Continuous Optical Measurement of Internal Dynamics in Drying Colloidal Droplets.

Accessing the colloidal dynamics non-invasively and continuously during the phase transition of a colloidal system is challenging but critical. Here, we demonstrate the use of spatiotemporal coherence-gated light scattering for studying the internal dynamics of drying colloidal droplets. The continuously acquired signal originates from a picoliter-sized volume located at the droplet-substrate interface. The measurement is non-contact, non-invasive, and label free and permits real-time observations of both optical and mechanical changes in the measurement volume. Additionally, some macroscopic descriptors of the drying process can be constructed from the microscopic measurement, providing ample information of the process. Implemented with an endoscopic-like probe, this system can be easily incorporated into the existing drop profiling instruments, which is potential for the full characterization of dynamic colloidal droplets.