14
2 Background
surface tension acting on a fluid–gas interface or between two immiscible fluids
(Ca = μ u/γ where μ is the viscosity, u is the velocity of the flow, and γ is the
surface or interfacial tension).
Exploiting these different characteristics of fluids in the microscale started in the
1990s where first developments of microfluidics were mostly based on continuous
flow platforms [55]. Today, a large variety of platforms exist for realizing a microfluidic device. The following list briefly describes important microfluidic platforms,
which are based on different principles to drive and control the fluids [85]:
• Pressure-driven continuous flow devices use applied pressure gradients to drive
the fluids through microchannels. The source of the pressure can be located on
as well as off device.
• Lateral flow devices (also called test strips) use capillary forces to drive the fluids.
• Linear actuated devices use some sort of mechanical displacement to control the
fluid flow.
• Microfluidic large-scale integration based devices use integrated valves to
control the flow through microchannels.
• Centrifugal devices use a rotating micro-structured disc, which is controlled by
a spin protocol. This generates, e.g., centrifugal and Euler forces, which are used
to control the fluids.
• Electrokinetic-operated devices control fluids or unit operations by using electric
fields or gradients of electric fields.
• Electrowetting-operated devices allow to control droplets (which are immersed
in a second immiscible continuous gaseous or liquid phase) on a one- or twodimensional array of electrodes by changing the electrodes’ voltages. This
platform is often called digital microfluidics.
• Surface acoustic waves-operated devices use droplets on a hydrophobic surface.
The generated acoustic shock waves allow moving the droplets.
• Paper-based devices use printed hydrophobic barriers on a paper, which represent the microchannel network. The fluids are then driven by capillary forces
(i.e., paper wicking).
• Two-phase flow (also called segmented flow) devices use droplets or plugs as
micro-reactors, which are injected in a second immiscible continuous phase.
The flow of the droplets or plugs inside the continuous phase through the
microchannels is driven by pressure gradients. In the microfluidics community,
this platform is usually called droplet microfluidics.
This book considers droplet microfluidics and especially corresponding networks, which are reviewed in more detail next.
2 Background
surface tension acting on a fluid–gas interface or between two immiscible fluids
(Ca = μ u/γ where μ is the viscosity, u is the velocity of the flow, and γ is the
surface or interfacial tension).
Exploiting these different characteristics of fluids in the microscale started in the
1990s where first developments of microfluidics were mostly based on continuous
flow platforms [55]. Today, a large variety of platforms exist for realizing a microfluidic device. The following list briefly describes important microfluidic platforms,
which are based on different principles to drive and control the fluids [85]:
• Pressure-driven continuous flow devices use applied pressure gradients to drive
the fluids through microchannels. The source of the pressure can be located on
as well as off device.
• Lateral flow devices (also called test strips) use capillary forces to drive the fluids.
• Linear actuated devices use some sort of mechanical displacement to control the
fluid flow.
• Microfluidic large-scale integration based devices use integrated valves to
control the flow through microchannels.
• Centrifugal devices use a rotating micro-structured disc, which is controlled by
a spin protocol. This generates, e.g., centrifugal and Euler forces, which are used
to control the fluids.
• Electrokinetic-operated devices control fluids or unit operations by using electric
fields or gradients of electric fields.
• Electrowetting-operated devices allow to control droplets (which are immersed
in a second immiscible continuous gaseous or liquid phase) on a one- or twodimensional array of electrodes by changing the electrodes’ voltages. This
platform is often called digital microfluidics.
• Surface acoustic waves-operated devices use droplets on a hydrophobic surface.
The generated acoustic shock waves allow moving the droplets.
• Paper-based devices use printed hydrophobic barriers on a paper, which represent the microchannel network. The fluids are then driven by capillary forces
(i.e., paper wicking).
• Two-phase flow (also called segmented flow) devices use droplets or plugs as
micro-reactors, which are injected in a second immiscible continuous phase.
The flow of the droplets or plugs inside the continuous phase through the
microchannels is driven by pressure gradients. In the microfluidics community,
this platform is usually called droplet microfluidics.
This book considers droplet microfluidics and especially corresponding networks, which are reviewed in more detail next.