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We propose a novel osmotic soft valve consisting of an osmosis membrane and hydrogel films. In our osmotic valve system, material selectivity is determined by the osmosis membrane, and the hydrogel film, which deforms depending on the ion concentration of the surrounding solution, controls the passage area of the membrane. Independently controlling the material selectivity and permeability allowed us to design an osmotic soft valve with an osmotic flow rate that increases with osmotic pressure at low pressures but decreases with osmotic pressure at high pressures. We demonstrate a representative application of our hydrogel valve system in a portable power generator utilizing reverse electrodialysis (RED). As the permeability varied with concentration, the hydrogel valve was able to maintain the electric power of the RED for 30 min with only an ∼10% change. Our study provides techniques to build osmotic soft valves that can serve as gating membranes in various osmosis and dialysis systems.

A parent's capacity to understand the mental states of both him/herself and the child is considered to play a significant role in various aspects of parent-child relationship-e.g., lowering parental stress and supporting cognitive development of the child. We propose Dyadic Mirror, a wearable smart mirror which is designed to foster the aforementioned parental capacity in everyday parent-child interaction. Its key feature is to provide a parent with a second-person live-view from the child, i.e., the parent's own face as seen by the child, during their face-to-face interaction. Dyadic Mirror serves as a straightforward cue that helps the parent be aware of (1) his/her emotional state, and (2) the way he/she would be now being seen by the child, thereby facilitate the parent to infer the child's mental state. To evaluate Dyadic Mirror under unconstrained parent-child interactions in real-life, we implemented the working prototype of Dyadic Mirror and deployed it to 6 families over 4 weeks. The participating parents reported extensive experiences with Dyadic Mirror, supporting that Dyadic Mirror has helped them be aware of their recurring but unconscious behaviors, understand their children's feelings, reason with the children's behaviors, and find self-driven momenta to better their attitude and expressions towards their children.

We investigate the dynamics of liquid imbibition through partially soluble porous sheets. Using a polymeric porous sheet composed of polyethylene oxide (PEO) nanofibers, we examine the imbibition of mixtures of ethylene glycol (EG) that does not dissolve PEO and water that dissolves PEO. For aqueous EG mixtures with a concentration of greater than 70 wt% that only partially dissolves a PEO nanofibrous sheet, imbibition with dissolution is markedly slower than without dissolution. By developing a mathematical model to predict the imbibition velocity, we elucidate how dissolution changes the viscosity of the invading liquid and affects the dynamics of imbibition. This study guides us to extend the classical theoretical framework for liquid imbibition through porous media to complex porous media involving dissolution. Thus, a useful background is provided for long-term drug delivery using partially soluble porous materials and for engineering processes, such as geological sequestration and exploration, and coffee percolation.

Regulating the fluid flow in microfluidic devices enables a wide range of assay protocols for analytical applications. A programmable, photo-paper-based microfluidic device fabricated by using a method of cutting and laminating, followed by printing, is reported. The flow distance of fluid in the photo-paper-based channel was linearly proportional to time. By printing silver nanoparticle (AgNP) and poly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole-co-tetrafluoroethylene] (PTFE) patterns on the surface of a photo-paper-based channel, we were able to either increase or decrease the fluid flow in the fabricated microfluidic devices, while maintaining the linearity in the flow distance-time relation. In comparison to the speed of fluid flow in a pristine channel, by using hydrophilic AgNP patterns, we were able to increase the speed in the channel by up to 15 times while we were able to slow the speed by a factor of 3 when using hydrophobic PTFE dots. We then further demonstrated a single-step protocol for detecting glucose and a multi-step protocol for detecting methyl paraoxon (MPO) with our methods in photo-paper-based microfluidic devices. This approach can lead to improved fluid handling techniques to achieve a wide range of complex, but programmable, assays without the need for any additional auxiliary devices for automated operation.

Water-absorbing materials consisting of hydrogel particles are widely used for agricultural substrates, hygienic products, drug delivery systems, and microfluidic devices. Hydrogel particles absorb water with expansion, leading to dynamics involving complicated interactions of intergranular liquid flow and hydrogel deformation. It is thus difficult to predict the water absorption rate of these materials. Water flow in hydrogel-coated capillary tubes also occurs with the deformation of flow passages, so it can be analogous to capillary flow through porous structures made of hydrogels. Here we report a combined experimental and theoretical investigation of water flow through capillaries whose inner surface is covered with a hydrogel layer. We experimentally measured the flow rate of water through hydrogel-coated capillary tubes and the volume of water absorbed by the hydrogel layer. We developed a mathematical model for capillary flow including the effects of the water absorption and swelling of the hydrogel layer. Our model is in good agreement with the measurements of the water flow rate and elucidates how the absorption and swelling of hydrogel regulate capillary flow. This study provides a theoretical framework for understanding the dynamics of capillary flow through porous structures composed of hydrogels, leading to insights into the engineering applications of porous hydrogel materials.

Gallium-based liquid metals (GBLMs) have both high fluidity and electrical conductivity and have recently raised great hopes with the promise of realizing soft electronics. By soft electronics, we mean electronic components that can retain their function while being bent and elongated. However, a nanometer-thick oxide skin forms on the GBLM surface, causing atypical interfacial behaviors, and the underlying physics remains unclear. Here, we present the results of a combined experimental and theoretical investigation of the dependence of contact angle of liquid phase eutectic gallium–indium (EGaIn) with oxide skin on van der Waals attraction. Our experiments showed that when EGaIn has an oxide skin, only the static advancing contact angle, not the equilibrium and static receding contact angles, can be specified. We suggest a mathematical model that explains how the static advancing contact angle of EGaIn depends on van der Waals force and the tension exerted on the oxide skin, elucidating the physics that determines the contact angle of EGaIn with the oxide skin in microscopic analysis. Our study helps us to better understand the interface of GBLMs, providing new insight into microfabrication techniques for GBLMs.

