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. Fabrica- and 3.6 (25). The definition of the Ba,cr implies that when the tion tolerances are challenging for acoustic resonances (Methods), vrms increases, the Rs has to decrease strongly. For a driving but future purely technical improvements in platform fabrication frequency of 24 kHz, the theoretical upper size limit for water will certainly help reduce such fluctuations, which, although pos- and hydrocarbons is around 2.7 mm and 1.6 mm in radius, sibly presenting a problem to individual sample analysis (22), are respectively (24). Approaching the upper limit of the static lev- not expected to affect sample advancing and processing seriously. itated droplet, our method allows transport and mixing of two Before node merging, the velocity of the droplets equals that droplets with a large size ratio. of the nodes. After node merging, the droplets approach one A–C 2 Fig. 3 shows the different behaviors observed during mixing another with a primary acceleration, €xp in the range of 0.1–1 m/s D fi of two water droplets and two tetradecane droplets. For head-on (Fig. 2 ), due to the primary scattering eld described by the merging of droplets, the different regimes of coalescence, bounc- acoustic potential. These acceleration values are in agreement 2 ing, and separation depend on the Weber number, We = 2Rsu ρs/σ, with the model, with the horizontal force being one order of where u is the relative impact velocity (26). The two water SI Text magnitude lower than the vertical force ( , section 3,and droplets coalesce at We = 0.42 (Fig. 3A and Movie S3). Shown in – Figs. S1 S3). When two droplets come into close proximity (Fig. Fig. 3B are two droplets for which, before merging, the low value C 2 ), the collision velocity increases sharply, with an additional of Ba (1.85 ± 0.47) prevents atomization. However, after merg- €x – 2 acceleration s of the order of 1 10 m/s , which cannot be solely ing, due to the larger size of the resulting combined droplet, Ba fi explained with the primary acoustic eld. Understanding of the increases above the critical point (2.33 ± 0.59), yielding explo- underlying physics is critical to realize acoustophoretic merging sive atomization (Movie S4). In Movie S5, explosive atomization of droplets. occurs at the very beginning of the merging process (Ba = 2.03 ± fi A secondary acoustic eld originating from the scattering of 0.72 and Ba = 1.93 ± 0.68). The slow motion of this movie cap- the primary waves on the two levitated objects significantly con- tures visually the effect of the secondary acoustic force discussed tributes to the collision dynamics (the total acceleration is calcu- earlier, causing significant deformation of the droplet just before lated as €x =€xp +€xs). The theoretical derivation of such a secondary merging. Fig. 3C shows that two tetradecane droplets first bounce force for the simple case of two closely placed spheres in an off (We = 0.875), the acoustic force then brings them back to- oscillating flow dates back to more than a century ago (23). The gether (double bouncing), and they coalesce at the second en- present work provides a unique platform to observe and inves- counter due to the lower impact velocity (We = 0.25; Movie S6). tigate this physical phenomenon. To this end, the present results The above-mentioned material independence of acoustic lev- constitute experimental quantitative confirmation of such secondary itation allows an unprecedented flexibility in the handling and acoustic force (SI Text, section 4). Assuming that the density of the interactions of solid and liquid entities. Fig. 3 D and E shows spheres ρs is much larger than that of ρ0 (as in the case of a liquid representative solid–liquid interaction scenarios. When a solid 3 droplet levitated in air), the attraction force on either sphere Fr, polystyrene particle (Rs = 0.5 mm, ρs = 1g/cm)andaliquid when the angle between the direction of wave propagation and (water) with a high surface tension collide, they do not merge

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Fig. 2. Controlled approach of two droplets in air. (A) Levitation potential inside a five-LPT device by varying the driving voltage of the LPTs (the levitation nodes are shown in blue). The small ellipses illustrate the experimental droplet positions. For clarity, only the emitting surfaces of the LPTs are shown. (B) Experimental results for the horizontal position of two acoustophoretically transported and eventually coalesced water droplets 0.84 mm in diameter for four traveling velocities, along with the numerical predictions (Figs. S6–S8). The oscillations of the droplets during translation are due to the very low damping effect of the surrounding fluid (air). (C) Experimental movement velocity (x_ ) of one of the two approaching water droplets. (D) Analytical and experimental values of total € acceleration (x) of the droplets near collision, with respect to the center-to-center droplet distance r (V0 = 2.6 m/s, H/λ = 0.496). The dotted line marks the primary € acceleration xp due to the acoustic potential field. The experimental uncertainty in the estimation of vrms is reflected in the error bars of the analytical data.

