Lateral Capillary Forces Research Articles

Braiding, twisting, and weaving microscale fibers with capillary forces.

Soft materials made from braided or woven microscale fibers can display unique properties that can be exploited in electromagnetic, mechanical, and biomedical applications. These properties depend on the topology of the braids or weaves-that is, the order in which fibers cross one another. Current industrial braiding and weaving machines cannot easily braid or weave micrometer-scale fibers into controllable topologies; they typically apply forces that are large enough to break the fibers, and each machine can typically make only one topology. Here we use a 3D-printed device called a "capillary machine" to manipulate micrometer-scale fibers without breaking them. The operating principle is the physics of capillary forces: as the machines move vertically, they exert lateral capillary forces on floating objects, which in turn move small fibers connected to them. We present a new type of capillary machine that is based on principles of braid theory. It implements all the possible fiber-swapping operations for a set of four fibers and can therefore make any four-strand topology, including braids, twists, hierarchical twists, and weaves. We make these different topologies by changing the pattern of vertical motion of the machine. This approach is a mechanically simple, yet versatile way to make micro- and nano-textiles. We describe the prospects and limitations of this new type of machine for applications.

Measuring lateral capillary forces on floating particles using the Moses effect.

This study presents a novel and user-friendly technique for detecting the lateral capillary force on a floating spherical particle. The technique leverages the interplay between the capillary attracting forces, hydrostatic pressure forces, and magnetic repulsion forces. A magnetic field is applied to induce a surface curvature in the liquid, resulting in a non-uniform distribution of capillary and hydrostatic pressure forces across the particle's surface. This leads to a stable equilibrium position of the particle at a specific distance from the magnet. The study analyzes the equilibrium position and other relevant parameters in comparison with the developed theory. Classical mechanics and intermolecular forces are applied to establish the theoretical basis for the method, modeling the behavior of the particle in response to the magnetic field, surface curvature, and hydrostatic pressure. The equilibrium position of the particle is determined by numerically solving the balance of forces equation.

Pairing-induced motion of source and inert particles driven by surface tension.

We experimentally and theoretically investigate systems with a pair of source and inert particles that interact through a concentration field. The experimental system comprises a camphor disk as the source particle and a metal washer as the inert particle. Both are floated on an aqueous solution of glycerol at various concentrations, where the glycerol modifies the viscosity of the aqueous phase. The particles form a pair owing to the attractive lateral capillary force. As the camphor disk spreads surface-active molecules at the aqueous surface, the camphor disk and metal washer move together, driven by the surface tension gradient. The washer is situated in the front of the camphor disk, keeping the distance constant during their motion, which we call a pairing-induced motion. The pairing-induced motion exhibited a transition between circular and straight motions as the glycerol concentration in the aqueous phase changed. Numerical calculations using a model that considers forces caused by the surface tension gradient and lateral capillary interaction reproduced the observed transition in the pairing-induced motion. Moreover, this transition agrees with the result of the linear stability analysis on the reduced dynamical system obtained by the expansion with respect to the particle velocity. Our results reveal that the effect of the particle velocity cannot be overlooked to describe the interaction through the concentration field.

Growth of self-aligned nonlayered TaC nanosheets on liquid copper by a solid phase diffusion strategy

Compared with layered 2D materials, nonlayered 2D materials combine atomic thickness and high-activity surface, leading to many unique properties. Herein, a solid phase diffusion strategy is raised to obtain self-aligned nonlayered TaC single crystals on liquid copper. Because of the immiscibility of Cu and Ta, a β-Ta nanolayer with a thickness of ∼7 nm would form on the surface of liquid copper by a long-time solid phase diffusion process. After introducing CH4, the β-Ta nanolayer could be converted into 10 μm TaC single crystals with a thickness of 10 nm. With the extension of the reaction time, TaC single crystals would form an ordered self-aligned state spontaneously under the action of lateral capillary force. Finally, the self-aligned TaC nanosheets are assembled into large-size TaC films with a thickness of 10 nm. This strategy paves a new way to grow large-size nonlayered 2D transition metal carbides.

Lateral and Normal Capillary Force Evolution of a Reciprocating Liquid Bridge.

