Programmable droplet manipulation and wetting with soft magnetic carpets

Abstract

The ability to regulate interfacial and wetting properties is highly demanded in anti-icing, anti-biofouling, and medical and energy applications. Recent work on liquid-infused systems achieved switching wetting properties, which allow us to turn between slip and pin states. However, patterning the wetting of surfaces in a dynamic fashion still remains a challenge. In this work, we use programmable wetting to activate and propel droplets over large distances. We achieve this with liquid-infused soft magnetic carpets (SMCs) that consist of pillars that are responsive to external magnetic stimuli. Liquid-infused SMCs, which are sticky for a water droplet, become slippery upon application of a magnetic field. Application of a patterned magnetic field results in a patterned wetting on the SMC. A traveling magnetic field wave translates the patterned wetting on the substrate, which allows droplet manipulation. The droplet speed increases with an increased contact angle and with the droplet size, which offers a potential method to sort and separate droplets with respect to their contact angle or size. Furthermore, programmable control of the droplet allows us to conduct reactions by combining droplets loaded with reagents. Such an ability of conducting small-scale reactions on SMCs has the potential to be used for automated analytical testing, diagnostics, and screening, with a potential to reduce the chemical waste.

Keywords: droplet manipulation; liquid-infused surface; magnetic fields; soft actuators; soft materials.

Fig. 1.

Fabrication of soft, magnetic carpets with an infusion layer. Schematics of the soft carpet fabrication. (A) A prepolymer mixed with NdFeB particles is spread over a substrate. (B) Formation of pillar-like structures featuring a magnetic polarization upon exposition to a magnetic field. While being kept at this configuration, the prepolymer is cured at 60 °C, resulting in soft standing pillars with a magnetic dipole determined by the magnet beneath. (C) Structures infused with a hydrophobic liquid will exhibit pillars that are straight and reach out of the infusion liquid, thus pinning water droplets on their tips. (D) Bending of the pillars upon exposition to a magnetic field opposed to the one in B allows for a direct contact of the droplet to the infusion liquid. (E) Micrograph of the soft pillars in the slip state, in which they are exposed to a magnetic field of opposite polarization. (F) Micrograph of the as-casted soft pillars in the pin state. The inset shows a three-dimensional reconstruction of images taken in reflection mode at different heights of the pillars. (Scale bars, 200 μm.)

Fig. 2.

Switchable and dynamically patternable wetting of the surface via lubricant infusion. (A) A sketch depicting the wetting states of the liquid-infused, magnetic carpet without and with a magnetic field of opposite polarization. N and S stand for the north and south pole of the magnet, respectively. (B) Time lapse of a 30-μL droplet on a silicon oil-infused carpet. Because the carpet is tilted, the droplet slips when the magnetic field is on and stays pinned when it is off. (C) The wetting properties at the surface can be patterned by placing the carpet on a magnet array with alternating field polarizations. Time lapse shows that droplets slip or stay pinned depending on the local wetting of a carpet tilted by 30°. The magnet array can be shifted with respect to the carpet to switch the slip and pin states. (D) Contact angle of a droplet on an infused surface as a function of the external magnetic field. (E) Velocity of a 30-μL droplet as a function of the tilt angle of the substrate with a silicon oil viscosity of 10,000 cSt. (F) Dependence of the minimal slip angle on the viscosity of the silicon oil. (Scale bars, 5 mm.)

Fig. 3.

Droplet transport via relocation of the pinning point. (A) Sketches depicting the pinning and the motion of the droplet by the backstroke of the pillar where the droplet pins. Note that the location of the droplet shifts as the pillars wave from left to right. The curved black arrow shows the motion of the pillar tip in time between the frames I through VI in B and C. (B) Snapshots of a single-droplet motion due to the wave movement of the pillars. (C) Droplet displacement plotted over time based on the time lapse in B. (D) A sketch showing the orientation of the pillars exposed to a magnet array. Time lapse of 40-μL droplets moving with a magnetic field wave on a soft carpet: top (E) and side views (F). The standing pillars that pin the droplet are clearly visible in F. (G) Magnetic potential of a set of magnets arranged in a rotor shape, which was used for the circular motion of a droplet. (H) Time lapse evidencing the droplet motion achieved with rotation of the rotor-shaped magnet shown in G set underneath the soft carpet. (I) Time lapse of two competing droplets under the influence of the same magnetic field. The deionized (DI) water droplet has a higher-contact angle and moves faster than the 2% PAA droplet. (J) Efficiency of the water droplet motion with respect to the translated magnetic field for different PAA surfactant concentrations. The efficiency is taken as the droplet displacement with respect to the displacement of the traveling magnetic field and varies from 0 to 1. (K) Time lapse of two droplets that are manipulated to merge and are driven to an opposite direction afterward. The slow pink droplet is merged with a faster water droplet. The PAA droplet is labeled with a rhodamine dye. (L) Sketch demonstrating droplet reactions. A neutral droplet carrying a pH indicator (phenolphthalein, HIn) is first merged with a high-pH droplet, changing the color of the neutral droplet to pink because of a pH increase. Next, a low-pH droplet is added to the system, decreasing the pH to an acidic one and yielding a transparent droplet. (M) Time lapse of the millifluidic reaction experiment described in L, in which the three droplets on the soft carpet are sequentially brought together. The neutral droplet becomes pink and then transparent again. (Scale bars, 5 mm.)

Fig. 4.

Droplet transport with a ferrofluid liquid infusion. (A) Magnetic flux density over an array of rod-shaped magnets computed with finite element analysis. This magnetic flux density generates the surface topography formed by the ferrofluid infused on the soft carpet. The planes xz and xy indicate the side and top views, respectively. (B) Finite element analysis of the magnetic scalar potential formed on top of a magnet array. The polarization direction (N/S depicts the north and south pole of the magnet, respectively) along the z-axis of each magnet in the array is indicated in the sketch. (C) Schematics depicting the case of excess ferrofluid in the layer used to infuse the soft carpet. Here, the system stays at the slip state, as the pillar edges are constantly covered by the infusion layer. (D) The motion of the surface topography with a moving magnetic wave carries the 20-μL droplet along the same direction of the wave motion. Snapshots show the droplet transport at the slip state over time. (E) A sketch depicting the case of a soft carpet infused with a layer poor in ferrofluid. Here, the surface exhibits both pin and slip states. Droplets pin quickly, and the strokes of these pinning pillars drive the droplet motion, similar to the motion mechanism in Fig. 3. (F) The strokes of the pinning pillars push the 60-μL droplet in an opposite direction to the moving magnetic field wave. Snapshots show the droplet transport at the pin state over time. (G) A rotor-shaped magnet array manipulates ten 50-μL droplets with a ferrofluid infusion. Magnetic flux density of the rotor-shaped magnet array placed under the infused substrate (H) and time lapse of a 50-μL water droplet being manipulated on a circular path (I). (Scale bars, 5 mm.)