(Movie S7). On the other hand, particle encapsulation is ob- with pH 12 are mixed. The resulting solution becomes neutral, served if a low surface tension liquid, such as tetradecane, is used which maximizes the fluorescent emission. (Fig. 3D and Movie S8). The delicate nature of the acousto- The spatial and temporal modulation of the acoustic nodes of phoretic forces is advantageous when dealing with fragile mate- the present method allows the levitation and transport functions rials, such as crystals and porous media. In this respect, two typical of such nodes to be manifested in collaborative manner, wherein configurations of porous particle contactless transport and in- several nodes act as a group and can even merge to form a single teraction with a liquid drop are considered. First, if the particle is elongated node and carry out a task. This extends the capability large enough, it acts as a sponge, absorbing the liquid (Movie S9). of the present concept beyond the handling of spherical or near- With an evaporation rate of 0.0036 mm2/s for water (27), the spherical particle and droplet transport and to the controlled droplet volume reduction due to evaporation (1.1% over 10 s) motion of very elongated, wire-like objects. As a result, an ex- does not play a significant role. Alternatively, small porous par- tremely elongated object with a characteristic length L much ticles (e.g., instant coffee granule) dissolve in the water droplet larger than λ can be levitated, propelled, transported, and ro- (Fig. 3E and Movie S10). If the droplet solution is kept in the tated in a controllable manner, going beyond the widely accepted device and the water is allowed to evaporate completely, a bowl- limit (9, 11, 14, 15, 27) of λ/2 for the maximum size of an acous- shaped particle is formed. The behavior of stationary evaporating tically levitated sample (Fig. 4 and Movie S11). It is worth noting droplets containing diluted particles has only recently been studied that, to the best of our knowledge, no previously reported acoustic (28), and the findings are qualitatively comparable to those shown levitation devices, including near-field acoustic levitators (29), in Fig. 3E. have even been shown to levitate elongated objects of the kind The inherent substrate-independent and contamination-free showninFig.4. characteristics of the presented concept can be beneficial to major areas of biomedical and biochemical processes. Therefore, its bio- Conclusions compatibility was tested by performing contactless DNA trans- The presented concept paves the way for new classes of processes, fection in a temperature- and humidity-controlled environment ranging from substrate-free biological and chemical reactions to (Methods). Fig. 3F shows the schematic of the contactless DNA novel containerless materials processing methods, by acous- transfection and the transfected cells with blurred edges. Dem- tophoretically transporting and mixing droplets and particles. onstrating similar efficiency and viability as the standard process, Matter can now be manipulated at atmospheric conditions by our contactless transport method is shown to be biocompatible counteracting the effect of the gravitational force, without re- and suitable for DNA transfection. The occurrence of a photo- quiring a microgravity environment to remove its presence, and chemical switch in a contactless manner is shown in Fig. 3G, the contactless material handling can be extended to hazardous, where a fluorescein solution with pH 3 and a physiological solution chemical, or radioactive samples. Because the occurrence of a

Foresti et al. PNAS Early Edition | 3of6 Downloaded by guest on September 28, 2021 Fig. 3. Series of representative experiments with droplets or particles using the present acoustophoretic concept. (A) Stable water droplet coalescence (We = 0.42). (B) Explosive atomization after water droplet coalescence. (C) Tetradecane droplets bouncing (We = 0.875) and subsequently coalescing (We = 0.25). (D) Polystyrene particle collision with tetradecane droplet (We = 0.66). (E) Collision of a porous particle (instant coffee) and a water droplet (We = 0.24). The colored image at the bottom shows an instant coffee particle before and after the mixing/evaporation process. (F) Schematic of the contactless DNA transfection process and micrographs of the transfected cells with blurred edges. The transfection agent (TA) was premixed with the DNA solution. (G) Mixing

experiment of ﬂuorescein droplet (Left), with a logarithmic acid dissociation constant of pKa = 6.4 and pH 3, and of a droplet of physiological solution with pH 12 (Right). (Scale bars: B–E, 1 mm.)

phase change in the samples is not expected to affect their handling for controlling the amplitude, and with d = 16 mm, the theoretical move- − by the acoustophoretic force, related exothermal and endothermal ment step size is calculated to be 16 × 10 3/256 ≈ 62 μm. Due to the non- reactions do not limit the field of applications of the presented linearity of the LPTs and the node translation process, the actual positioning concept. Magnetic, electrostatic, diamagnetic, and optical forces resolution is lower than this value; the minimum step size at the gap between the two LPTs for the case shown in Fig. 2B was found to be around can be coupled with the acoustophoretic handling to enhance 400 μm. The V0 was measured using a laser vibrometer Polytec CLV-2534. even further the stability and the degrees of freedom of the Small holes with a diameter of 1.2 mm in the Plexiglas reflector allowed manipulation. droplet injection. Methods The Numerical Model. The Simulia Abaqus (6.9) package was used to calculate Experimental Setup. An LPT is composed of a back brass mass, a front alu- the levitation potential for different vibration amplitudes of the LPTs, and the minum mass, and a set of piezoelectric elements between the two, clamped sample position was estimated by determining the levitation potential by a bolt (SI Text, section 6, and Fig. S9). minima. A 3D finite element model was implemented to solve the linear In the optimized design, the emitting surface of LPTs had dimensions of acoustic equations in the frequency domain (SI Text, section 2). The linear 15 mm × 15 mm. The working frequency was f = 24.3 kHz, corresponding to acoustic pressure and particle velocity are sufficient to determine the radi- a wavelength in air of λ = 14.2 mm. The amplitude of the excitation voltage ation force (nonlinear) based on the levitation potential approach (SI Text, was adjusted using an in-house-designed microcontroller-based potenti- section 1). Due to symmetry, only half of the domain was modeled. The ometer and a LabVIEW program (National Instruments), and it was amplified radiating plate and the reflector were implemented as rigid shells, ac- to the desired value with an amplifier (DPA-4000T; ECLER), as shown in Fig. 1. counting for acoustic-structural coupling. On the radiating plate, the dis-

