Michael J. Vogel

Capillarity-based switchable adhesion.

Drawing inspiration from the adhesion abilities of a leaf beetle found in nature, we have engineered a switchable adhesion device. The device combines two concepts: The surface tension force from a large number of small liquid bridges can be significant (capillarity-based adhesion) and these contacts can be quickly made or broken with electronic control (switchable). The device grabs or releases a substrate in a fraction of a second via a low-voltage pulse that drives electroosmotic flow. Energy consumption is minimal because both the grabbed and released states are stable equilibria that persist with no energy added to the system. Notably, the device maintains the integrity of an array of hundreds to thousands of distinct interfaces during active reconfiguration from droplets to bridges and back, despite the natural tendency of the liquid toward coalescence. We demonstrate the scaling of adhesion strength with the inverse of liquid contact size. This suggests that strengths approaching those of permanent bonding adhesives are possible as feature size is scaled down. In addition, controllability is fast and efficient because the attachment time and required voltage also scale down favorably. The device features compact size, no solid moving parts, and is made of common materials.

Low-dissipation capillary switches at small scales

A system of two coupled capillary surfaces is made to switch between its stable states via mechanical and electrochemical disturbances. The bistable switch is experimentally demonstrated using water droplets, where the mechanical activation or “toggle” is achieved by a momentary air-pressure change. Requirements for capillary switches to avoid viscous dissipation are described and strategies for utilizing capillary switches for transporting other liquids or solids are discussed. Addressability of individual switches is achieved using electrochemical activation via a redox surfactant, where surface tension of one element of the switch is changed relative to the other.

Determination of the Zeta Potential of Porous Substrates by Droplet Deflection. I. The Influence of Ionic Strength and pH Value of an Aqueous Electrolyte in Contact with a Borosilicate Surface

This paper presents a new method to determine the zeta potential of porous substrates in contact with a liquid. Electroosmosis, arising near the solid/liquid boundaries within a fully saturated porous substrate, pumps against the capillary pressure arising from the surface tension of a droplet placed in series with the pump. The method is based on measuring the liquid/gas interface deflection due to the imposed electric potential difference. The distinguishing features of our technique are accuracy, speed, and reliability, accomplished with a straightforward and cost-effective setup. In this particular setup, a bistable configuration of two opposing droplets is used. The energy barrier between the stable states defines the range of capillary resistance and can be tuned by the total droplet volume. The electroosmotic pump is placed between the droplets. The large surface area-to-volume ratio of the porous substrate enables the pumping strength to exceed the capillary resistance even for droplets small enough that their shapes are negligibly influenced by gravity. Using a relatively simple model for the flow within the porous substrate, the zeta potential resulting from the substrate-liquid combination is determined. Extensive measurements of a borosilicate substrate in contact with different aqueous electrolytes are made. The results of the measurements clarify the influence of the ionic strength and pH value on the zeta potential and yield an empirical relationship important to engineering approaches.

Spatio-temporal dynamics of a periodically driven cavity flow

The flow in a rectangular cavity driven by the sinusoidal motion of the floor in its own plane has been studied both experimentally and computationally over a broad range of parameters. The stability limits of the time-periodic two-dimensional base state are of primary interest in the present study, as it is within these limits that the flow can be used as a viable surface viscometer (as outlined theoretically in Lopez & Hirsa 2001). Three flow regimes have been found experimentally in the parameter space considered: an essentially two-dimensional time-periodic flow, a time-periodic three-dimensional flow with a cellular structure in the spanwise direction, and a three-dimensional irregular (in both space and time) flow. The system poses a space–time symmetry that consists of a reflection about the vertical mid-plane together with a half-period translation in time ( RT symmetry); the two-dimensional base state is invariant to this symmetry. Computations of the two-dimensional Navier–Stokes equations agree with experimentally measured velocity and vorticity to within experimental uncertainty in parameter regimes where the flow is essentially uniform in the spanwise direction, indicating that in this cavity with large spanwise aspect ratio, endwall effects are small and localized for these cases. Two classes of flows have been investigated, one with a rigid no-slip top and the other with a free surface. The basic states of these two cases are quite similar, but the free-surface case breaks RT symmetry at lower forcing amplitudes, and the structure of the three-dimensional states also differs significantly between the two classes.

