Session OC: Microfluidics XI: Dielectrophoresis and Surface Wetting

Physical and Design Parameters Influence On Nanoparticle DEP Dynamic Focusing

The manipulation of biological objects from micrometer to nanometer scale particles plays an important role in many biological and colloidal science applications. As nanoparticles are subject to Brownian motion, their trapping is challenging. However, dynamic trapping can be achieved by using the combined effect of diffusion, fluid flow and electrical forces. We present numerical simulations and experiments on nanoparticle dielectrophoretic (DEP) trapping in a linear electrode array microchannel. We derived a dynamical model using an advection-diffusion equation where the advective term consists of the sum of a conservative (related to the fluid flow) and a non-conservative term (related to the DEP force). The numerical solution of this PDE predicts the intensity of the focusing of particles as small as 10 nm in diameter which fits our experimental measurements. We observe that the existence and location of the trapping regions depend not only on the size of the particles but also on the physical and design parameters. We characterize those influences.   

Stretching DNA molecules via combined electrospraying and evaporation

The high elastic moduli of coiled DNAs stipulate that, without anchoring, they can be stretched hydrodynamically only if a high shear-rate gradient can be imposed longitudinally. We demonstrate that DNAs can be stretched by a combination of two micro-stagnation flows: Electrospraying of the DNA solution produces micro-drops that are impacted onto a positively charged surface and the drops then recede by evaporation on the same surface. The opposite surface charge helps retain the stretched conformation. Varying the spraying conditions alters the drop dimension and charge and consequently the evaporation rate as well as the impact force, leading to a variety of stain patterns with different stretching mechanisms and stretching ratio. A multi-ring DNA stain pattern, with stretched DNAs connecting the rings, is also observed when the DNAs aggregate at the contact-line and cause a stick-slip receding process with periodic depinning of the contact line. The effect of each stretching parameter and the role of electrospraying, drop impact and evaporation as individual stretching mechanisms are studied through high-speed confocal scanning microscopy.   

The effect of time-scales on the liquid meniscus during AC electrospraying.

Electrospraying under an AC electric field displays different spray modes. The liquid meniscus exhibits resonance, multi-cone spraying, dripping, tip-streaming, jetting, steady conical shapes or discharge assisted ejections at certain voltage-frequency-conductivity-viscosity windows. The dramatic variation in the meniscus behavior suggests interplay of different charging mechanisms taking place both in the liquid and gas phases. The observation of quasi--steady cones exhibiting growth in the longitudinal dimension point to charge entrainment in the liquid phase causing charge accumulation over successive half cycles leading to the meniscus growth. The effect of liquid conductivity on the occurrence of these cones reflects the role of charge relaxation time scale which must be significantly larger than the period of the applied field. On the other hand, the gas phase charging behavior is seen in the observation of corona discharge induced glow and the consequent ejection event. The coupling of these charging behaviors hence gives rise to various spraying modes suggesting a simple method to produce droplets with specific dimensions for different liquids.   

Reversible Control of Anisotropic Electrical Conductivity Using Colloidal Microfluidic Networks

In this talk, we present controllable electrical conductivity changes in thin-films using the directed assembly of metallic colloidal particles suspended in microfluidic circuits. Our preliminary experiments have shown that for a given concentration of colloidal particles in the microfabricated device with interdigitating electrodes, we can effectively and reversibly tune colloidal interactions to yield various steady-state configurations, by navigating the AC frequency-amplitude phase space. The device has two operation modes: 1) variable capacitor, and 2) variable resistor, and it can reversibly and rapidly switch between the two modes. Growth of colloidal wires spanning the gap between the two electrodes, establishes a conductive network circuit, and variable resistance can be realized by controlling the growth of colloidal wire interconnects. A variable capacitive circuit can be realized by concentrating colloidal particles in between the electrodes, and forming a vertically spanning colloidal network, that changes the effective spacing between the electrodes, and thereby the capacitance. A scaling analysis that includes gravity, Brownian motion, electrokinetic and electrohydrodynamic forces is used to interpret the relative influence of each force on the equilibrium response of the colloidal system.   

Effect of surface wettability on liquid structure and mobility near a solid surface

Using molecular dynamics and Langevin dynamics simulations, we study how surface wettability affects the density, structure, diffusion, and migration of a monatomic liquid near a solid surface. The liquid atoms near the solid surface experience different dynamics than those in the bulk. The resulting structure and density variations generate an anisotropic self- diffusion coefficient tensor which, in theory, can be tuned to bias liquid movement towards or away from the surface. The self- diffusion coefficient parallel to the surface is found to be inversely proportional to the local liquid structure factor and the self-diffusion coefficient parallel to the solid surface is found to be proportional to the local liquid density. These findings suggest that altering the wettability of a micro/nanochannel could provide a means for passive control of the diffusion of select targets towards a functionalized surface.   

Asymptotic Behavior of EWOD Force Distribution at a Contact Line

The exact nature of the electrostatic force distribution near the contact line of an EWOD-activated droplet can be expected to play a major role in such phenomena as contact angle hysteresis, contact angle saturation and dielectric breakdown. The electric field very close to a fluid contact line located at the interface of two materials with differing dielectric constants is investigated analytically. This is accomplished via a conformal mapping of the solution domain, as well as series expansions of the electric potential very close to the contact line. The field within each material near the tri-phase boundary is examined and used to discuss the relative importance of such phenomena as charge trapping, corona discharge, local dielectric breakdown and finite fluid resistivity to contact angle saturation.   

Numeric Simulation of EWOD and DEP Force Distributions

Two primary methods for electrostatically actuating microdroplets in channels currently exist: Dielectrophoresis (DEP) for electrically insulating liquids, and electrowetting on dielectric (EWOD) for conducting fluids. In each case, a transverse electric field is used to create an electrostatic pressure for transporting individual liquid slugs. This paper examines the nature of the force distribution for both EWOD- and DEP-actuated droplets. The effects of system parameters such as dielectric constant, dielectric thickness, contact angle and channel height on the shape of the force density and its net intergral are considered. A comparison of the scaling properties of the DEP and EWOD methods for applications in digital microfluidics is presented.   

Digitized Heat Transfer via Liquid Alloy Electrowetting

This presentation examines the use of liquid alloy electrowetting on dielectric (EWOD) as a novel method for active thermal management of compact electronic devices. The digitized nature of EWOD allows microscopic fluid slugs to be manipulated in a highly efficient and programmable manner. In addition, the characteristically high thermal conductivities of liquid metals and alloys allow them to support orders-of-magnitude higher heat fluxes than traditional coolants such as air or water. Consequently, we propose the use of EWOD- activated liquid alloy slugs for digitized heat transfer (DHT), a novel paradigm in micro scale thermal management. The hydrodynamics and heat transfer of EWOD-actuated alloy slugs are discussed, as well as preliminary experimental results.