Yit Fatt Yap

Thermally mediated droplet formation in microchannels

Precise dispensing of microdroplets is an important process for droplet-based microfluidics. The dropletformation by shear force between two immiscible fluids depends on their flow rates, the viscosities, and the interfacial tension. In this letter, the authors report the use of integrated microheater and temperature sensor for controlling the dropletformation process. The technique exploits the dependency on temperature of viscosities and interfacial tension. Using a relatively low heating temperature ranging from 25 to 70 ° C , the droplet diameter can be adjusted to over two times of its original value. The relatively low temperature range makes sure that this concept is applicable for droplets containing biological samples.

Thermally mediated breakup of drops in microchannels

The authors used thermally induced surface tension gradients to manipulate aqueous droplets in microchannels. Control of the droplet breakup process was demonstrated. Droplet sorting can be achieved with temperatures above a critical value. Numerical simulation using a two-dimensional model agrees qualitatively well with the experimental results. The used control temperature of less than 55°C shows that this active control concept is suitable for biochemical applications. Thermal control promises to be a simple and effective manipulation method for droplet-based lab on a chip.

Thermally mediated control of liquid microdroplets at a bifurcation

The ability to precisely control the motion of droplets is essential in droplet-based microfluidics. It serves as the basis for various droplet-based devices. This paper presents a thermal control technique for microdroplets at a bifurcation. Control was achieved using an integrated microheater that simultaneously induces a reduction in fluidic resistance and thermocapillarity. The temperature of the heater was monitored by an integrated temperature sensor. At a bifurcation with symmetric branches, a droplet can be split into two daughter droplets of controllable sizes or entirely switched into a desired branch. The physics of this phenomenon was investigated with the help of a numerical model. Splitting and switching were demonstrated within an operational temperature range 25‐38 ◦ C. The relatively low operational temperature range allows this technique to be used for droplets containing biological samples. The present control concept is not limited to bifurcations, but can be employed in other geometries.

A Global Mass Correction Scheme for the Level-Set Method

The level-set method is used to study the evolution of a bubble carried by a primary phase in (1) a straight channel, (2) a double-bend channel, and (3) a constricted channel. Special attention is given to the conservation of mass for the phases. A global mass correction scheme is proposed to ensure mass conservation. Surface tension effect is modeled using the continuum surface force approach. A finite-volume method is used to solve the governing equations. The CLAM schemes are used to model the convection of the level-set equations. The results compare well with the solutions of the volume-of-fluid (VOF) method.

ACTIVE CONTROL FOR DROPLET-BASED MICROFLUIDICS

Active control of microdroplets in microchannels is an important task in droplet-based microfluidics. The breakup process of droplets at an T-junction is usually controlled passively by the fluidic resistance of the branches. We used thermal control to actively manipulate aqueous droplets in microchannels. The temperature affects both viscosity and interfacial tension between the phases. The concept was first simulated with a two-dimensional model. The simulation results show that increasing temperature at a branch can change the size ratio of the two daughter droplets from 0 to 1. That means, droplet switching is possible with this concept. Control of droplet size during the formation process and splitting process was demonstrated experimentally by varying the temperature of the branches. At a critical temperature, droplet switching can be achieved. The used control temperature of less than 40 ◦C shows that this active control concept is suitable for biochemical applications. Thermal control promises to be a simple and effective manipulation method for droplet-based lab on a chip.

Numerical study of the formation process of ferrofluid droplets

This paper numerically investigates the influence of a uniform magnetic field on the dropletformation process at a microfluidicflow focusing configuration. The mathematical model was formulated by considering the balance of forces such as interfacial tension, magnetic force, and viscous stress across the liquid/liquid interface. A linearly magnetizable fluid was assumed. The magnetic force acts as a body force where the magnetic permeability jumps across the interface. The governing equations were solved with finite volume method on a Cartesian fixed staggered grid. The evolution of the interface was captured by the particle level set method. The code was validated with the equilibrium steady state of a ferrofluiddroplet exposed to a uniform magnetic field. The evolution of the dropletformation in a flow focusing configuration was discussed. The paper mainly analyzes the effects of magnetic Bond number and the susceptibility on the velocity field and the droplet size. The droplet size increased with increasing magnetic strength and susceptibility.

