Vibrational modes of elongated sessile liquid droplets

Resonance Modes of Water Drops Pinned to a Vibrating Rectangular Post

We studied the effects of vertical vibrations on a water drop that was pinned to the sharp edges of a rectangular post. By varying the frequency and amplitude of the vertical displacement, distinct resonance peaks were observed using a simple optical technique. The vibrational spectra of the first two modes exhibited two closely spaced peaks, which corresponded to standing waves that exist along the major and minor contour lengths of the drops. The values of the resonance frequencies can be explained rather well by a simple model, which was originally proposed for axially symmetric drops.

Looking for future biological control agents: the comparative function of the deutosternal groove in mesostigmatid mites

The physics of fluid laminar flow through an idealised deutosternum assembly is used for the first time to review predatory feeding designs over 72 different-sized example species from 16 mesostigmatid families in order to inform the finding of new biological control agents. Gnathosomal data are digitised from published sources. Relevant gnathosomal macro- and micro-features are compared and contrasted in detail which may subtly impact the control of channel- or 'pipe'-based transport of prey liquids around various gnathosomal locations. Relative deutosternal groove width on the mesostigmatid subcapitulum is important but appears unrelated to the closing velocity ratio of the moveable digit. Big mites are adapted for handling large and watery prey. The repeated regular distance between deutosternal transverse ridges ('Querleisten') supports the idea of them enabling a regular fluctuating bulging or pulsing droplet-based fluid wave 'sticking' and 'slipping' along the groove. Phytoseiids are an outlier functional group with a low deutosternal pipe flow per body size designed for slot-like microchannel transport in low volume fluid threads arising from daintily nibbling nearby prey klinorhynchidly. Deutosternal groove denticles are orientated topographically in order to synergise flow and possible mixing of coxal gland-derived droplets and circumcapitular reservoir fluids across the venter of the gnathosomal base back via the hypostome to the prey being masticated by the chelicerae. As well as working with the tritosternum to mechanically clean the deutosternum, denticles may suppress fluid drag. Shallow grooves may support edge-crawling viscous flow. Lateral features may facilitate handling unusual amounts of fluid arising from opportunistic feeding on atypical prey. Various conjectures for confirmatory follow-up are highlighted. Suggestions as to how to triage non-uropodoid species as candidate plant pest control agents are included.

Audible Sound from Vibrating Sessile Droplets for Monitoring Chemicals and Reactions in Liquid

To reduce environmental impact and sensor footprint, researchers need cost-effective and small-size surface tension and viscosity measurement devices. New measurement principles are needed for such sensors. We demonstrate that a sessile droplet's mechanical vibration can be transformed to audible sound, by recording the ultrasonic Doppler frequency shift in the form of an acoustic signal. The recorded sound wave reveals a droplet's surface tension and its viscosity, through its frequency spectrum and attenuation rate of the signal, respectively. Based on such sensors, two chemical measurements inside sessile droplets are shown: (I) titration of a Ni²⁺ and Co²⁺ mixture with a surface-active indicator (using surface tension) and (II) measurement of the molecular weight of a polymer in solution (using viscosity). Unlike the commercial technique, our ultrasound-based sensor is cost-effective in terms of equipment price and sample volume.

Hydrodynamic and physicochemical phenomena in liquid droplets under the action of nanosecond spark discharges: A review

This review presents experimental studies of phenomena occurring in droplets of various liquids under the effect of nanosecond spark discharges. Inorganic liquids and liquids of biological origin are considered here. Attention is payed to hydrodynamic and physico-chemical phenomena in droplets, including a movement of sessile droplets on a substrate under the effect of the discharges, internal flows in droplets (excited by the discharges), plasma capillary phenomena, features of the droplets drying under the effect of the discharges, traces (patterns) left by the droplets, exposed to the discharges, on the substrates etc.

Emanating Jets As Shaped by Surface Tension Forces

We show that emanating jets can be regarded as growing liquid towers, which are shaped by the twofold action of surface tension: first the emanated fluid is being accelerated back by surface tension force, herewith creating the boundary conditions to solve the shape of the liquid tower as a solution of an equation mathematically related to the hydrostatic Young-Laplace equation, known to give solutions for the shape of pending and sessile droplets, and wherein the only relevant forces are gravity g and surface tension γ. We explain that for an emanating jet under specific constraints all mass parts with density ρ will experience a uniform time dependent acceleration a( t). An asymptotic solution is subsequently numerically derived by making the corresponding Young-Laplace type equation dimensionless and by dividing all lengths by a generalized time dependent capillary length λ_c( t) = [Formula: see text]. The time dependent surface tension γ( t) can be derived by measuring both time dependent acceleration a( t) and time dependent capillary length λ_c( t). Jetting experiments with water and coffee show that the dynamic surface tension behavior according to the emanating jet method and with the well-known maximum bubble pressure method are the same, herewith verifying the proposed model.

