Phononic crystals for surface acoustic wave drivendroplet positioning

M. Travagliatia,b*, R. Shiltona, G. De Simonia,b, C. M. Lazzarinib, V. Piazzaa, F. Beltrama,b, M. Cecchinib

a Center for Nanotechnology Innovation @ NEST, Istituto Italiano di Tecnologiab NEST, Scuola Normale Superiore and Istituto di Nanoscienze - CNR

*email: marco.travagliati@iit.it

References: [1] Friend et al. Rev. Mod. Phys. 83, 647 (2011) [2] Travagliati et al., Lab Chip 12, 2621 (2012)

Concept

When a travelling SAW impinges on a resonator there are two possibilities:1) its frequency matches an eigenfrequency of the cavity, it couples to an eigenmode and is fully transmitted (resonant condition)2) its frequency doesn’t match the resonantor eigenfrequency, it doesn’t penetrate in the resonator and fully re�ected (non-resonant condition)

If the device is designed so that the SAW actuator and a resonator along the SAW propagation di-rection are resonantly coupled:

1) if a droplet is between the actuator and the resonator, it moves the droplet toward the resonator due to droplet-SAW interactions2) if a droplet is in the resonator it changes the eigenmodes spectrum, therefore the actuator and resonator will be in the non-resonant condition. The droplet-SAW interaction will be suppresed and the droplet remains in the resonator even if the actuator is turned on. A SAW will be re�ected back to the actuator signal if there is a droplet present in the resonator without interaction with it (interaction-free measurement).

200 400 600 800 1000 12000

40

X (µm)

Y (µ

m) 0 500250

Energy density (W/m3)

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Energy density (W/m3)

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rgy

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1029690Frequency (MHz)

with resonator without resonator

fc

b) Free delay line (fc)

c) Absorption in the resonator (fc)

a) Free delay line trasmission

Finite element simulation of a device operating at 100 MHz demonstrat-ing the interection-free measurement by SAW. Panel a) shows the trasmis-sion spectrum of a SAW delay with a resonator (green line) compared to a standard delay with a free substrate (red line). The trasmittivity is sup-pressed except in peaks corresponding to cavity eigenfrequencies. Panel b) elucidates this point showing that at the peak frequency fc the energy is stored in the resonator. Panel c) demonstrates that if the actuator is op-erated at this frequency and an absorber (such as a droplet) is introduced in the cavity, the SAW will have negligible interaction with the absorber and therefore be re�ected by the resonator.

Simulations

Experimental demonstration

Panel a) shows the transmission spectrum of a device. The frequency region in light blue is the cavity band-width due to the �nite re�ective bandwidth of the dis-tributed mirrors. The central frequency fc is the resonant frequency. f0 represent a frequency out of the cavity bandwidth having the transmissivity of fc. Panels b) and c) present selected frames of the actuation experiments carried out respectively at fc and f0. In panel d) the drop-let position (respectively blue and red lines) and size (light blue area) as a function of time over an entire rep-resentative experiment are shown. When the IDT is op-erated at f0 (transparent mirrors) the droplet crosses the cavity without any velocity reduction. When the IDT is operated at fc (resonant-condition) the droplet is auto-matically positioned in the cavity demonstrating the interaction-free routing. Final expulsion of the droplet occurs at the critical diameter of 550 mm, and is related to mirror design. This is explained in the resonant cou-pling scenario.

fc fo

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S12

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170155Frequency (MHz)

a) Free delay line trasmission

t=0.000 sec

t=0.033 sec

t=0.066 sec

SAW fc

SAW fc

SAW fc

b) Routing at fc

t=0.000 sec

t=0.033 sec

t=0.066 sec

SAW fo

SAW fo

SAW fo

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drop

let p

ositi

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-220 s

d) Droplet evolution

Time (s)

AbstractRecently surface acoustic wave (SAW) driven biochips have drawn particular attention due to their pump–chip integration, excellent mixing e�ciency, and particle manipulation capabilities [1]. A key opera-tion which must be realized for the widespread use of portable micro total analysis system (μTAS) devices is automatic liquid routing. We propose and demonstrate the �rst scheme for automatic and passive SAW driven �uid routing [2]. This scheme is based on the resonant coupling phenomenon between travelling waves and the stationary modes of a cavity.