Microfluidic rheometer based on hydrodynamic focusing

This paper reports the concept and the optimization of a microfluidic rheometer based on hydrodynamic focusing. In our microfluidic rheometer, a sample stream is sandwiched between two sheath streams. The width of the middle stream depends on the viscosity ratio and the flow rate ratio of the liquids involved. Fixing the flow rate ratio and using a known Newtonian liquid for the sheath streams, the viscosity and the shear stress of the sample stream can be determined by measuring its width and using a prediction algorithm that uses the known channel geometries, fluid properties, flow rates and the focused width as input parameters. The optimization reveals that a measurement channel with a high aspect ratio is more suitable for a sample liquid with viscosity higher than the reference value. For a sample liquid with viscosity of the same order of magnitude or lower than the reference, a low aspect ratio is more suitable. Furthermore, the measurement range and the relative error can be improved by adjusting the flow rate ratio between the core stream and the sheath stream. A microfluidic device was fabricated in polymethylmethacrylate (PMMA). Using this device, viscosities of deionized (DI) water and polyethylene oxide (PEO) solutions were measured and compared with results obtained from a commercial rheometer.

Modeling the Flows of Two Immiscible Fluids in a Three-Dimensional Square Channel Using the Level-Set Method

ABSTRACT A numerical solution of an annular two-phase flow in a square channel is presented. A combined formulation using only one set of conservation equations to treat both fluids is employed. The level-set (LS) method is used to capture the interface between the fluids. To overcome a weakness in the level-set method, the localized mass correction (LMC) scheme of [22] is applied to ensure mass conservation. The finite-volume method is used to solve the governing equations. Results are presented for two fluids of identical and different properties with two different inlet interface shapes. These results are compared with that of the volume-of-fluid (VOF) method and good agreement is achieved.

Particle Transport in Microchannels

Particle transport in microchannel is presented. This article focuses on situations in which the sizes of the particles are comparable to the sizes of the channels. These solid bodies are sufficiently large that momentum is exchanged between the bodies and the flowing fluid. As a result, the solid bodies affect the fluid flow significantly, and vice versa, resulting in a transient process in which the motions of the solid bodies and the flow field are strongly coupled. The flow field and the particulate flow must then be solved simultaneously. The solid bodies are modeled as a fluid constraint to move with rigid body motion. The solid–fluid interface is described using a distance function. For demonstration purposes, the finite-volume method is used to solve the resulting set of governing equations. The present approach is validated against (1) flow around stationary, (2) flow around forced rotating, (3) flow around freely rotating cylinders, and (4) sedimentation of a circular cylinder under gravity. Finally, the motion of particles carried by an incompressible fluid in a microchannel system is studied.

A procedure for the motion of particle-encapsulated droplets in microchannels

A fixed-grid approach for modeling the motion of a particle-encapsulated droplet carried by a pressure-driven immiscible carrier fluid in a microchannel is presented. Three phases (the carrier fluid, the droplet, and the particle) and two different moving boundaries (the droplet–carrier fluid and droplet–particle interfaces) are involved. This is a moving-boundaries problem with the motion of the three phases strongly coupled. In the present article, the particle is assumed to be a fluid of high viscosity and constrained to move with rigid body motion. A combined formulation using one set of governing equations to treat the three phases is employed. The droplet–carrier fluid interface is represented and evolved using a level-set method with a mass-correction scheme. Surface tension is modeled using the continuum surface force model. An additional signed distance function is employed to define the droplet–particle interface. Its evolution is determined from the particle motion governed by the Newton-Euler equations. The governing equations are solved numerically using a finite-volume method on a fixed Cartesian grid. For demonstration purposes, the flows of particle-encapsulated droplets through a constricted microchannel and through a microchannel system are presented.