Ultrasonic Levitator – a powerful tool for contact-free experiments
超声波悬浮器 – 用于微重力实验的强大工具
Tag: Acoustic Levitator 超声波悬浮器, contactless environment 物体悬浮, spectroscopy 光谱仪, microgravity 微重力, tec5 AG
Ever since that Bücks and Müller demonstrated their ultrasonic/ acoustic levitation experiment in 1933, this technique has slowly transformed into a powerful and versatile method for the study of droplets or particles in a contactless environment. The basic principle behind this technology is simple: a sound standing wave is generated between an ultrasonic transducer and a reflector (see Figure 1). Within this standing wave there are several pressure points that could counteract the gravitational force and levitate the samples. As a rule of thumb, samples with effective diameters of less than half of a wavelength (sound) will be levitated slightly below the pressure modes. When the sample’s size is approx. three times smaller than the sound wavelength, minimal acoustic power is required for the levitation.
自从Bücks和Müller在1933年展示了他们的超声波悬浮实验，这个技术慢慢地变成一种用于在非接触环境中研究液滴或颗粒通用的方法，。基本原理很简单：超声驻波在超声波传感器和反射器之间产生（见图1）。 在该驻波内的压力最小点可以抵消重力并悬浮样品。 根据经验，只要样品的有效直径大约小于波长（声音）的一半可以悬浮的。 当样品三倍小于比声波长，悬浮需要最小的声功率。
Attractive features of an ultrasonic levitator:
- Provide contactless environment
- Provide controllable gas environment
- Required a very small sample volume
- Can be easily combined with numerous analytic techniques, such as FTIR, UV-VIS-NIR, RAMAN spectroscopy, microscopy, X-ray scattering etc.
Figure 1: Left: Levitated droplet at the pressure node. Right: Schematic drawing of the ultrasound resonator.
Application Examples 应用示例：
CASE 1: The study of nanomaterial self-assembly using ultrasonic levitator and X-ray 使用超声波悬浮器和X射线研究纳米材料自组装
Seddon et al. and his coworker  have successfully demonstrated the levitation of soft nanomaterials and analyze them under contactless manner. In addition, they implemented the in-situ X-ray scattering (SAXS) as well as controllable gas-phase environment to study the crystalline phase of the investigated nanomaterials. Their study has shown that such a technique open -up new experimental approaches for study of materials without wall effect.
Seddon和他的同事  已经成功地悬浮了软纳米材料的，并以非接触方式分析它们。 此外，他们实施原位X射线散射（SAXS）以及可控的气相环境，研究纳米材料的结晶相。 他们的研究表明，这种技术开辟了新的实验方法来研究材料。
Figure 2: A setup for investigating the nanomaterial self-assembly within in a controlled gas phase and contract free environment 
Few month later Agthe group  uses the similar setup to perform the study of nanoparticles assembly into mesocrystals (see Figure 3). The experiment demonstrates how the crystallization process of a levitated nanocubes proceed using small angle X-ray scattering and ex situ scanning electron microscopy.
Figure 3: a) Schematic drawing of the experimental setup using ultrasonic levitator, camera and D-rays to study the assembly of levitated nanoparticles b-f) show the SAXS and SEM results .
Case 2: The study of droplet evaporation using ultrasonic levitator and heater/cooler使用超声波悬浮器和加热器/冷却器研究液滴的蒸发
Yan  has performed an interesting study using the ultrasonic levitator together with a heater/cooler. The goal was to study the freezing/melting of a droplet/ice suspended in air. This experiment has successfully investigated the relationships between percentage melting states of water droplets for varying residence times.
Figure 4. Experimental setup for the study of melting of frozen droplet (ice crystal) suspended in air.
A similar study was performed by Saha  in his PhD thesis (later chapters include the study of agglomeration of nanoparticle). But this time, near infrared and infrared laser is used as the heating source instead. The study has successfully described different stages of evaporation of droplets that contained Nano-suspensions or precursor solution.
