Chapter 2
Background
Microfluidics deals with the manipulation and control of small amounts of fluids [126] and is frequently applied for Lab-on-a-Chip (LoC) devices. This chapter
briefly reviews the background on microfluidics including an overview of possible
platforms. Afterwards, this chapter especially focuses on droplet microfluidic
networks, which are considered in this book. Finally, this chapter provides details
on the hardware and software setup, which has been applied for implementing and
evaluating the proposed design methods.
2.1 Microfluidics
In the microscale, characteristics of fluids change compared to the macroscale.
These characteristics are primarily based on the following physical properties:
• Laminar Flow: The dimensionless Reynolds number (Re) defines the ratio of
inertial forces to viscous forces (Re = ρ u L/μ where ρ is the density, u
is the linear velocity, L is a characteristic length, and μ is the viscosity). In
microfluidics, this number is generally small (Re ≤ 1) [55, 93] due to the small
channel sections and relatively small volumetric flow rates. Hence, the inertial
effects (i.e., the gravity, separation, secondary flow, and turbulence) are negligible
and yield a laminar flow regime.
• Surface and Interfacial Tension: The surface tension states the elastic tendency of
a fluid in a surface to contract the surface–air interface in order to reduce its free
energy [100]. For two immiscible fluids, this phenomenon is called interfacial
tension and, in the microscale, the interfacial tension dominates the gravity force.
• Capillary Forces: Capillarity describes the rise or depression of a fluid in a small
passage (e.g., a narrow channel/tube). Especially at the microscale, the capillary
forces are dominant which allow fluids to flow against the gravity [100]. The
Capillary number (Ca) describes the relative effect between viscous forces and
© Springer Nature Switzerland AG 2020
A. Grimmer, R. Wille, Designing Droplet Microfluidic Networks,
https://doi.org/10.1007/978-3-030-20713-7_2
13
Background
Microfluidics deals with the manipulation and control of small amounts of fluids [126] and is frequently applied for Lab-on-a-Chip (LoC) devices. This chapter
briefly reviews the background on microfluidics including an overview of possible
platforms. Afterwards, this chapter especially focuses on droplet microfluidic
networks, which are considered in this book. Finally, this chapter provides details
on the hardware and software setup, which has been applied for implementing and
evaluating the proposed design methods.
2.1 Microfluidics
In the microscale, characteristics of fluids change compared to the macroscale.
These characteristics are primarily based on the following physical properties:
• Laminar Flow: The dimensionless Reynolds number (Re) defines the ratio of
inertial forces to viscous forces (Re = ρ u L/μ where ρ is the density, u
is the linear velocity, L is a characteristic length, and μ is the viscosity). In
microfluidics, this number is generally small (Re ≤ 1) [55, 93] due to the small
channel sections and relatively small volumetric flow rates. Hence, the inertial
effects (i.e., the gravity, separation, secondary flow, and turbulence) are negligible
and yield a laminar flow regime.
• Surface and Interfacial Tension: The surface tension states the elastic tendency of
a fluid in a surface to contract the surface–air interface in order to reduce its free
energy [100]. For two immiscible fluids, this phenomenon is called interfacial
tension and, in the microscale, the interfacial tension dominates the gravity force.
• Capillary Forces: Capillarity describes the rise or depression of a fluid in a small
passage (e.g., a narrow channel/tube). Especially at the microscale, the capillary
forces are dominant which allow fluids to flow against the gravity [100]. The
Capillary number (Ca) describes the relative effect between viscous forces and
© Springer Nature Switzerland AG 2020
A. Grimmer, R. Wille, Designing Droplet Microfluidic Networks,
https://doi.org/10.1007/978-3-030-20713-7_2
13