Sound-controlled fluidic processor

Abstract

Precision processing of various liquids while maintaining their purity holds immense potential for many applications. However, liquids tend to leave residues that contaminate handling tools and compromise volumetric precision, necessitating contactless strategies to prevent liquid loss. Biological and chemical samples carried by fluids can be sensitive to physical stimuli, demanding mild but effective means to preserve integrity. Here, we report a sound-controlled fluidic processor for complete and well-controlled microfluidic functions, including moving, merging, mixing, and cleaving, in contactless and harmless manners. The processor generates an acoustophoretic force field that serves as a versatile toolbox for manipulating droplets with surface tension from 17.9 to 72 millinewtons per meter and volume from 1 nanoliter to 3 milliliters, offering a wealth of operations crucial to fundamental biomedical and chemical practices.

Fig. 1.. Design and demonstration of the SFP.

(A) Schematic of the SFP. The acoustic wave generated by the acoustic source in air drives liquid droplet motion on a slippery surface. (B) Schematics of droplet manipulation by the SFP, including pushing, pulling, merging, separating, and vortexing. The gray-blue arrows indicate the moving direction of the droplets, while the red arrows indicate the moving direction of the acoustic source. The red gradients with black curved lines indicate the acoustic field. (C) Chronophotographs showing acoustic manipulation of water droplets with different volumes. Here, the SFP pushes a 0.01-μl water droplet and a 1000-μl water puddle and pulls a 5-μl water droplet. (D) Chronophotographs showing manipulation of a 2.5-μl blue-dyed water droplet on a vertical slippery surface. The dashed red arrows indicate acoustic source and droplet moving directions, and the light blue arrow indicates the direction of gravity. (E) Sequential images showing on-demand patterning of randomly positioned oil droplets, including hexadecane (blue), mineral oil (brown), and silicon oil (pink). The droplets are dispensed onto the slippery surface together and then repositioned into a pattern of letters “H,” “K,” and “U,” using the SFP. (F) Droplet manipulation performance comparison with other techniques in terms of maximum velocity versus controllable volume range. Image credit: W.L. and H.Z., The University of Hong Kong.

Fig. 2.. Liquid operations based on the SFP.

(A) Schematics demonstrating navigation (pushing and pulling), separation, and merging operations based on the SFP. The red arrows represent droplet motion direction, while the dashed black arrows represent acoustic source motion direction. (B to G) Operation of silicon oil droplets dyed by oil red. (B) Chronophotographs showing long-distance (~65 mm) pulling of a 2.5-μl droplet. (C) Sequential images showing pushing of a 1-ml puddle. (D) Sequential images showing pulling of two ~1-nl droplets. The dashed red circles indicate the initial positions of ~1-nl droplets. (E) Sequential images showing pushing of a 2.5-μl droplet along a square path. (F) Schematics and sequential images showing controlled separation of a 10-μl droplet, where the separation process begins with droplet deformed to a “U” shape and the motion direction of the acoustic source determines the separation volume. (G) Sequential images showing fusion of two silicon oil droplets influenced by pulling (left) and pushing (right). Image credit: H.Z., The University of Hong Kong.

Fig. 3.. Working mechanism of the SFP.

(A) Three typical manipulation modes of the SFP presented by a 10-μl dodecane droplet. We defined the manipulation modes by two parameters: acoustic source center to droplet center distance D and acoustic source center to slippery surface distance H. The black arrows and circles with crosses indicate the movement direction of the acoustic source. (B) Top view of acoustic pressure distribution on the slippery surface and the dodecane droplet at three typical manipulation positions (D = 0.3, 1.5, and 4.4 mm) at H = 2.7 mm. (C) Top and side view of the acoustic pressure and force distribution on the droplet surface at the corresponding manipulation positions. The direction and the length of red arrows represent the force direction and relative amplitude. (D) Net acoustic acceleration in droplets’ motion direction integrated over the surfaces of a 10-μl dodecane droplet and a 10-μl water droplet placed at H = 2.7 mm with respect to D. The positive acceleration values (red) represent pushing of the droplet, and the negative values (blue) represent pulling of the droplet. (E) The net acoustic acceleration of a droplet is determined by the position relative to the acoustic source, namely, D and H. Image credit: H.Z., The University of Hong Kong.

Fig. 4.. Acoustic vortexing and versatility of the SFP.

(A) Simulation of flow field, acoustic pressure distribution, and force distribution in a 200-μl water droplet placed at H = 2.7 mm and D = 7.6 mm relative to acoustic source tilted 45°. The gray lines with arrows indicate the flow direction, the red arrows indicate the surface force direction, and their lengths indicate relative magnitude. (B) Sequential images demonstrating flow field by acoustic vortexing and diffusion of a 200-μl water droplet merged with a 5-μl blue-dyed water droplet. The dashed black arrow represents the moving direction of the acoustic source. (C) Acoustic vortexing facilitates accelerated mixing performance compared to diffusion. Here, combinations of blue-dyed ink droplets and water droplets, adding up to 200 μl, are mixed simultaneously using acoustic vortexing and diffusion. After the completion of acoustic vortexing, we measure the mixing index, which is defined as one minus the ratio of the SD of the gray values within the droplet after and before mixing. a.u., arbitrary units. (D and E) Versatile chemical reactions performed by the SFP on various substrates of choice. The dashed arrows indicate droplet motion directions. (D) Sequential images showing acoustic vortexing of a potassium hydroxide droplet 1 with a thymol blue droplet 2 and then a hydrochloride droplet 3 on a copper substrate. (E) Sequential images showing acoustic vortexing of iodine saturated solution in 5% (w/v) sodium hydroxide on a nitrile rubber substrate with sodium hydroxide droplets i and ii. The precursor droplets react with acetone droplet iii and HCl droplet iv to form iodoform suspension and iodine precipitation, respectively. Image credit: H.Z., The University of Hong Kong.

Fig. 5.. Biodetection and organoid screening with the SFP.

(A and B) Schematic diagram and sequential images demonstrating protein detection via on-demand separation of biuret reagent, a high surface tension fluid, with the aid of a separation barrier. A 20-μl biuret reagent reservoir is separated into 5-μl droplets and mixed with 20-μl droplets with bovine albumin at different concentrations. The dashed black arrows represent the synchrotronic motion of the droplet and the acoustic source. The dashed purple circles indicate the initial droplets of clear bovine albumin solution. (C to E) Liver organoid culture and functional analysis with the SFP. (C) Schematic diagram illustrating mouse liver organoid culturing and functionality characterization. The dashed black arrows represent the synchrotronic motion of the droplet and the acoustic source. The black arrows represent the process flow of the SFP. (D) Immunofluorescent images and fluorescent intensity comparing verapamil’s influence on rhodamine 123 localization capability of mouse liver organoids. (E) Immunofluorescent images comparing biliatresone toxicity to liver organoids, where dimethyl sulfoxide (DMSO) and biliatresone with concentrations at 2 and 20 μg ml⁻¹ in DMSO are added. The green fluorescence indicates microfilaments F-actin distributions in cytoskeletons, and the blue fluorescence indicates cell nuclei arrangements and distributions. DAPI, 4′,6-diamidino-2-phenylindole. **** indicate P value less than 0.0001, making the fluorescent intensity difference statistically significant. Image credit: H.Z., The University of Hong Kong.