Mixing in Colliding, Ultrasonically Levitated Drops

Transcription

1 Mixing in Colliding, Ultrasonically Levitated Drops Supporting information Details of acoustic levitation. Delivering drops into the acoustic levitation zone is easily ignored as a challenging first step in carrying out reactions in the levitated drops. However, it is difficult to introduce drops without disturbing the acoustic field, failing to get the drop to detach from the syringe tip on which it was formed, or failing to have the drop entrained in the acoustic trap. Since the drops tend to stick to the tip of the syringe instead of detaching from the tip, several approaches were tried. First, the syringe tip was coated with hydrophobic paraffin wax to assist detachment of the drop; however, the increased diameter of the coated tip proved counterproductive. Then, Teflon tubing (AlphaWire, Elizabeth, NJ) was heat-shrunk to generate a very small diameter, hydrophobic tip. After stretching, the tube was cut with a razor and the residual hole diameter inspected with a microscope. Several nested Teflon tubes (AlphaWire TFT-200 series) were used to shrink the tip diameter. Several different tip shapes were tried, but the straight thin tip of a syringe with PTFE tubes works best since a small, straight tube induces fewer acoustic disturbances than other geometries. In addition, curving the tip upward helps facilitate drop detachment. For example, three nested tubes were shrunk and the last tube was stretched while heating and pulling by hand to generate thinner tube tips. The drop has a diameter of approximately 1 mm and the tip of the pipette is 200µm in diameter. To further ease detachment, reflector height was slightly increased from the optimum resonance position; after the drop floated free, the height of the reflector was returned to optimum resonance. The key to delivering the drops to the acoustic levitation zone is increasing the disturbance force for detachment, then, decreasing force by returning the height to optimum resonance. A built-in cooling fan on the camera body was found to perturb drop position since it exhausts forward, toward the levitated drop. By diverting the blast from the cooling fan, drop positional stability was recovered. The drop was illuminated with a white LED array. While heating of the drop and its surroundings by the LED was considered as a possible source of levitation instability, such heating appears not to be significant since the LED is behind the levitated droplet and is placed outside the transparent acrylic chamber. Still, the remaining equipment inside the chamber raised the temperature about 0.2 C/min, so the experiment was halted every 30 minutes. Modulation of the acoustic field was achieved by using a second function generator (Agilent 3320A, Agilent Technologies, Santa Clara, CA) to generate a TTL-level sync waveform at the assumed resonance frequency of the levitated drop ( Hz). The sync waveform was gated by a pulse from second digital channel of the USB-6009 was routed to the modulation input of the ultrasonic function generator levitation. This resulted in square-wave modulation of the ultrasonic frequency. The schematic and images of a levitated drop under modulation are shown in Figure S1. Increasing modulation power beyond a threshold causes the drop to explode, as shown in Figure S2. Image Processing. Video captured by the camera was exported as image sequences in 24-bit image bitmaps. The acquired images were analyzed off-line to obtain droplet dimensions using a program written with the Vision Builder for Automated Inspection (VBAI) machine vision application (National Instruments, Austin, TX). Droplet volume was calculated based on an oblate-spheroid model. For any oblate spheroid

2 2 4π r r 1 2 V = (1) 3 where, r 1 is the radius perpendicular to the levitator axis and r 2 is the radius aligned with the levitator axis for the levitated drop. The radii were obtained from the automated measurements of the drop in each video frame. The Region of Interest (ROI) had to be adjusted for each series of images, but once set, generally results in successful determination of drop dimensions for a majority of the images. Not every frame results in a successful determination of volume as the droplet was oscillating following impact by the ballistically-injected droplet, and in some instances, the droplet goes out of frame. In fact, until oscillations damped, the drop was no longer ellipsoidal, and accurate volume measurements could not be made. Once drop shape oscillations ceased, the combined drop was still orbiting the levitator axis, passing in and out of focus as it orbited. The image was also distorted by glare. Therefore, some manual manipulation to guide the radius calculation was necessary. The ballistically-launched droplet is approximately spherical, removing distinctions between radii. Figure S4 shows a screen capture of the images being processed by Vision Builder software where volumes are determined from images of the addition of three ballistically-added droplets. The incoming droplet is more circular than the primary levitated drop. Insets (a-f) show the volume calculation from just before the 1 st, 2 nd and 3 rd collisions of KOH droplets with the titrant drop, and (g) is the grid image used for optical system calibration. Since Vision Builder as-programmed only calculates dimensions for one object at a time, the volumes of the two colliding drops had to be calculated separately. Substantial programming could modify the code to recognize the two drops and calculate their volumes in parallel. Vision Builder was also used to automatically recognize areas of light transmission in the drop and calculate the average intensity of each area for each video frame.

3 PC running LabVIEW program Ballistic injector Acoustic levitator NI USB-6009 (launch control) Modulation input Agilent 33210A (modulation frequency) TTL sync out Gating and level shift circuit Agilent 33220A (piezo drive source) Figure S1. The block diagram of the electronics that comprise the modulation circuit. The modulation of the acoustic wave is synchronized to the launch and impact of the droplet with the levitated drop.

4 Figure S2. A sequence of frames showing a levitated drop undergoing collision with a droplet while at the same time undergoing acoustic modulation: Top events prior to and soon after drop collision around 9 milliseconds after launch of droplet. Bottom post collision, where the force of the colliding droplet combined with the modulation power causes the drop to shatter.

5 Figure S3. Screen capture of Vision Builder window showing the automated determination of drop and droplet major and minor axis used to calculate the volume. (a) First ballistic droplet, (b) Same as (a), focusing on levitated drop, (c) Second ballistic droplet, (d) Same as (c), focusing on levitated drop, (e) Third ballistic droplet, (f) Same as (e), focusing on levitated drop, (g) Calibration mesh (1 x 1 mm) image.

6 A. Match the center spot (reference image size 16 11). The values shown are (in order): index number of region of interest (ROI) coordinate center angle of tilt (in degrees) likeness value (a ranking from 0 to 1000 showing the degree of matching to the region of interest, where the threshold to pass was set to 900). B. Match the bright line below it (reference image size 72 10).

7 C. Two reference coordinates are set at the two matched region, origins are at the center of each matched region. D. An oval shaped region of interest is created (size 18 12), positioned to center at the origin of the coordinate made for the bright spot. A rectangular region of interest is created (size 53 7), positioned to center at the origin of the coordinate made for the line.

8 E. Vision Builder then calculates an average intensity and standard deviation of the regions of interest. Figure S4. Automated Analysis of Drop Intensity (using NI Vision Builder). Image frame size is pixels.

9 Figure S5. Ultraviolet/visible spectrum of phenolphthalein in alkaline solution.

10 Videos No Modulation Trial : Movie shows drop collision and mixing without the modulation of acoustic drive. Test 1 trimmed : Movie shows drop distortions due to modulation of acoustic drive. The modulation ends before droplet collides. Test 2 trimmed : Movie shows drop modulated at critical amplitude, where large distortions of drop shape can be seen. Upon collision with ballistic droplet, the drop explodes and daughter droplets are expelled. Test 3 trimmed : Movie shows drop modulated at sub-critical amplitude, continued post-collision. Full_Traj_rotated : Movie showing ballistic trajectory of droplet. Beginning with launch from capillary tip, to impact with levitated drop. The original frame rate is 1949 frames-per-second, played back at 10 fps.