Axisymmetric and Nonaxisymmetric Oscillations of Sessile Compound Droplets in an Open Digital Microfluidic Platform

Manipulating droplets of biological fluids in an electrowetting on dielectric (EWOD)-based digital microfluidic platform is a significant challenge because of biofouling and surface contamination. This problem is often addressed by operating in an oil environment. We study an alternate configuration of sessile compound droplets having an aqueous core surrounded by a smaller oil shell. In contrast to the conventional EWOD platform, an open digital microfluidic platform enabled by the core-shell configuration will allow electrical, mechanical, or optical probes to get unrestricted access to the droplet, thus enabling highly flexible and dynamically reconfigurable lab-on-chip systems. Understanding droplet oscillations is essential as they are known to enhance mixing. To our knowledge, this is the first study of axisymmetric and nonaxisymmetric oscillations of compound droplets actuated using EWOD platforms. Mode shapes for both axisymmetric and nonaxisymmetric oscillations were studied and explained. Enhancement in the axisymmetric oscillation of the core by decreasing the shell volume was obtained experimentally and modeled theoretically. Smaller shell volumes reduce the damping losses, allowing the appearance of nonaxisymmetric modes over a larger range of operating parameters. The oscillation frequency regime for obtaining prominent nonaxisymmetric oscillations for different shell volumes was identified. Compound droplets provide a mechanism to reduce biofouling, sample contamination, and evaporation. We demonstrate axisymmetric and nonaxisymmetric oscillations of compound droplets with the biological core of red blood cells, providing crucial first steps for promoting applications such as rapid efficient assays, mixing of biological fluids, and fluidic photonics on hysteresis-free surfaces.

Oscillating Ferrofluid Droplet Microrheology of Liquid-Immersed Sessile Droplets

The damped oscillations of liquid-immersed ferrofluid sessile droplets were studied with high-speed imaging experiments and analytical modeling to develop a novel microrheology technique. Droplet oscillations were induced with an external magnetic field, thereby avoiding transients in the resulting vibrational response of the droplet. By following the droplet relaxation with a high-speed camera, the frequency and relaxation time of the damped harmonic oscillations were measured. We extend upon existing analytical theories to describe our liquid-immersed sessile droplet system, and directly quantify the droplet relaxation with the viscosity of the internal and external fluid as well as the interfacial tension between these. The easily controllable magnetic droplets make our oscillating ferrofluid droplet technique a potential candidate for high-throughput microrheology and tensiometry in the future.

Dielectrowetting manipulation for digital microfluidics: creating, transporting, splitting, and merging of droplets

Generating, splitting, transporting, and merging droplets are fundamental and critical unit operations for digital (droplet-based) microfluidics. State-of-the-art digital microfluidics performs such operations commonly using electrowetting-on-dielectric (EWOD) in the typical configuration of two parallel channel plates. This paper presents such operations using dielectrowetting (derived from liquid dielectrophoresis), not EWOD, with an array of interdigitated electrodes. The major and unique feature is that the present droplet manipulations are effective for conductive (water with/without surfactant) and non-conductive (propylene carbonate) fluids. An equally important aspect is that the manipulations are performed in an open space without the covering top plate. This behavior is attributed to the intrinsic nature of dielectrowetting to generate stronger wetting forces than EWOD (with the ability to achieve complete wetting with contact angle = 0° to form a thin film). Using dielectrowetting, micro-droplets of various volumes are created from a large droplet and transported. Splitting a single droplet as well as multiple droplets and merging them are also achieved, even when the droplets are smaller than the electrode pads. The above splitting, transport, and merging operations are effective for propylene carbonate as well as DI water with/without surfactant, though the creating operation is proven only for propylene carbonate at this moment. All the above manipulations are successfully carried out on a single plate, which not only simplifies the structure and operation procedure, but could also eliminate the restriction to the volume of fluid handled.

Optovibrometry: tracking changes in the surface tension and viscosity of multicomponent droplets in real-time

An instrument was developed for measuring real time changes in the surface tension and viscosity of multicomponent droplets of miscible liquids and other soft materials. Droplets containing glycerol and water were supported on superamphiphobic surfaces and vibrated by applying a short mechanical impulse. Laser light was refracted through the droplets and allowed to fall on the surface of a photodiode. Time dependent variations in the intensity measured by the photodiode during vibration were used to monitor the decay of the droplet oscillations. The frequencies and spectral widths of the droplet vibrational resonances were then obtained from Fourier transforms of these time dependent intensity signals. A recently developed model of viscoelastic droplet vibration was used along with these values and measurements of the drop dimensions to extract the surface tension and viscosity of the drops as they evaporated. Collection of data was automated and values of frequency, spectral width, drop size, surface tension and viscosity were obtained with a time resolution of three seconds over a period of thirty minutes. The values of surface tension and viscosity obtained were shown to be in good agreement with literature values obtained from bulk glycerol/water solutions; thus validating the technique for wider application to other multicomponent liquids and soft matter systems.

Mechanical vibrations of pendant liquid droplets

A simple optical deflection technique was used to monitor the vibrations of microlitre pendant droplets of deuterium oxide, formamide, and 1,1,2,2-tetrabromoethane. Droplets of different volumes of each liquid were suspended from the end of a microlitre pipette and vibrated using a small puff of nitrogen gas. A laser was passed through the droplets and the scattered light was collected using a photodiode. Vibration of the droplets resulted in the motion of the scattered beam and time-dependent intensity variations were recorded using the photodiode. These time-dependent variations were Fourier transformed and the frequencies and widths of the mechanical droplet resonances were extracted. A simple model of vibrations in pendant/sessile drops was used to relate these parameters to the surface tension, density and viscosity of the liquid droplets. The surface tension values obtained from this method were found to be in good agreement with results obtained using the standard pendant drop technique. Damping of capillary waves on pendant drops was shown to be similar to that observed for deep liquid baths and the kinematic viscosities obtained were in agreement with literature values for all three liquids studied.