Self-assembly is a process in which interacting bodies are autonomously driven into ordered structures. Static structures such as crystals often form through simple energy minimization, whereas ...dynamic ones require continuous energy input to grow and sustain. Dynamic systems are ubiquitous in nature and biology but have proven challenging to understand and engineer. Here, we bridge the gap from static to dynamic self-assembly by introducing a model system based on ferrofluid droplets on superhydrophobic surfaces. The droplets self-assemble under a static external magnetic field into simple patterns that can be switched to complicated dynamic dissipative structures by applying a time-varying magnetic field. The transition between the static and dynamic patterns involves kinetic trapping and shows complexity that can be directly visualized.
Reliable characterization of wetting properties is essential for the development and optimization of superhydrophobic surfaces. Here, the dynamics of superhydrophobicity is studied including droplet ...friction and wetting transitions by using droplet oscillations on micropillared surfaces. Analyzing droplet oscillations by high‐speed camera makes it possible to obtain energy dissipation parameters such as contact angle hysteresis force and viscous damping coefficients, which indicate pinning and viscous losses, respectively. It is shown that the dissipative forces increase with increasing solid fraction and magnetic force. For 10 µm diameter pillars, the solid fraction range within which droplet oscillations are possible is between 0.97% and 2.18%. Beyond the upper limit, the oscillations become heavily damped due to high friction force. Below the lower limit, the droplet is no longer supported by the pillar tops and undergoes a Cassie–Wenzel transition. This transition is found to occur at lower pressure for a moving droplet than for a static droplet. The findings can help to optimize micropillared surfaces for low‐friction droplet transport.
Superhydrophobicity of micropillared surfaces is evaluated using transverse droplet oscillations. The well‐defined structure of the surfaces makes it possible to relate the measured droplet friction and energy dissipation to the solid fraction. Additionally, controlling the normal force exerted on the magnetic droplet allows the measurement of the impalement pressure and the induced wetting transition for a droplet in motion.
Manipulation of small-scale matter is a fundamental topic in micro and nanorobotics. Numerous magnetic robotic systems have been developed for the manipulation of microparticles in an ambient ...environment, liquid, as well as on the air-liquid interface. These systems move intrinsically magnetic or magnetically tagged objects by inducing a magnetic torque or force. However, most of the materials found in nature are nonmagnetic. Here, we report a ferrofluidic manipulator for automatic 2-D manipulation of nonmagnetic objects floating on top of a ferrofluid. The manipulation system employs 8-cm-scale solenoids, which can move nonmagnetic particles by deforming the air-ferrofluid interface. Using linear programming, we can control the motion of the nonmagnetic particles with a predefined trajectory of a line, square, and circle with a precision of 25.1 <inline-formula><tex-math notation="LaTeX"> \pm </tex-math></inline-formula> 19.5, 34.4 <inline-formula><tex-math notation="LaTeX"> \pm </tex-math></inline-formula> 28.4, and 33.4 <inline-formula><tex-math notation="LaTeX"> \pm </tex-math></inline-formula> 26.6 μm, respectively. The ferrofluidic manipulator is versatile with the materials and the shapes of the objects under manipulation. We have successfully manipulated particles made of polyethylene, polystyrene, a silicon chip, and poppy and sesame seeds. This article shows a promising venue for the manipulation of living and nonliving matter at the air-liquid interface.
Abstract
Surface wetting is a multiscale phenomenon where properties at the macroscale are determined by features at much smaller length scales, such as nanoscale surface topographies. Traditionally, ...the wetting of surfaces is quantified by the macroscopic contact angle that a liquid droplet makes, but this approach suffers from various limitations. In recent years, several techniques have been developed to address these shortcomings, ranging from direct measurements of pinning forces using cantilever-based force probes to atomic force microscopy methods. In this review, we will discuss how these new techniques allow for the probing of surface wetting properties in far greater detail. Advances in surface characterization techniques will improve our understanding of surface wetting and facilitate the design of functional surfaces and materials, including for antifogging and antifouling applications.
Accurate wetting characterization is crucial for the development of next‐generation superhydrophobic surfaces. Traditionally, wetting properties are measured with a contact angle goniometer (CAG) ...suitable for a broad range of surfaces. However, due to optical errors and challenges in baseline positioning, the CAG method suffers from inaccuracies on superhydrophobic surfaces. Here we present an improved version of the oscillating droplet tribometer (ODT), which can reliably assess wetting properties on superhydrophobic surfaces by measuring the frictional forces of a water‐based ferrofluid droplet oscillating in a magnetic field. We demonstrate that ODT has superior accuracy compared to CAG by measuring the wetting properties of four different superhydrophobic surfaces (commercial Glaco and Hydrobead coatings, black silicon coated with fluoropolymer, and nanostructured copper modified with lauric acid). We show that ODT can detect the small but significant changes in wetting properties caused by the thermal restructuring of surfaces that are undetectable by CAG. Even more, unlike any other wetting characterization technique, ODT features an inverse sensitivity: the more repellent the surface, the lower the error of measurement, which was demonstrated by experiments and simulations.