Capillary forces of a shearing liquid bridge can significantly affect the friction and adhesion of interacting surfaces, but the underlying mechanisms remain unclear. We custom built a surface force apparatus (SFA, ±2 μN) equipped with in situ optical microscopy and performed normal and lateral force measurements on a reciprocating water bridge formed between two flat plates. A modified wedge method was developed to correct the unique force measurement errors caused by the changing bridge geometry and position. The results found (1) strong linear relations among the bridge shear displacement, the cosine difference between the left and right contact angles, and the lateral adhesion force and (2) the normal adhesion force increased monotonically up to 13% as the bridge geometry approached its axisymmetric state. Quasi-static force analyses based on a newly developed decahedral model showed good agreement with the experiments and improved accuracy compared with that of cylindrical or rectangular column models previously proposed in the literature. Although limited in certain aspects, this study may (1) prove helpful to the design and analysis of liquid bridge force experiments on platforms similar to the SFA used in this study and (2) help to bridge the gap between friction and liquid bridge physics in the literature.

Thinning air–water films stabilized by bacterial particles

The ability of particles to stabilize liquid films has broad applications in many fields, such as drug delivery, biofuel catalysis, and crude-oil separation. The mechanism of particles to stabilize emulsions has been widely studied. However, how particles affect the stability of suspension films is still unclear. Here, the dynamics of bacterial particles on free liquid films during evaporation is obtained by direct microscopic observation. We find that the initial concentration of the bacterial solution affects the evolution of the shapes and stabilities of liquid films. The liquid film with a low bacterial concentration exhibits a drainage and rupture process similar to pure water, and bacteria would be evacuated during the evaporation of liquid film. However, when the bacterial concentration is high, bacteria form a single-layer array in the center of the film and greatly enhance the stability of the liquid film. We demonstrate that these two distinct film evaporation dynamics are caused by the difference in the lateral capillary force of the particles at different positions on the liquid film, which is not affected by the active motility of particle. Together, our research reveals the critical role of bacteria concentration in stabilizing suspension liquid film.

Fine stripe coating and micro patterning of organic thin films using double nozzles for OLEDs

The aim of this work is to investigate the feasibility of coating and patterning of fine organic thin-film stripes using double nozzles. For fine stripe coating of an aqueous poly (3,4-ethylenedioxythiophene):poly (4-styrenesulfonate) (PEDOT:PSS), we have changed the outer surface of the needle from hydrophilic to hydrophobic in such a way that lateral capillary force is suppressed and thus the PEDOT:PSS stripe becomes narrow. Using a table coater with the hydrophobic needle (inner diameter = 51 μm), we have achieved the 54-μm-wide PEDOT:PSS stripe at a maximum coating speed of 30 mm/s. For micro patterning of PEDOT:PSS thin-film stripes, we have employed double needles; one for discharging the main solvent of the underlying thin film and the other for sucking the dissolved thin film. It is found that the pattern width decreases as the patterning speed increases and the pattern factor (the ratio of film thickness to slope length) is of the order of 10 −3 due to the isotropic patterning characteristics. Such a patterning scheme is also shown to remove non-aqueous poly (N-vinylcarbazole) (PVK) films coated in the presence of a bank structure. With the coating and patterning process, we have successfully fabricated multiple OLED stripes, an OLED device with the patterned light-emitting stripe, and OLED pixels, demonstrating its potential applicability to solution-processable OLEDs. • Suppressed lateral capillary flow by a hydrophobic needle for fine stripe coating. • Fabricated 54-μm-wide PEDOT:PSS stripe at a maximum coating speed of 30 mm/s. • Performed micro patterning of aqueous and non-aqueous thin films using double needles. • Achieved the pattern width of 260 μm and the pattern factor of 10 −3 using PEDOT:PSS. • Achieved light emission from OLED pixels and OLEDs with patterned or unpatterned stripes.

Two Floating Camphor Particles Interacting through the Lateral Capillary Force

We consider a mathematical model for a two-particle system driven by the spatial gradient of a concentration field of chemicals with conservative attractive interactions in one dimension. This setup corresponds to an experimental system with floating camphor particles at a water surface. Repulsive interaction is introduced, as well as self-propelling force, through the concentration field of camphor molecules at the water surface. Here we newly adopt the attractive lateral capillary force due to the deformation of the water surface. The particles experience competing dissipative repulsion and conservative attraction. We numerically investigated the mathematical model, and found six different modes of motion. The theoretical approach revealed that some of such mode transitions can be understood in terms of bifurcation.

