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Acoustophoretic Contactless Transport and Handling of Matter in Air

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Acoustophoretic Contactless Transport and Handling of Matter in Air Acoustophoretic contactless transport and handling of matter in air Daniele Foresti, Majid Nabavi, Mirko Klingauf, Aldo Ferrari, and Dimos Poulikakos1 Department of Mechanical and Process Engineering, Laboratory of Thermodynamics in Emerging Technologies, Eidgenössische Technische Hochschule Zürich, CH-8092 Zurich, Switzerland Edited by William R. Schowalter, Princeton University, Princeton, NJ, and approved June 8, 2013 (received for review January 30, 2013) Levitation and controlled motion of matter in air have a wealth of levitation potential [the sum of the acoustic potential (20) and potential applications ranging from materials processing to bio- the gravitational potential; SI Text, section 1]. This varies non- chemistry and pharmaceuticals. We present a unique acoustopho- monotonically between the emitting surface and reflector. If it is retic concept for the contactless transport and handling of matter strong enough to overcome the gravitational force, small amounts in air. Spatiotemporal modulation of the levitation acoustic field of matter can be levitated and trapped in its minima (nodes). An allows continuous planar transport and processing of multiple acoustic potential node can correspond to an acoustic pressure objects, from near-spherical (volume of 0.1–10 μL) to wire-like, node or antinode, depending on the density and compressibility without being limited by the acoustic wavelength. The indepen- of the levitated sample (ρs and βs) and of the surrounding me- dence of the handling principle from special material properties dium [ρ0 and β0 (21)]. To this end, note that ρs > ρ0 and βs < β0 (magnetic, optical, or electrical) is illustrated with a wide palette of in the overwhelming majority of envisioned applications in air application experiments, such as contactless droplet coalescence and (SI Text,section1). mixing, solid–liquid encapsulation, absorption, dissolution, and The acoustic levitation and handling concept is realized with DNA transfection. More than a century after the pioneering work the help of a discretized planar resonator platform and a single of Lord Rayleigh on acoustic radiation pressure, a path-breaking flat reflector placed at a uniform distance H. Each discrete res- concept is proposed to harvest the significant benefits of acoustic onator element is a specially designed and optimized Langevin levitation in air. piezoelectric transducer (LPT) excited by a single sinusoidal SCIENCES signal voltage of ultrasound frequency f (Fig. 1 and Methods). A APPLIED PHYSICAL acoustics | fluid | ultrasounds | manipulation | microfluidics newly developed excitation mechanism allows controllable and smooth propulsion of the levitation potential nodes from one LPT ontactless transport and handling of matter are of funda- or a group of LPTs to the next. In this mechanism, the amplitudes A t A t mental importance to the study of many physical phenomena of the sinusoidal inputs 1( )and 2( ) of the two adjacent LPTs C T (1, 2) and biochemical processes (3). Typical state-of-the-art are varied over the travel period (time needed for the object to d methods of contactless handling of matter are based on electro- travel the distance between the centers of two adjacent LPTs) magnetic (4–6) principles and have interesting capabilities but also in a parabolic manner, as shown in Fig. 1. As a result, a nearly clear limitations in terms of particle size (micrometer range) and/ constant acoustic force magnitude during movement is obtained due to the proportionality of the acoustic force to the square of or inherent requirements of special material properties. Methods Acoustic levitation (7–11) is both contact-free and material- the driving voltage amplitude ( ). Movie S1 demonstrates independent, also without requiring laborious sample preparation. the capability of the present mechanism to perform a multistep Although significant progress has been made in the handling planar process in air acoustophoretically, where two water drop- of microsized particles suspended in a liquid (12, 13), where lets are introduced, transported from opposite directions, mixed, transported in the orthogonal direction to be mixed with a third buoyancy forces are almost entirely responsible for the levitation, droplet, and finally collected. We are not aware of any other method various drawbacks in the state of the art of acoustic levitation have or technology able to perform such multidroplet transport and prohibited the realization of controlled and reliable transport of handling in a gaseous environment. matter in air, thereby limiting the remarkable potential and utility The physical principle behind the demonstrated controlled of acoustic levitation (14–16). Its full exploitation requires funda- acoustophoretic transport of matter lies in the spatiotemporal mental advances leading to material transport with high control- modulation of the levitation acoustic field, shown in Fig. 2A