Millimeter-Sized Hole Damming.

Valves used to control liquid filling and draining processes from storage typically need to be actuated. Here, we show that similar flow enabling and restricting operations can be achieved through millimeter scale holes that function according to the amount of hydrostatic pressure applied without any other intervention. This phenomena is exhibited using receptacles where the base is made of either a hydrophilic or superhydrophobic substrate with hole sizes ranging from 1.0-2.0 mm. The construction is such that the drainage flow velocities are of the same order in both substrates and follow Torricelli's law trends. Nevertheless, the primary mechanisms responsible for resisting the onset of flow in each substrate are different; nonbreaching of the advancing contact angle threshold in the former, and stable maintenance of an elastic-like deformation of the liquid-gas interface that is connected to the surrounding plastron in the latter. These differences are demonstrated using an upward jet of water delivered to the orifice, where a discharging flow from the hydrophilic base occurred before the threshold hydrostatic pressure condition was attained, while liquid from the jet is subsumed into the liquid body of the receptacle with the superhydrophobic base without any leakage. These findings portend advantages in simplicity and robustness for a myriad of liquid-related processes.

Drops on a Superhydrophobic Hole Hanging On under Evaporation.

Drops with larger volumes placed over a superhydrophobic (SH) surface with a hole do not fall through unless they are evaporated to a size that is small enough. This feature offers the ability to preconcentrate samples for biochemical analysis. In this work, the influence of pinning on the behavior of drops placed on a 0.1 mm thick SH substrate with a 2 mm diameter hole as they evaporated was investigated. With 16 µL of water dispensed, the sessile drop component volume was initially higher than that of the overhanging drop component and maintained this until the later stages where almost identical shapes were attained and full evaporation was achieved without falling off the hole. With 15 µL of water dispensed, the volume of the sessile drop was initially higher than that of the overhanging drop component but the liquid body was able to squeeze through the hole after 180 s due to the contact line not having sufficient pinning strength when it encountered the edge of the hole. This resulted in the liquid body either falling through the hole or remaining pinned with an oval-like shape. When it did not fall-off, the liquid body had volume and contact angle characteristics for the sessile drop and overhanging drop components that were reversed. In the later stages, however, nearly identical shapes were again attained and full evaporation was achieved without falling off the hole. The effects of pinning, despite the substrate being SH, offer another path toward achieving practical outcomes with liquid bodies without the need for chemical surface functionalization. Similarities and differences could be seen in the behavior of a sessile drop on a SH plate that was inclined at 30° to the horizontal and evaporated.

Linear Stepper Actuation Driving Drop Resonance and Modifying Hysteresis.

In this work, 2 µL water drops are placed on substrates that are created to have a circular hydrophilic region bounded by superhydrophobicity so that they exhibit high contact angles. When the substrate is translated by a linear stepper actuator, the random force components present in the actuator are shown to cause the drop to rock resonantly. When the substrate is translated downward at inclination angles of up to 6° with respect to the horizontal, the contact angle hysteresis increases progressively to a limiting condition. When the substrate is moved up at inclined angles, alternatively, the contact angle hysteresis increases initially to the limiting condition before it is progressively restored to its static state. These behaviors are accounted for by the reversible micro-Cassie to Wenzel wetting state transformations that are made possible by the hierarchical microscale and nanoscale structures present in the superhydrophobic regions.

Stabilized dried blood spot collection.

During the collection phase of the dried blood spot method, practitioners need to ensure that there is no smearing of the blood sample on the filter paper or else readings from it will be invalid. This can be difficult to accomplish in the field if there is relative motion between the site of blood discharge on the finger and the filter paper. In this article, a gyroscope stabilization method is introduced and demonstrated to provide consistent and improved dried blood spot collection within a circular guide region notwithstanding the presence of rocking.

Adapting a Low-Cost Selective Compliant Articulated Robotic Arm for Spillage Avoidance.

Flexible automation systems provide the needed adaptability to serve shorter-term projects and specialty applications in biochemical analysis. A low-cost selective compliant articulated robotic arm designed for liquid spillage avoidance is developed here. In the vertical-plane robotic arm movement test, the signals from an inertial measurement unit (IMU) and accelerometer were able to sense collisions. In the horizontal movement test, however, only the signals from the IMU enabled collision to be detected. Using a calculation method developed, it was possible to chart the regions where the obstacle was likely to be located when a collision occurred. The low cost of the IMU and its easy incorporation into the robotic arm offer the potential to meet the pressures of lowering operating costs, apply laboratory automation in resource-limited venues, and obviate human intervention in response to sudden disease outbreaks.

Liquid-body resonance while contacting a rotating superhydrophobic surface.