Capillary interactions between soft capsules protruding through thin fluid films.

When a suspension dries, the suspending fluid evaporates, leaving behind a dry film composed of the suspended particles. During the final stages of drying, the height of the fluid film on the substrate drops below the particle size, inducing local interface deformations that lead to strong capillary interactions among the particles. Although capillary interactions between rigid particles are well studied, much is still to be understood about the behaviour of soft particles and the role of their softness during the final stages of film drying. Here, we use our recently-introduced numerical method that couples a fluid described using the lattice Boltzmann approach to a finite element description of deformable objects to investigate the drying process of a film with suspended soft particles. Our measured menisci deformations and lateral capillary forces, which agree well with previous theoretical and experimental works in case of rigid particles, show that the deformations become smaller with increasing particle softness, resulting in weaker lateral interaction forces. At large interparticle distances, the force approaches that of rigid particles. Finally, we investigate the time dependent formation of particle clusters at the late stages of the film drying.

Colloidal films of SiO2 in elastomeric polyacrylates by photopolymerization: A strain sensor application

Thin layer SiO2 colloidal films show angle-dependent coloration (iridescence) based on constructive interference, rather than absorption, without the existence of pigments. The transfer of thin layered colloidal film into a transparent elastomeric matrix maintaining its color may allow the fabrication of colorimetric strain sensors. In this study, trilayer SiO2 colloidal films were prepared by Langmuir-Blodgett deposition using a binary solvent system (chloroform/methanol) and this structure is successfully transferred into poly(ethylene glycol) phenyl ether acrylate elastomer via lateral capillary force. The resulting composite films exhibit iridescence depending on the particle size, therefore, film thickness as similar in mere colloidal films with a slight difference due to change in efficient refractive index (neff). Uniaxial extension of the composite film up to 50 % strain causes a remarkable linear shift in reflection signal from 568 to 496 nm. The change in thickness of the composite film accordingly intercolloidal distance normal to the application of mechanical stretching causes variation of the reflection of light.

Life-Like Motion of Oil Drops at the Air-Liquid Interface.

Generally, interactions of oil drops at the air-liquid interface mainly have two features, namely, attraction and repulsion. However, in our study, we find that the oil drops at the air-liquid interface have other interacting features, that is, the atomic-like motion and the "capture" motion. For the atomic-like motion, oil drops attract each other at a long distance, but repel when they are about to come into contact with each other. For the "capture" motion, a big oil drop can actively "capture" oil droplets like a zooplankton. In our research, we analyze interfacial forces among the oil drops. Based on the experiments and analyses, we demonstrate that the atomic-like motion of oil drops is mainly due to the lateral capillary force and the surface tension force, and the "capture" motion is mainly due to the unbalanced impact force of flow fluid around the drops. In addition, based on our results, we use the oil drops to perform many functions at the air-liquid interface. For example, the oil drops can drive an object with linear and rotational motion. When a carbon tetrachloride drop is suspended above the air-liquid interface, it can be used to control an oil droplet to pass through serpentine grooves and obstacles. In addition, the suspended carbon tetrachloride drops also can be used to rank multiple droplets with a special shape. Based on the results, our study makes it possible to use oil drops to transport materials, drive objects, and even collect droplets at the air-liquid interface.

Interactions of Oil Drops Induced by the Lateral Capillary Force and Surface Tension Gradients.

The interacting forces on the objects at the air-liquid interface are important for researching self-assembly of objects. Due to the interacting forces among objects, the objects self-assemble at the air-liquid interface and organize into ordered structures. In general, for the interacting of oil drops, they can attract coalescence or repel dispersion. However, according to our former research, we find that interactions of oil drops can be motion like interactions of gas atoms. The oil drops cannot coalesce to form a big oil drop or separate to form ordered structures. They attract at a long distance but repel when they almost contact. In this study, according to our simulation, we demonstrate that the atomic-like motion of oil drops is mainly caused by the surface tension gradient and the lateral capillary force. Based on the results, our research will facilitate the fundamental understanding about interactions among oil drops. Apart from giving detailed demonstration about the interacting forces among oil drops, our research may also put forward research about self-assembly, oil emulsification, and microfluidics.

