Magnetic field induced motion of water droplets and bubbles on the lubricant coated surface

Dynamics of droplet motion over hydrophobic surfaces with functionalized and non-functionalized ferro particles

Dynamically manipulating droplet motion on hydrophobic surfaces is crucial in various fields, including biomedical, sensing, actuation, and oil-water separation applications. Ferrofluid droplets can be manipulated and controlled using external magnetic forces. The creation of ferrofluids involves multiple procedures that can affect the functionality and stability of droplet manipulation, limiting their use in sustainable applications. This study investigates the dynamics of droplet motion over functionalized and non-functionalized ferroparticles, considering different droplet volumes, ferroparticle layer widths, and wt% concentrations. The translational and sliding velocities of the droplets are measured using high-speed camera recording with a tracker application. The finding revealed the transformation of a droplet sliding motion into a rolling motion with propulsion under the magnetic influence. The sliding velocity increases for the droplets moving over the ordinary ferroparticles on the hydrophobic surface. However, the droplet motion is dominated by rolling in the case of hydrophobic ferro particles. The droplet sliding velocity rises sharply at high concentrations (or layer width) of ferroparticle as the magnetic bond number rises sharply to 3. A newborn droplet adheres to the magnet surface during droplet rolling and sliding motion.

Versatile bubble maneuvering on photopyroelectric slippery surfaces

Contactless bubble manipulation with a high spatiotemporal resolution brings a qualitative leap forward in a variety of applications. Despite considerable advances, light-induced bubble maneuvering remains challenging in terms of robust transportation, splitting and detachment. Here, a photopyroelectric slippery surface (PESS) with a sandwich structure is constructed to achieve the versatile bubble manipulation. Due to the generated dielectric wetting and nonuniform electric field under the irradiation of near infrared (NIR) light, a bubble is subject to both the Laplace force and dielectrophoresis force, enabling a high-efficiency bubble steering. We demonstrate that the splitting, merging and detachment of underwater bubbles can be achieved with high flexibility and precision, high velocity and agile direction maneuverability. We further extend the capability of bubble control to microrobots for cargo transportation, micropart assembly and transmission of gear structures. We envision this robust bubble manipulation strategy on the PESS would provide a valuable platform for various bubble-involved processes, ranging from microfluidic devices to soft robotics.

Magnetocontrollable droplet mobility on liquid crystal-infused porous surfaces

Magnetocontrollable droplet mobility on surfaces of both solids and simple fluids have been widely used in a wide range of applications. However, little is understood about the effect of the magnetic field on the wettability and mobility of droplets on structured fluids. Here, we report the manipulation of the dynamic behaviors of water droplets on a film of thermotropic liquid crystals (LCs). We find that the static wetting behavior and static friction of water droplets on a 4'-octyl-4-biphenylcarbonitrile (8CB) film strongly depend on the LC mesophases, and that a magnetic field caused no measurable change to these properties. However, we find that the droplet dynamics can be affected by a magnetic field as it slides on a nematic 8CB film, but not on isotropic 8CB, and is dependent on both the direction and strength of the magnetic field. By measuring the dynamic friction of a droplet sliding on a nematic 8CB film, we find that a magnetic field alters the internal orientational ordering of the 8CB which in turn affects its viscosity. We support this interpretation with a scaling argument using the LC magnetic coherence length that includes (i) the elastic energy from the long-range orientational ordering of 8CB and (ii) the free energy from the interaction between 8CB and a magnetic field. Overall, these results advance our understanding of droplet mobility on LC films and enable new designs for responsive surfaces that can manipulate the mobility of water droplets.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (further details of the stability of LCIPS against water-induced dewetting, the interfacial tension and contact angle measurement using a goniometer, the estimation of the thickness of LC wrapping layer at air-water interface on droplets, SEM measurements, the average sliding velocity of a water droplet on 5CB, E7, silicone oil, and mineral oil films with and without a magnetic field, representative force diagram (F _d versus time) of a 3-µL water droplet moving at a speed of 0.1 mm/s on a nematic 8CB film, F _dynamic acting on 3 µL water droplets moving at speeds of 0.1-1 mm/s on an isotropic 8CB film, the calculated magnetic coherence length as a function of the magnitude of the magnetic field applied to the nematic LCIPS, and the apparent advancing and receding contact angles of a moving water droplet on nematic LCIPS as a function of time, and polarized light micrographs (top view) of a nematic 8CB film between two DMOAP-functionalized glass slides before and after applying a horizontal magnetic field) is available in the online version of this article at 10.1007/s12274-022-5318-y.