Saha  在他的博士论文中进行了类似的研究（后面的章节包括纳米颗粒聚集的研究）。 但这次，近红外和红外激光器被用作加热源。 研究成功地描述了纳米悬浮液或前体溶液滴的蒸发不同阶段。
Figure 5. Experiement setup for the study of levitated droplet’s evaporation via laser heating
Case 3: The study of levitated dye laser 悬浮染料激光
A research group from Denmark  has reported a very interesting experiment using an ultrasonic levitator and demonstrate a lasing action in a levitated dye droplet (see Figure 6). In this case, the droplet itself acts as a resonator and a frequency doubled Nd:YAG laser was used as the optical pump. The laser emission is captured and analyzed by using a spectrometer.
Figure 6. Lasing action of a dye droplet suspended in air
Case 4: The study of nanoparticles using ultrasonic levitation combined with various optical spectroscopy techniques 使用超声波悬浮和各种光谱技术研究纳米颗粒
One of the holy grail for experimentalist is to be able to perform and analyze an untainted specimen using different type analytical tools simultaneously. This has been shown to be possible by Dr. Schenk in his PhD dissertation . Figure 7 shows the schematic drawing of his setup, where he combined different techniques such as RAMAN, UV-VIS and fluorescence spectroscopy in one setup and investigate the levitated samples.
实验者的圣杯之一是能够同时使用不同类型的工具析未受污染的样本。 这已经被Schenk博士在他的博士论文 中证明是可能的。 图7显示了他的设置示意图，他在一个设置中结合了不同的技术，如拉曼，UV-VIS和荧光光谱，并研究悬浮样品。
Figure 7. Schematic drawing of a levitation setup in combination with RAMAN, UV_VIS and fluoreschence spectroscopy.
Ultrasonic levitator has been proven to be a very useful tool for the study of microfluidic as well as nanomaterials in contact free environment. Such a device is commercially available and can be purchased under: https://www.tec5.com/en/products/custom-solutions/levitator
- Levitator with transducer frequency of 58 KHz
- Levitator with transducer frequency of 100 KHz
- Levitator with integrated free jet Noozle
- Levitator with Integrated Piezo sensor
- 传感器频率58 KHz
- 传感器频率100 KHz
- Bücks, K., Müller, H.: Über einige Beobachtungen und Piezoquarzen schwingenden Ihrem Shallfeld. Z. Phys 84, 75-86 (1933).
- Seddon A. M., Richardson et. al.: Control of Nanomaterial Self-Assembly in Ultrasonically Levitated Droplets, J. Phys. Chem. Lett. 2016, 7, 1341−1345
- Following in Real Time the Two-Step Assembly of Nanoparticles into Mesocrystals in Levitating Drops. Michael Agthe, Tomás S. Plivelic, Ana Labrador, Lennart Bergström, and German Salazar-Alvare Nano Letters Article
- Yan, S. (2016) Experimental impact dynamics testing of a glaciated water drop and quantification of partial melting with varying residence time (Master dissertation). Retrieved from https://etda.libraries.psu.edu/files/final_submissions/12701
- Abhishek, S. (2012) Evaporation, Precipitation Dynamics and Instability of Acoustically Levitated Functional Droplets (Doctoral dissertation). Retrieved from https://etd.fcla.edu/CF/CFE0004436/PhD_Asaha_submission.pdf
- Azzouz, L. Alkhafadiji, S. Balslev, J. Johansson, N. A. Mortensen, S. Nilsson, and A. Kristensen, “Levitated droplet dye laser,” Opt. Express 14, 4374-4379 (2006)
- Jonas, S (2014) Optische Spektroskopie in der Ultraschallfalle (Doctoral dissertation). Retrieved from https://hu-berlin.de/dissertationen/schenk-jonas-2014-10-02/PDF/schenk.pdf