Atomic-like motion of coverslips at the air-water interface

Atomic-like motion of coverslips (their surfaces drip with 70 vol% aqueous ethanol) is that the coverslips attract each other over a long distance, but repel when they are almost in contact. When they are placed at a considerable distance, the two coverslips get attracted towards each other. However, when the separating distance between them is reduced to a near contact point, they start to repel. According to our research, there are mainly two opposite forces acting on the coverslips. The attraction is mainly due to the lateral capillary force, while the repulsion is mainly due to the Marangoni force. In our study, we analyze interfacial forces of the coverslips, and experimentally and analytically demonstrate that atomic-like motion of coverslips is mainly due to the Marangoni force and the lateral capillary force.

Hydrogel micromotors with catalyst-containing liquid core and shell

Methacrylic anhydride-derived hydrogel microcapsules have unique properties, including reversibly tunable permeation, purification, and separation of dissolved molecular species. Endowing these dynamic encapsulant systems with autonomous motion will significantly enhance their efficiency and applicability. Here, hydrogel micromotors are realized using complex water-in-oil-in-water double emulsion drops and oil-in-water emulsion drops from glass capillary microfluidics and subsequent photopolymerization. Three hydrogel micromotor strategies are explored: microcapsules with thin shells and liquid cores with dispersed catalytic Pt nanoparticles, as well as water-cored microcapsules and homogeneous microparticles selectively coated with Ti/Pt catalytic layers. Autonomous motion of hydrogel particles and capsules is realized in hydrogen peroxide solutions, where generated oxygen microbubbles propel the dynamically responsive micromotors. The micromotors are balanced by weight, buoyancy, lateral capillary forces and show specific autonomous behaviours that significantly extend short range dynamic responses of hydrogels. Drop-based microfluidics represent a paradigm shift in the integration of multifunctional subsystems and high-throughput design of chemical micromachines in reasonable quantities towards their desired biomedical, environmental and flow/diffusion microreactor applications.

3D‐Printed Silicone Soft Architectures with Programmed Magneto‐Capillary Reconfiguration

AbstractSoft intelligent structures that are programmed to reshape and reconfigure under magnetic field can find applications such as in soft robotics and biomedical devices. Here, a new class of smart elastomeric architectures that undergo complex reconfiguration and shape change in applied magnetic fields, while floating on the surface of water, is reported. These magnetoactive soft actuators are fabricated by 3D printing with homocomposite silicone capillary ink. The ultrasoft actuators easily deform by the magnetic force exerted on carbonyl iron particles embedded in the silicone, as well as lateral capillary forces. The tensile and compressive moduli of the actuators are easily determined by their topological design through 3D printing. As a result, their responses can be engineered by the interplay of the intensity of the magnetic field gradient and the programmable moduli. 3D printing allows us to fabricate soft architectures with different actuation modes, such as isotropic/anisotropic contraction and multiple shape changes, as well as functional reconfiguration. Meshes that reconfigure in magnetic fields and respond to external stimuli by reshaping could serve as active tissue scaffolds for cell cultures and soft robots mimicking creatures that live on the surface of water.

Electrically Conductive Thin Films Derived from Bulk Graphite and Liquid–Liquid Interface Assembly

AbstractConfinement of particles to fluid–fluid interfaces provides a unique interaction environment, allowing the directed assembly of particles using lateral capillary forces. The particle laden interfacial layers can be deposited onto a variety of substrates for the fabrication of thin film coatings, designed to have structural or functional properties resulting from the interface‐specific structures. For the fabrication of electrically conducting films and specifically graphene‐based coatings, interfacial deposition techniques could offer a low cost and environmentally favorable alternative to conventional gas phase production methods, with possibly a broader choice of substrates. In this work liquid‐phase electrochemical exfoliation is used to produce platelets of few‐layer graphene from bulk graphite which are directly introduced to the water–air interface. These interfaces are characterized through compression experiments and interfacial shear rheology to probe the mechanical properties of the resulting monolayer films and identify mechanical percolation. Further, in situ measurements of electrical conductivity are integrated as a direct indication of electrical percolation. This directly verifies sufficient film quality as an important characteristic of the deposition process. Once deposited on a solid substrate, these films retain their electrical conductivity. But the mechanical properties of the films allow for facile production of freestanding microporous graphene membranes.