Magnetocontrollable droplet mobility on liquid crystal-infused porous surfaces

Magnetocontrollable droplet mobility on surfaces of both solids and simple fluids have been widely used in a wide range of applications. However, little is understood about the effect of the magnetic field on the wettability and mobility of droplets on structured fluids. Here, we report the manipulation of the dynamic behaviors of water droplets on a film of thermotropic liquid crystals (LCs). We find that the static wetting behavior and static friction of water droplets on a 4'-octyl-4-biphenylcarbonitrile (8CB) film strongly depend on the LC mesophases, and that a magnetic field caused no measurable change to these properties. However, we find that the droplet dynamics can be affected by a magnetic field as it slides on a nematic 8CB film, but not on isotropic 8CB, and is dependent on both the direction and strength of the magnetic field. By measuring the dynamic friction of a droplet sliding on a nematic 8CB film, we find that a magnetic field alters the internal orientational ordering of the 8CB which in turn affects its viscosity. We support this interpretation with a scaling argument using the LC magnetic coherence length that includes (i) the elastic energy from the long-range orientational ordering of 8CB and (ii) the free energy from the interaction between 8CB and a magnetic field. Overall, these results advance our understanding of droplet mobility on LC films and enable new designs for responsive surfaces that can manipulate the mobility of water droplets.

Electronic Supplementary Material

Supplementary material (further details of the stability of LCIPS against water-induced dewetting, the interfacial tension and contact angle measurement using a goniometer, the estimation of the thickness of LC wrapping layer at air-water interface on droplets, SEM measurements, the average sliding velocity of a water droplet on 5CB, E7, silicone oil, and mineral oil films with and without a magnetic field, representative force diagram (Fd versus time) of a 3-µL water droplet moving at a speed of 0.1 mm/s on a nematic 8CB film, Fdynamic acting on 3 µL water droplets moving at speeds of 0.1-1 mm/s on an isotropic 8CB film, the calculated magnetic coherence length as a function of the magnitude of the magnetic field applied to the nematic LCIPS, and the apparent advancing and receding contact angles of a moving water droplet on nematic LCIPS as a function of time, and polarized light micrographs (top view) of a nematic 8CB film between two DMOAP-functionalized glass slides before and after applying a horizontal magnetic field) is available in the online version of this article at 10.1007/s12274-022-5318-y.

Sliding and Rolling Motion of a Ferro-Liquid Droplet on the Hydrophobic Surface under Magnetic Influence

The ferro-liquid droplet manipulation on hydrophobic surfaces remains vital for various applications in biomedicine, sensors and actuators, and oil-water separation. The magnetic influence of ferro-liquid droplets on the hydrophobic surface is elucidated. The mechanisms of a newborn droplet formation under the magnetic force are explored. The sliding and rolling dynamics of the ferro-liquid droplets are assessed for the various concentrations wt % of ferro-particles. High-speed recording and a tracker program are used to evaluate the droplet sliding and translational velocities. It is demonstrated that the mode of droplet motion changes from sliding to rolling as the magnetic Bond number increases, in which case, the droplet position becomes close to the magnet surface. The translational velocity of the droplet under rolling mode increases as the ferro-particle concentration in the droplet fluid increases. A further increase of the magnetic Bond number results in the creation of a newborn droplet attached to the magnet surface.

Directional motion of the foam carrying oils driven by the magnetic field

Foams are substances widely used the foam flooding technology, which aim to greatly improve the residual oil recovery. In the present study, we perform a comprehensive investigation on the oil removal process driven by the foam embedded with magnetic particles, under the action of the magnetic force. The experiment shows that the addition of magnetic particles has little effect on the stability of the foam. During the motion of the foam, its maximum displacement and maximum acceleration are fully explored. Such factors as the volume of the foam, the volume of the oil droplet, the mass concentration of magnetic particles, and the Young's contact angle of surfactant on solid are surveyed in detail. The function curves of the maximum displacement and the maximum acceleration with respect to these variables are obtained in the experiment, and the selection of some optimal parameters is advised. Moreover, the dimensional analysis has been conducted and several scaling laws are given, which are in agreement with the experimental results. These findings are beneficial to understand the oil displacement with the aid of magnetic field, which also provide some inspirations on drug delivery, robots and micro-fluidics.

Composite Liquid Marbles as a Macroscopic Model System Representing Shedding of Enveloped Viruses

A model macroscopic system imitating the entry of viruses into living cells is suggested. The system represents the contact of a composite (core-shell) liquid marble with hydrophobic/hydrophilic particles. Composite liquid marbles are water droplets coated with silicone oil armored with nanometer-sized hydrophobic particles serving as an interfacial model of a living cell. Composite marbles absorbed hydrophilic polymer particles but prevented hydrophobic particles from entering their core. Swallowing of hydrophilic particles by composite marbles resembles the penetration of viruses into living cells. The interfacial mechanism of absorption is suggested.