We advance a scheme in which a liquid body on a stationary tip in contact with a rotating superhydrophobic surface is able to maintain resonance primarily from stick-slip events. With tip-to-surface spacing in the range 2.73 ≤ h < 2.45 mm for a volume of 10 µL, the liquid body was found to exhibit resonance independent of the speed of the drum. The mechanics were found to be due to a surface-tension-controlled vibration mode based on the natural frequency values determined. With spacing in the range 2.45 ≤ h < 2.15 mm imposed for a volume of 10 µL, the contact length of the liquid body was found to vary with rotation of the SH drum. This was due to the stick-slip events being able to generate higher energy fluctuations causing the liquid-solid contact areas to vary since the almost oblate spheroid shape of the liquid body had intrinsically higher surface energies. This resulted in the natural frequency perturbations being frequency- and amplitude-modulated over a lower frequency carrier. These findings have positive implications for microfluidic sensing.

Controlled transport of captive bubbles on plastrons.

Captive bubbles that reside on superhydrophobic surfaces with plastrons move uncontrollably when tilted. A system based on creating moveable local apexes on flexible superhydrophobic foils is shown to allow controlled transport. Simulations done reveal that specific bubble transport speeds are needed to form concentration gradients suited for aerotaxis study and sensing.

Concentrating nanoparticles in environmental monitoring.

There are significant challenges in assessing the toxicity of nanoparticles in the environment in which effective methods for detection are crucial. An inexpensive method that uses superhydrophobic well with an evaporating droplet followed by a simple squeeze flow is described here and found to provide practical high nanoparticle collection from samples for detection. The process could be hastened by placing a radiant heater close to the droplet if temperature rises in the sample can be tolerated.

Plastron-Mediated Growth of Captive Bubbles on Superhydrophobic Surfaces.

Captive bubbles on a superhydrophobic (SH) surface have been shown to increase in volume via injection of air through the surrounding plastron. The experimental contact diameter against volume trends were found to follow that predicted by the Surface Evolver simulation generally but corresponded with the simulated data at contact angle (CA) = 158° when the volume was 20 µL but that at CA = 170° when the volume was increased to 180 µL. In this regime, there was a simultaneous outward movement of the contact line as well as a small reduction in the slope that the liquid-air interface makes with the horizontal as air was injected. At volumes higher than 180 µL, air injection caused the diameter to reduce progressively until detachment. The inward movement of the contact line in this regime allowed the bubble body to undergo shape deformations to stay attached onto the substrate with larger volumes (300 µL) than predicted (220 µL at CA = 170°) using simulation. In experiments to investigate the effect of translating the SH surface, movement of captive bubbles was possible with 280 µL volume but not with 80 µL volume. This pointed to the possibility of transporting gas-phase samples on SH surfaces using larger captive bubble volumes.

Particles replaced axially in an optical trap.

Flow-based measures to automate optical trapping have significant limitations. A scheme is advanced here where a spherical bead is first located in a trap, and a second bead below the focus point is selectively drawn into the trap to replace the original particle. Experimentation conducted showed that it was possible to do so with little perturbation of other surrounding particles. Simulations done allowed for a clearer description of the exchange mechanism.

Drop transfer between superhydrophobic wells using air logic control.

Superhydrophobic surfaces aid biochemical analysis by limiting sample loss. A system based on wells here tolerated tilting up to 20° and allowed air logic transfer with evidence of mixing. Conditions for intact transfer on 15 to 60 µL drops using compressed air pressure operation were also mapped.

Optical trapping map of dielectric spheres.

Many applications use a focused Gaussian laser beam to manipulate spherical dielectric particles. The axial trapping efficiency of this process is a function of (i) the particle radius r, (ii) the ratio of the refractive index of particle over the medium, and (iii) the numerical aperture of the delivered light beam. During what we believe is the first comprehensive simulation of its kind, we uncovered optical trapping regions in the three-dimensional (3D) parameter space forming an iso-surface landscape with ridge-like contours. Using specific points in the parameter space, we drew attention to difficulties in using the trapping efficiency and stiffness metrics in defining how well particles are drawn into and held in the trap. We have proposed an alternative calculation based on the maximum forward and restoration values of the trapping efficiency in the axial sense, called the trapping quality. We also discuss the manner in which the ridge regions may be harnessed for effective particle sorting, how the optical trapping blind spots can be used in applications that seek to eschew photothermal damage, and how trapping can proceed when many parameters change, such as when swelling occurs.

Optical stirring in a droplet cell bioreactor.

In the context of a bioreactor, cells are sensitive to cues from other cells and mechanical stimuli from movement. The ability to provide the latter in a discrete fluidic system presents a significant challenge. From a prior finding that the location of the focus of a laser below particles relative to the beam axis producing a pushing effect in a predominant lateral sense, we advance an approach here that generates a gentle and tunable stirring effect. Computer simulation studies show that we are able to characterize this effect from the parameters that govern the optical forces and the movement of the particles. Experimental results with polystyrene microbeads and red blood cells confirm the notions from the simulations.

Sorting of brownian rods by the use of an asymmetric potential.

We present here a method for sorting nanometer scale brownian rods by using a switching asymmetric periodic potential. A two stage sorting process is used to isolate particles with specific dimensions, with acceptable sorting times as well as realizable potential barrier lengths. The method was tested using computer simulations. The ability to sort the nanometer scale anisotropic particles, such as gold nanorods, portends important applications in large scale data recording, photothermal surgery, and bioimaging.