Shaping Liquid Droplets on an Active Air–Ferrofluid Interface

An air–liquid interface is important in many biological and industrial applications, where the manipulation of liquids on the air–liquid interface can have a significant impact. However, current manipulation techniques on the interface are mostly limited to transportation and trapping. Here, we report a magnetic liquid shaping method that can squeeze, rotate, and shape nonmagnetic liquids on an air–ferrofluid interface with programmable deformation. We can control the aspect ratio of the ellipse and generate repeatable quasi-static shapes of a hexadecane oil droplet. We can rotate droplets and stir liquids into spiral-like structures. We can also shape phase-changing liquids and fabricate shape-programmed thin films at the air–ferrofluid interface. The proposed method may potentially open up new possibilities for film fabrication, tissue engineering, and biological experiments that can be carried out at an air–liquid interface.


Technical note S1
The spreading coefficients for the manipulated liquids can be described with = − − , where , represent the surface tensions of the ferrofluid and oil droplet and represent the interfacial tension of the oil droplet and the ferrofluid. Hexadecane and silicon oil with a viscosity of 10 cSt have spreading coefficients of 1.64 and 16.71, respectively.

Technical note S2
Energy dispersive spectroscopy (EDS) data suggests that iron content was generally within several wt%, but there were local differences. Edges of the polystyrene film shown in Figure S6A contained higher concentration of iron (1.4 wt%), while some areas were found to contain no iron at all ( Figure S6B). In some cases, the iron particles formed aggregates on the surface of the film increasing the local iron content substantially (19.4 wt%) (see Figure S6C). Other major elements were carbon and oxygen, which likely originate from polystyrene and Fe2O3 nanoparticles in the ferrofluid, respectively. However, it should be S-7 noted that quantitative analysis of light elements with EDS is prone to error and thus the wt% values are approximations.

Technical note S3
The solenoid coils are comprised of 300-400 turns of SWG 27 copper wire in six layers wrapped around a martensitic steel core with a diameter of 1 mm. To achieve a circular arrangement of the solenoids and due to geometric considerations, four of the solenoids have shorter tips and the other four have longer tips. All solenoids are placed about 1 mm above the air-ferrofluid surface. The analog output sample rate was set to 1 kHz for generating low-frequency signals (f < 100Hz) and 10 kHz for generating high-frequency signals (f > 100Hz). The current controllers are powered with 12 V fixed voltage using a low noise power supply (E36233A, Keysight, USA). For calibration purposes, the magnetic flux density at the tip of each solenoid was measured using a hall effect sensor (SS495A1, Honeywell, USA). The resultant magnetic flux density for actuation currents up to 2 A is shown in the supplementary Figure S2. The B-I relations of individual solenoids were fitted to fifth-degree polynomials to generate currents needed to keep a consistent field strength at the tip of each solenoid. During simultaneous actuation of short and long solenoids for quasi-static shaping of droplets, we used 25 mT magnetic flux density at the solenoid tips. The magnetic flux density for shaping droplets was kept at 25 mT to avoid pushing the droplet outside the workspace during the shaping process. We used a 39 mT peak field strength at the solenid tips for droplet rotation and stirring experiments (see Supplementary Figure S5).

Supplementary Movies
S1: Squeezing a hexadecane droplet at the air-ferrofluid interface using two opposite solenoid actuations.
S2: Shaping of a hexadecane droplet at the air-ferrofluid interface using multiple solenoid actuations.
S3: Rotation of a hexadecane droplet at the air-ferrofluid interface.
S4: Stirring of liquid dispersed at the air-ferrofluid interface.
S5: Formation of polystyrene films at the air-ferrofluid interface.