Concentration measurements downstream of an insoluble monolayer front

The surfactant concentration distribution on a planar uniform flow with a surface-piercing barrier was measured via the nonlinear optical technique of second-harmonic generation. The measurements were performed for an insoluble surfactant monolayer on the air/water interface. A theoretical model balancing surface elasticity and bulk shear at the interface was developed to predict the concentration profile for any insoluble monolayer. Measured equations of state, relating the surface tension to the surfactant concentration, were used in the model along with velocity data obtained using boundary-fitted digital particle image velocimetry. Theoretical concentration profiles were in agreement with experimental results. Additionally, global predictions from the model for four different insoluble surfactant systems also showed agreement with experimental measurements.

Determination of the zeta potential of porous substrates by droplet deflection: II. Generation of electrokinetic flow in a nonpolar liquid.

Here we study the nature and extent of free electrical charges in nonpolar liquids, using a recently introduced technique of observing droplet deflection generated by electrokinetic flow in a porous substrate. In the presence of dispersed water, surfactant molecules agglomerate and inverted micelles are generated which may act as charge carriers. In the present work, the conductivities of solutions of a nonpolar liquid with several concentrations of a dissolved surfactant are measured by electrical transients. The induced current densities are proportional to the applied voltage, indicating that the solutions represent an ohmic system. The conductivity does not scale simply with the surfactant concentration, though. It is inferred that different micellization mechanisms exist depending on the surfactant concentration, and a model is sketched. Further experiments reveal that flows of such solutions can be generated within saturated porous substrates when they are subjected to moderate electric fields. An investigation of the phenomena leads to the conclusion that these flows exist due to the presence of an electrical double layer; that is, they are of electrokinetic (electroosmotic) origin. Hence, the measured electrokinetic flow rates can be related to the zeta potential of the porous substrate saturated with the solution. Plotting the zeta potential against the logarithm of the ionic strength reveals a linear relationship.

Simultaneous measurement of free-surface velocity and surfactant concentration via a common laser probe

A new technique was developed to simultaneously measure the velocity and surfactant surface concentration at an air/water interface via a common laser probe. The surfactant concentration is measured using the nonlinear optical technique of reflected second-harmonic generation (SHG), utilizing an Nd:YAG laser. The transmitted portion of the same laser beam provides illumination for velocity measurements at the interface using the technique of boundary-fitted digital particle image velocimetry (DPIV). The combined SHG and DPIV system was utilized to scan along the free surface of a uniform water flow with a surface barrier trapping hemicyanine, an insoluble monolayer. Measurements were made downstream of the Reynolds ridge, which marks the contamination front and the leading edge of a free-surface boundary layer.

Electromagnetic control of coupled droplets

Electromagnetism offers several advantages for moving capillary surfaces, including energy efficiency, fast response, and device integrability. Here, we demonstrate electromagnetic control of a pinned-contact, coupled droplet system using aqueous ferrofluids. A time-varying magnetic field provides the necessary perturbation to toggle millimeter scale capillary switches. Furthermore, periodic magnetic fields can drive coupled droplets at resonant frequencies approaching 100 Hz using only 1 V by balancing capillary forces with liquid inertia. These addressable devices may find applications in adaptive optics, fluidic actuators, and read-write arrays.

Optical performance of an oscillating, pinned-contact double droplet liquid lens

Liquid droplets can produce spherical interfaces that are smooth down to the molecular scale due to surface tension. For typical gas/liquid systems, spherical droplets occur on the millimeter and smaller scales. By coupling two droplets, with contact lines pinned at each edge of a cylindrical hole through a plate, a biconvex lens is created. Using a sinusoidal external pressure, this double droplet system (DDS) can be readily forced to oscillate at resonance. The resulting change in the curvatures of the droplets produces a time-varying focal length. Such an oscillating DDS was introduced in 2008 [Nat. Photonics 2, 610 (2008)]. Here we provide a more comprehensive description of the systems optical performance, showing the effects of liquid volume and driving pressure amplitude on the back focal distance, radii of curvature, object distance, and image sharpness.

Beetle-inspired adhesion by capillary-bridge arrays: pull-off detachment

Adhesion by capillarity (‘wet’ adhesion) depends on the surface tension of an array of many small liquid bridges acting simultaneously against a substrate. A particular leaf beetle has been previously shown to defend itself using wet adhesion, and a man-made device, inspired by this beetle, has been previously demonstrated to exhibit electronically controlled switchable wet adhesion. In both cases, measurements of detachment under load have been reported as pull-off strengths. In this paper, we pose models for pull-off failure of adhesion and discuss the predictions of these models in relationship to available observations. The focus is on the role of array geometry and how net adhesive failure relates to the instability of a single liquid bridge.