and lability and movement resolution, long transport length, versatility, Movie S2. The transport and mixing of two droplets in a device and multidimensionality. consisting of a 1D array of five LPTs are numerically analyzed Here, we present a unique acoustophoretic concept, enabling using a validated 3D finite element model (Methods). The model the continuous planar transport and processing of multiple acous- calculates the levitation potential inside the system as a function tically levitated droplets and particles in air. This is a significant of the vibrational velocity of the emitting surface V0. Fig. 2B advance in the area of contactless handling in air, making it a viable shows the experimental results of the horizontal position of the complementary methodology to existing advanced approaches in two approaching droplets with four traveling velocities (0.6, 1.1, a liquid environment (17, 18), one that has its own unique features, 2.2, and 4.9 mm/s), along with the numerical predictions. Note expanding the horizon of possible applications. The concept is that air has a very low damping effect and oscillation of the based on the ability to modulate the acoustic node regions in the samples is present. However, the droplets are stably kept at the acoustic field spatially and temporally, enabling the reversible transition from material trapping to transport. Additionally, the levitation and handling of extremely elongated objects with Author contributions: D.F. and D.P. designed research; D.F. performed research; D.F., a characteristic length much longer than the acoustic wavelength M.K., and A.F. contributed new reagents/analytic tools; D.F., M.N., and D.P. analyzed is demonstrated through their transport and rotation. data; and D.F., M.N., and D.P. wrote the paper. The authors declare no conflict of interest. Results and Discussion This article is a PNAS Direct Submission. In acoustic levitation, a standing wave is established between an 1To whom correspondence should be addressed. E-mail: [email protected]. fl emitting surface and a re ector (9, 11, 19). The radiation pres- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. sure, a nonlinear property of the acoustic field, engenders the 1073/pnas.1301860110/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1301860110 PNAS Early Edition | 1of6 Downloaded by guest on September 28, 2021 the axis connecting two spheres is θ = 90° (our configuration), is calculated as (23): 3 3 2 4 Fr = -3πρ0Rs1 Rs2 vrms=r ; [1] where r is the center-to-center distance of spheres; Rs1 and Rs2 are the radii of the two spheres; and vrms is the rms acoustic velocity of the surrounding fluid, which cannot be measured di- rectly here. SI Text, section 4 explains the numerical model used to estimate vrms at the levitation nodes, where the mixing takes place (Fig. S4). Fig. 2D shows the analytical and experimental values of €x for two water droplets of Rs = 0.84 mm, which agree very well. The agreement is also excellent for droplets of differ- 3 3 ent densities (ρs = 1 g/cm for water and ρs = 0.76 g/cm for tetradecane) and radii, spanning over a wide range of acceleration (€x = 0.4–20 m/s2; SI Text, section 4, Fig. S5 A–D,andTable S1). The presented features of local control in space, time, and magnitude of the acoustic forces and the wide range of possi- bilities of matter that can be simultaneously handled provide a rich palette of combinations of liquid–liquid, liquid–solid, and solid–solid transport and interaction. One distinctive advancement of the present acoustophoretic method is that it features an almost constant acoustic potential magnitude during the node transition between two transducers, which is an important requirement for stable levitation of liquids Fig. 1. Schematic of the contactless multidrop manipulator and its excita- (24). In fact, not only does the acoustic force have to be strong tion mechanism. In the illustrative example, droplets are introduced into the fi enough to overcome the gravitational force, but it has to be below system at three locations (inlets 1, 2, and 3) in a ve-one-two LPT levitator. fi Their numbers correspond to the number of LPTs in a certain row. All rows the threshold of atomization of the droplet in the acoustic eld. are on the same plane, parallel to the reflector plane. The droplets move and Indeed, when the acoustic force is stronger than the interfacial mix, and the final sample is delivered to the outlet. The introduction of force, the droplet atomizes explosively. The ratio of acoustic droplets into the system can be achieved either manually with a micropi- forces to surface forces for a levitated droplet scales with Rs and 2 pette or with an automatized syringe pump and a glass capillary (Methods). is described by the acoustic Bond number (25), Ba = 2v rmsρ0Rs/σ, fl The re ector height H is adjusted with a linear micrometer stage. where σ is the surface tension of the liquid. The maximum Rs that can be levitated depends on the critical acoustic Bond number Ba,cr, which was determined experimentally to be between 2.5 middle of a node for a wide range of transport velocities.
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