Development of dark-field dynamic light scattering microscopy and its application: Tracking dynamics of particles in condensed slurries spreading on planar/nonplanar substrates

Real-time three-dimensional micro-imaging of liquid front in spreading sessile droplet under transient spreading states

Dynamic wetting behaviors of sessile droplets on substrates play crucial roles for various industrial chemical processes. In the case of complete wetting, it has been proposed that a precursor film which is nanometer-order thickness and micrometer-order length further expands outside the macroscopic contact line of the sessile droplet. While the time evolution of the precursor film is believed to strikingly affect the macroscopic wetting behavior (e.g., spreading velocity), it had been hard to visualize the three-dimensional shape of the precursor film at the early stage of wetting. Further, although the spreading velocity rapidly decreases upon time at the early stage of the wetting (transient state) and then converged to be a constant at later time (steady state), conventional fluid mechanics theories generally describe only the wetting behavior at the steady state. Therefore, experimental observation of time evolution of the precursor film shape in the transient state is essential to proceed the theories to the next step. Here, a monochromatic laser interference microscope was developed to visualize three-dimensional shape of the wetting front of sessile droplets in real time. By detecting an interference of the laser reflected at the liquid and substrate surfaces, the precursor film was successfully visualized with a time resolution of 20 ms and a thickness resolution of about 3.5 nm at worst. For 10 cSt silicone oil on Si substrate, a 60-nm-thick and 70-μm-long precursor film was observed for 2-10 min after dropping and its spreading velocity which decreased with time was quantitatively analyzed.

Label-free tracking of whole-cell response on RGD functionalized surfaces to varied flow velocities generated by fluidic rotation

Fluidic flow plays important roles in colloid and interface sciences. Measuring adsorption, aggregation processes and living cell behavior under a fluidic environment with varied flow velocities in a parallel and high-throughput manner remains to be a challenging task. Here a method is introduced to monitor cell response to well-defined flow with varied velocities over an array of label-free resonant waveguide grating (RWG) based optical biosensors. The arrangement consists of a circular well with an array of biosensors at the bottom surface. By rotating the liquid over the biosensor array using a magnetic stirrer bar, flow velocities from zero to a predefined maximum can be easily established over different locations within the biosensor array as characterized in detail by numerical simulations. Cell adhesion and detachment measurements on an Arg-Gly-Asp (RGD) peptide functionalized surface were performed to demonstrate i) measurements at a wide range of simultaneous flow velocities over the same interface; ii) the possibility of parallel measurements at the same flow conditions in one run; and iii) the simple tuning of the employed range of flow velocities. Our setup made it possible to analyze the magnitude and rate of cell detachment at various flow velocities in parallel and determine the critical velocity and force where cells start to detach from the RGD motif displaying biomimetic surface. Furthermore, cellular response to simultaneous mechanical (flow) and chemical stimulation was also investigated using trypsin as a model. This study opens a new possibility to investigate interface phenomena under predefined and conveniently varied flow conditions.