traps are alternately turned on and off such that the probe particles are displaced by the flow of the surrounding fluid and then retrapped. The particles’ displacements are monitored by digital video microscopy and directly converted into velocity field values. This technique enables the measurement of a two-dimensional flow field at points arbitrarily distributed in a three-dimensional volume. The validity of the technique is demonstrated for the case of the flow around a spinning sphere and the flow at the outlet of a microchannel.

Figure 1

The optical setup for the multipoint flow measurement technique. The 532 nm laser is chopped and reflects off the SLM before passing through the microscope objective. The second laser, in this case 1064 nm, is used to cause fluid flow by trapping and rotating a birefringent particle. The inset shows a sample hologram used to create multiple traps.

Figure 2

Measured velocity $\Delta r/\Delta t$ versus microscope translation stage velocity $v_0$. The right axis represents the corresponding probe particle displacements $\Delta r$.

Figure 3

(a) Measured flow field (arrows) around a $4\mu \mathrm{m}$ radius vaterite particle spinning at $\nu =2.4\mathrm{Hz}$. (b) Measured flow field (arrows) at six different radial distances from a $3.7\mu \mathrm{m}$ radius vaterite particle spinning at 5.2 Hz. Arrows in the bottom left corners indicate velocity units.

Figure 4

Azimuthal components of measured flow velocities versus the corresponding radial distance from the center of the spinning particle in Fig. 3b. The gray shaded area represents the range of the expected velocity values for a sphere of radius $R=3.7\pm 0.2\mu \mathrm{m}$ spinning at 5.2 Hz.

Figure 5

Azimuthal components of measured flow velocities at a radial distance $r=5.3\mu \mathrm{m}$ from the center of a spinning sphere as a function of the $z$ coordinate normal to the imaging plane. The gray shaded area represents the range of the expected velocity values for a sphere of radius $R=1.9\pm 0.2\mu \mathrm{m}$ spinning at 3 Hz.

Figure 6

Measured flow field (arrows) at the exit of a $15\mu \mathrm{m}$ wide PDMS microchannel. A water flow is produced by a pressure driven syringe.