Controlled Chemistry via Contactless Manipulation and Merging of Droplets in an Acoustic Levitator

A unique, versatile, and material-independent approach to manipulate contactlessly and merge two chemically distinct droplets suspended in an acoustic levitator is reported. Large-amplitude axial oscillations are induced in the top droplet by low-frequency amplitude modulation of the ultrasonic carrier wave, which causes the top sample to merge with the sample in the pressure minimum below. The levitator is enclosed within a pressure-compatible process chamber to enable control of the environmental conditions. The merging technique permits precise control of the substances affecting the chemical reactions, the sample temperature, the volumes of the liquid reactants down to the picoliter range, and the mixing locations in space and time. The performance of this approach is demonstrated by merging droplets of water (H₂O) and ethanol (C₂H₅OH), conducting an acid-base reaction between aqueous droplets of sodium hydroxycarbonate (NaHCO₃) and acetic acid (CH₃COOH), the hypergolic explosion produced via merging a droplet of an ionic liquid with nitric acid (HNO₃), and the coalescence of a solid particle (CuSO₄·5H₂O) and a water droplet followed by dehydration using a carbon dioxide laser. The physical and chemical changes produced by the merging are traced in real time via complementary Raman, Fourier-transform infrared, and ultraviolet-visible spectroscopies. The concept of the contactless manipulation of liquid droplets and solid particles may fundamentally change how scientists control and study chemical reactions relevant to, for example, combustion systems, material sciences, medicinal chemistry, planetary sciences, and biochemistry.