Capillary interactions between dynamically forced particles adsorbed at a planar interface and on a bubble.

We investigate the dynamic interfacial deformation induced by micrometric particles exerting a periodic force on a planar interface or on a bubble, and the resulting lateral capillary interactions. Assuming that the deformation of the interface is small, neglecting the effect of viscosity, and assuming point particles, we derive analytical formulas for the dynamic deformation of the interface. For the case of a planar interface the dynamic point force simply generates capillary waves, while for the case of a bubble it excites shape oscillations, with a dominat deformation mode that depends on the bubble radius for a given forcing frequency. We evaluate the lateral capillary force acting between two particles, by superimposing the deformations induced by two point forces. We find that the lateral capillary forces experienced by dynamically forced particles are non monotonic and can be repulsive. The results are applicable to micrometric particles driven by different dynamic forcing mechanisms such as magnetic, electric or acoustic fields.

Microrobotic Manipulation at Time-Varying Air–Liquid Interface for High-Precise Watch-Hand Alignment

Micromanipulation is a core technique in precise manufacturing and mechatronics. However, current researchers mainly focus on the time-invariant normal environment, whereas complex manipulation at time-varying air–liquid interface is rarely studied, such as the watch-hand alignment problem in industry. To address the above challenge, this paper presents a microrobotic system and corresponding control strategy by modeling the watch-hand movement dynamically at time-varying air–liquid interface. In this paper, a microrobotic manipulation system with five degrees of freedom is developed and integrated with an optical microscopy. Then, the dynamic model of watch-hand is built through Lagrange equation, including the viscoelastic force from glue and the lateral capillary force at the air–liquid interface. After that, a proportional–integral–derivative controller is designed to guarantee the accuracy of the alignment process. Lastly, the accuracy and efficiency of the proposed system is verified through both simulation and experiments. This research provides a common solution for the precise micromanipulation at time-varying air–liquid interface, which would greatly promote the micromanufacturing and microfabrication in lab and industry.

Large-scale fabrication of commercially available, nonpolar linear polymer film with a highly ordered honeycomb pattern.

Highly ordered, hexagonally patterned poly(methyl methacrylate) (PMMA) thin film is successfully fabricated using an improved phase separation method. A mixture of chloroform and methanol, which is used as a volatile solvent/nonsolvent pair, effectively controls the surface morphology and sensitively determines the ordered pattern. In particular, the methanol accumulation, which induces the formation of a gel-like protective layer and enhances the lateral capillary force, is crucial in the formation of the highly ordered hexagonal pattern even when using a nonpolar polymer such as PMMA. The convergence of cost-effective and large-scale production of highly ordered micropatterned film has wide potential for application, and it can enable new prospects for the commercialization of future high-tech devices that require specific multifunctionality.

Microcavity substrates casted from self-assembled microsphere monolayers for spheroid cell culture.

Multicellular spheroids are an important 3-dimensional cell culture model that reflects many key aspects of in vivo microenvironments. This paper presents a scalable, self-assembly based approach for fabricating microcavity substrates for multicellular spheroid cell culture. Hydrophobic glass microbeads were self-assembled into a tightly packed monolayer through the combined actions of surface tension, gravity, and lateral capillary forces at the water-air interface of a polymer solution. The packed bead monolayer was subsequently embedded in the dried polymer layer. The surface was used as a template for replicating microcavity substrates with perfect spherical shapes. We demonstrated the use of the substrate in monitoring the formation process of tumor spheroids, a proof-of-concept scale-up fabrication procedure into standard microplate formats, and its application in testing cancer drug responses in the context of bone marrow stromal cells. The presented technique offers a simple and effective way of forming high-density uniformly-sized spheroids without microfabrication equipment for biological and drug screening applications.