Wetting characterization is typically carried out with a contact angle goniometer measuring the advancing and receding contact angles. However, the contact angle goniometer does not have sufficient sensitivity for superhydrophobic surfaces. Here, we use an oscillating droplet tribometer to accurately characterize superhydrophobic surfaces, demonstrating its superior sensitivity compared to a contact angle goniometer.
Ferrofluids exhibit a unique combination of liquid properties and strong magnetic response, which leads to a rich variety of interesting functional properties. Here, the magnetic‐field‐induced ...splitting of ferrofluid droplets immersed in an immiscible liquid is presented, and related fascinating dynamics and applications are discussed. A magnetic field created by a permanent magnet induces instability on a mother droplet, which divides into two daughter droplets in less than 0.1 s. During the splitting process, the droplet undergoes a Plateau–Rayleigh‐like instability, which is investigated using high‐speed imaging. The dynamics of the resulting satellite droplet formation is shown to depend on the roughness of the supporting surface. Further increasing the field results in additional splitting events and self‐assembly of microdroplet populations, which can be magnetically actuated. The effects of magnetization and interfacial tension are systematically investigated by varying magnetic nanoparticles and surfactant concentrations, and a variety of outcomes from labyrinthine patterns to discrete droplets are observed. As the splitting process depends on interfacial tension, the droplet splitting can be used as a measure for interfacial tension as low as 0.1 mN m−1. Finally, a population‐based digital microfluidics concept based on the self‐assembled microdroplets is presented.
Magnetic‐field‐induced instability triggers a splitting of a ferrofluid droplet immersed in immiscible liquid, creating two daughter droplets and several tiny satellite droplets. Further increasing the field results in additional splitting events, creating a self‐assembled microdroplet population, which can be used in population‐based digital microfluidics. This phenomenon can also be used to measure interfacial tension in liquid–liquid systems.
Wetting of ferrofluids: Phenomena and control Latikka, Mika; Backholm, Matilda; Timonen, Jaakko V.I. ...
Current opinion in colloid & interface science,
July 2018, 2018-07-00, Letnik:
36
Journal Article
Recenzirano
Ferrofluids are liquids exhibiting remarkably strong response to magnetic fields, which leads to fascinating properties useful in various applications. Understanding the wetting properties and ...spreading of ferrofluids is important for their use in microfluidics and magnetic actuation. However, this is challenging as magnetically induced deformation of the ferrofluid surface can affect contact angles, which are commonly used to characterize wetting properties in other systems. In addition, interaction of the magnetic nanoparticles and solid surface at nanoscale can have surprising effects on ferrofluid spreading. In this review we discuss these issues with focus on interpretation of ferrofluid contact angles. We review recent literature examining ferrofluid wetting phenomena and outline novel wetting related ferrofluid applications. To better understand wetting of ferrofluids, more careful experimental work is needed.
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•Understanding ferrofluid wetting properties is essential for many applications.•Ferrofluid contact angle measurement under magnetic field is challenging.•Interaction between nanoparticles and surface can affect ferrofluid spreading.•Further work on advancing and receding ferrofluid contact angles is needed.
We present an efficient approach to develop cellulose nanocrystal (CNC) hybrids with magnetically responsive Fe
O
nanoparticles that were synthesized using the (Fe
/Fe
) coprecipitation. After ...2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-catalyzed oxidation of CNC, carbodiimide (EDC/NHS) was used for coupling amine-containing iron oxide nanoparticles that were achieved by dopamine ligand exchange (NH
-Fe
O
NPs). The as-prepared hybrids (Fe
O
@CNC) were further complexed with Cu(II) ions to produce specific protein binding sites. The performance of magnetically responsive Cu-Fe
O
@CNC hybrids was assessed by selectively separating lysozyme from aqueous media. The hybrid system displayed a remarkable binding capacity with lysozyme of 860.6 ± 14.6 mg/g while near full protein recovery (∼98%) was achieved by simple elution. Moreover, the regeneration of Fe
O
@CNC hybrids and efficient reutilization for protein separation was demonstrated. Finally, lysozyme separation from matrices containing egg white was achieved, thus revealing the specificity and potential of the presented method.
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