Highlights • The cleaning effects of gas and vapor bubbles in ultrasound fields are compared. • The cleaning effect is assessed in terms of adhesion strength and wettability. • The substrates advantageously cleaned with gas or vapor bubbles are identified., The dynamic actions of cavitation bubbles in ultrasonic fields can clean surfaces. Gas and vapor cavitation bubbles exhibit different dynamic behaviors in ultrasonic fields, yet little attention has been given to the distinctive cleaning effects of gas and vapor bubbles. We present an experimental investigation of surface cleaning by gas and vapor bubbles in an ultrasonic field. Using high-speed videography, we found that the primary motions of gas and vapor bubbles responsible for surface cleaning differ. Our cleaning tests under different contamination conditions in terms of contaminant adhesion strength and surface wettability reveal that vapor and gas bubbles are more effective at removing contaminants with strong and weak adhesion, respectively, and furthermore that hydrophobic substrates are better cleaned by vapor bubbles. Our study not only provides a better physical understanding of the ultrasonic cleaning process, but also proposes novel techniques to improve ultrasonic cleaning by selectively employing gas and vapor bubbles depending on the characteristics of the surface to be cleaned.

Public spaces are equipped with 'public actuators', e.g., HVAC, lighting fixtures, speakers, or streaming TV channels to ensure their visitors' comfort. However, many public actuators rarely allow the visitors to adjust their operation, limiting their utility and fairness across the visitors. Also, the social bar is often too high to speak up one's preference and attempt to change an actuator's operation. Social control and use of IoT devices is an underexplored new direction of research even with its huge potential and implication, but comes with high complexity and scale. This paper proposes a novel architecture, namely, Social Control-and-Use Architecture for IoT Devices, which provides a systematic view and an effective tool to handle the complication and intricacy in system design. It also proposes Hivemind, a first-of-a-kind system developed, upon the architecture, for sharing IoT-enabled actuators in a public space. It transforms an exclusively-controlled actuator in a public space into a true public actuator, supporting visitors to instantly participate in the democratic collective control. Also, a myriad of off-the-shelf actuators are easily incorporated without modification to their implementation. The field deployment of Hivemind shows its comprehensive service coverage as well as the users' approval on the democratic collective control of public actuators.

We report an experimental and theoretical investigation of bubble collisions on parallel arranged fibers. Using high speed videography, we visualized the impact outcomes and classified them into three modes: capturing, single bubble rising, and splitting. The experimental results demonstrate that the impact mode is mainly determined by the distance between the fibers as well as the rising speed of the bubble. We present the results in a regime map, displaying the dependence of the impact outcomes on dimensionless parameters: Web = ρUb2D/σ, Bo = ρgD2/σ, and d/D in the range of 0 ≤ Web ≤ 25, 0.4 ≤ Bo ≤ 0.7, 0.30 ≤ d/D ≤ 0.75, where ρ is the liquid density, Ub is the collision speed of bubble, D is the bubble diameter, σ is the surface tension, g is the gravitational acceleration, and d is the inter-fiber distance. The observed regime boundaries are explained with scale analysis. We also examined the size of daughter bubbles for splitting cases, and the results unveil the critical dependence of the daughter bubble size on the impact offset rather than on the collision speed. The results are expected to be widely used to control bubble filtration or generation in a variety of engineering systems aimed at improving heat and mass transfer using bubbles.

While the power-law model has been used widely to predict hemolysis numerically, it has a few inherent limitations that may result in inaccurate prediction. As an alternative, the viscoelastic model is a strain-based model derived using the mechanical properties of the red blood cell and erythrocyte. The model assumes two different time scales that lead to modified exponential trends versus the exposure time. As a zero-dimensional model, the viscoelastic model is not applicable to a complex multi-dimensional problem. In the present study, it is extended to a form based on partial differential equations for multi-dimensional hemodynamic flows such as cannulas and blood pumps. The extended method is revised further to comply with a physical constraint. When evaluated using experimental data and relative hemolysis indices, the extended viscoelastic model does not show a clear advantage over the power-law model for cannula problems with relatively short residence times. In contrast, the viscoelastic model shows improved agreement with experimental data consistently for flows inside blood pumps with relatively long residence times. Considering that the viscoelastic model was targeted originally at improving prediction at a long residence time, the present results are consistent with a previous study. As a result, it is implied that the extended viscoelastic model is useful for specific applications such as blood pumps compared to the previous methods.

Liquids can invade fibrous porous media when the fibres are either wettable or soluble, and the infiltration rate can differ depending on spatial distribution of fibres as well as liquid properties. With continuing developments in dissolution-driven release mechanisms of porous drugs and chemical pattern formations, the understanding of how liquids spontaneously infiltrate into soluble fibrous media is strongly called for. Here we show that unlike capillarity-driven insoluble wicking (exhibiting diffusive growth of wetting distance with time), the wicking distance in soluble porous media grows linearly with time as dominated by liquid viscosity rather than surface tension. Such soluble wicking is highly sensitive to flow orientation relative to fibre alignment, so that it arises only in the crosswise direction while being strongly inhibited in the lengthwise direction. We present a theoretical model to explain the experimentally measured wicking rates in soluble porous media.