Automated Droplet Manipulation Using Closed Loop Axisymmetric Drop Shape Analysis.
Droplet manipulation plays an important role in a wide range of scientific and industrial applications, such as synthesis of thin film materials, control of interfacial reactions, and operation of digital microfluidics. Compared to micron sized droplets, which are commonly considered as spherical beads, millimeter sized droplets are generally deformable by gravity, thus introducing nonlinearity into control of droplet properties. Such a nonlinear drop shape effect is especially crucial for droplet manipulation, even for small droplets, at the presence of surfactants. In this paper, we have developed a novel closed loop axisymmetric drop shape analysis (ADSA) integrated into a constrained drop surfactometer (CDS) for manipulating millimeter sized droplets. The closed loop ADSA generalizes applications of the traditional drop shape analysis from a surface tension measurement methodology to a sophisticated tool for manipulating droplets in real time. We have demonstrated the feasibility and advantages of the closed loop ADSA in three applications, including control of drop volume by automatically compensating natural evaporation, precise control of surface area variations for high fidelity biophysical simulations of natural pulmonary surfactant, and steady control of surface pressure for in situ Langmuir Blodgett transfer from droplets. All these applications have demonstrated the accuracy, versatility, applicability, and automation of this new ADSA based droplet manipulation technique. Combining with CDS, the closed loop ADSA holds great promise for advancing droplet manipulation in a variety of material and surface science applications, such as thin film fabrication, self assembly, and biophysical study of pulmonary surfactant.
Langmuir the ACS journal of surfaces and colloids