The amplitude resolution of the driving signal Ai(t) determines the move- placement boundary condition was applied. Inﬁnite elements were used at the ment resolution of the object. We used an eight-bit digital signal (256 levels) outer boundary of the acoustic medium. An additional acoustic medium

4of6 | www.pnas.org/cgi/doi/10.1073/pnas.1301860110 Foresti et al. Downloaded by guest on September 28, 2021 Fig. 4. Contactless transport of an elongated object (a toothpick with L= 8cm≈ 6λ,H≈ λ). Controlled rotation: top view (A) and side view (B). (C) Controlled translation: top view. In principle, there is no limit to the length of the object that can be handled.

volume was added to the model to avoid the influence of the nonreflecting surface tension, Rs,min decreases further. The lower limit is set by the critical boundary on the core of the levitator. The model was validated against the acoustic Bond number: if the acoustic field is too strong, atomization occurs SCIENCES experimental values of force acting on a sphere inside an axisymmetrical before droplet detachment.

levitator (30). The implemented quasistatic model suffices for our analysis. APPLIED PHYSICAL The characteristic time scale of the acoustophoretic forces is related to the DNA Transfection. HeLa cells were transfected in our device by mixing a drop τ characteristic time of the radiation pressure rad . The radiation pressure of cells (at a concentration of 3,000 cells per microliter) in fully supplemented acting on a sphere in a standing wave becomes fully established after a pe- medium with serum (DMEM; Sigma) and a drop of a stock solution containing riod of τ = N · τ , with N being the number of acoustic wave cycles rad wave the transfection reagent (3 μL of XtremeGENE HP; Roche) and a DNA needed to reach steady state and τ = f −1 = 41 μs being the wave period. wave plasmid (1 μg) encoding for enhanced GFP in 50 μL(100μL) of serum-free N was determined numerically to be between 50 and 100 (30, 31). The medium. The two drops fused by levitation were each 2.5 μL in volume. characteristic time of a sample transported at an average linear speed x_ is _ In general, the technique can also be applied to the study of incremental defined as τmov = λ=x, and for our case, λ∼d → τmov ∼ T. When using the transfections, in which several DNA plasmids can be introduced in the same acoustic potential for the dynamic analysis, we assume τmov τrad .Inour experiments, the highest linear transport speed of 4.9 mm/s corresponds to cells or in a multiple of the cell line. The temperature of the chamber was ± τmov min = Tmin = 3 s, which is much larger than τrad = 400 μs, justifying our maintained at 36 2 °C, and the relative humidity was close to saturation. assumptions. A humid environment is necessary to slow down evaporation. The process was fully viable; the cells were mixed with the DNA solution in our Droplet Input Method. In the feasibility studies carried out in this paper, contactless manipulation platform (requiring a few seconds for merging droplet input into the acoustic manipulator was easily achieved using a Teflon and remaining for 1–5 min in a steady levitation position) and were sub- = μ micropipette with an outer radius Rc 150 m(Movie S1). The acoustic force sequently plated into a 96-well plate containing 50 μL of fully supplemented acts on the droplet at the outlet of the micropipette and scales with its medium. After 18 h of incubation, GFP-positive cells were clearly detected 3 volume Rs , as the gravitational force (SI Text, section 1). When the sum of showing high levels of expression. The transfection efficiency of the levita- the two forces (gravitational and acoustic) overcomes the capillary force, the tion protocol was comparable to that of the standard protocol con- droplet is detached. The capillary force acting on a droplet at the outlet of ducted in microwells, whereas the amount of reagents used was reduced a pipette of radius R is F = 2πR σ. If gravity is the only force acting on a c c c by 50–75%. water droplet, by simply equating the surface tension force to the gravitational force at the point of detachment, the radius of the detached droplet is ACKNOWLEDGMENTS. We thank the Swiss National Science Founda- determined to be R = 1.18 mm, corresponding to a volume of 6.9 μL. Be- s tion (Grant 144397) for financial support. We also thank Paolo D’Aleo cause Rc is much smaller than the capillary length of water, 3.8 mm, no cor- [Eidgenössische Technische Hochschule (ETH) Zürich] for support with electronic rection to this calculation is needed (32). The presence of the additional controls, Bruno Kramer (ETH Zürich) for the manufacturing of parts of the acoustic force can reduce the droplet size at detachment (e.g., in Fig. S5A, experimental device, and Dr. Manish Tiwari (Laboratory of Thermodynamics Rs,min = 0.63 mm, volume = 1.05 μL). For hydrocarbons with a smaller in Emerging Technologies, ETH Zürich) for helpful discussions.

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