28 Feb Acoustic manipulation of endoskeletal droplets on a microfluidic platform
As droplet microfluidics progresses, more sophisticated applications get developed by researchers which in turn require more complex techniques for manipulation. One class of such droplets is endoskeletal droplets. Endoskeletal droplets are complex colloids in which a solid phase is cast within a liquid emulsion. This solid phase facilitates more controllability over the shape and the orientation of the droplets in response to external stimuli. In this week’s research highlight, we will introduce a microfluidic chip that enables researchers to manipulate disk-in-sphere endoskeletal droplets using acoustic waves to form clusters.
“Manipulation of internal structure of droplet/particle is rarely explored and remains challenging due to its complicated nature. Here we demonstrated the manipulation of internal structure of disk-in-sphere endoskeletal droplets using acoustic wave. We developed a model to investigate the physical mechanisms behind this interesting phenomenon. Theoretical analysis of the acoustic interactions indicated that these assembly dynamics arise from a balance of the primary and secondary radiation forces.”, the authors explained.
The working mechanism of the microfluidic device
The proposed microfluidic device consisted of two separate microfluidic chips both microfabricated with PDMS using standard photo/soft lithography techniques. The first microfluidic chip was a flow-focusing microfluidic droplet generator that was pre-coated to make the surface more hydrophilic. The other microfluidic chip was microfabricated for SAW experiments. The solutions were pre-warmed before running them through the microfluidic chip to melt the reagents. The syringes were covered with a syringe heater to keep the reagents warm. Therefore, the microfluidic flow-focusing chip made ~5 µm isotropic droplets. The droplets were then cooled down so the PFDD could solidify and create the disk shape structure within the droplets. Microscope images of the droplets confirmed that the solid disk shape structures were formed successfully and could move inside the droplets with no hassle. Next, the droplets were diluted 10x and a 10 µL fraction was pipetted at the edge of the PDMS microfluidic channel which then flowed through the device through the microfluidic device via capillary force where they were exposed to the SAW electrodes.
Having transferred the endoskeletal droplets to the microfluidic chamber, the team then exposed them to acoustic waves. The surface acoustic waves (SAW) with a center frequency of 20 MHz were generated to examine the effect on the collective behaviour of the droplets. It was shown that the droplets formed clusters as soon as the SAW was turned on. Depending on the number of droplets in the cluster, the orientation of the droplets changes with respect to the substrate.
“This reversible on-demand manipulation of the disk orientation can potentially be utilized in various filtering as well as imaging applications. This distinctive dynamic manipulation could potentially provide further opportunities for directed colloidal assembly with dynamic and acoustically tunable internal structures and pave the way towards manipulation of the internal structures of organoids and cells.”, the authors concluded.
Figures and the abstract are reproduced from Shakya, G., Yang, T., Gao, Y. et al. Acoustically manipulating internal structure of disk-in-sphere endoskeletal droplets. Nat Commun 13, 987 (2022). https://doi.org/10.1038/s41467-022-28574-4 under Creative Commons Attribution 4.0 International License
Read the original article: Acoustically manipulating internal structure of disk-in-sphere endoskeletal droplets