Mixing in Colliding, Ultrasonically Levitated Drops

Abstract

Lab-in-a-drop, using ultrasonic levitation, has been actively investigated for the last two decades. Benefits include lack of contact between solutions and an apparatus and a lack of sample cross-contamination. Understanding and controlling mixing in the levitated drop is necessary for using an acoustically levitated drop as a microreactor, particularly for studying kinetics. A pulsed electrostatic delivery system enables addition and mixing of a desired-volume droplet with the levitated drop. Measurement of mixing kinetics is obtained by high-speed video monitoring of a titration reaction. Drop heterogeneity is visualized as 370 nl of 0.25 M KOH (pH: 13.4) was added to 3.7 μL of 0.058 M HCl (pH: 1.24). Spontaneous mixing time is about 2 s. Following droplet impact, the mixed drop orbits the levitator axis at about 5 Hz during homogenization. The video's green channel (maximum response near 540 nm) shows the color change due to phenolphthalein absorption. While mixing is at least an order of magnitude faster in the levitated drop compared with three-dimensional diffusion, modulation of the acoustic waveform near the surface acoustic wave resonance frequency of the levitated drop does not substantially reduce mixing time.

Automatic contactless injection, transportation, merging, and ejection of droplets with a multifocal point acoustic levitator

“…In the state-of-the-art levitation systems, 11,14,22,23,28 samples are manually inserted and removed with a syringe, requiring a skilled person to avoid sample losses and preventing the automation of the processes. Therefore, executing different fluidic operations such as injection, 19,29,30 transportation, 14,23,24 merging 14,26,28 and ejection 31 in a single levitation device would open up multiple possibilities, enabling the automatic processing of droplets in mid-air.…”

Increasing Protein Charge State When Using Laser Electrospray Mass Spectrometry

“…The fact that the Z avg values for LEMS measurements were approximately the same suggests that the solution phase processes within the droplet are not important for laser vaporized protein. A droplet mixing experiment performed by colliding a 370 nL droplet of a basic solution containing phenolphthalein into an acoustically levitated 4 μL droplet of an acid showed that the basic solution remained on the surface for more than 2 s [51]. This time scale is larger than the~100 ms time scale that the protein spends with the ES droplet during the transit to the capillary inlet in LEMS measurements.…”

Stimuli‐Responsive Liquid Marbles: Controlling Structure, Shape, Stability, and Motion

“…During levitation, the gravitational force acting on the levitated droplet is balanced by the acoustic radiation force exerted on the droplet surface . Attractive features of the ultrasonic levitation include the compatibility with small droplet volumes (typically 5−10 μL, which is comparable to typical liquid marble volumes) and the ability to introduce contact‐less mixing and to change the droplet shape by changing the ultrasonic pressure. Ultrasonic levitation enables a container‐free environment that can resist surface confinement and contamination of the droplets and is highly desirable for sample manipulation, material fabrication and chemical analysis.…”