Microfluidics for Biologists Fundamentals and Applications

For static liquid, Fnet¼0; therefore, it can be deduced as

Fgρo¼FBρf ð 1 : 9 Þ

Where,‘o’and‘f’denote‘object’and‘fluid’, respectively.
Now, based on (1.9) If,
ρo>ρf, the object will submerge and settle down to the bottom of the fluid.
However, an object will float but submerged withρo¼ρf, and will float on the
surface with aρo <ρf.
This knowledge becomes the basis of buoyancy-dependent mixing in certain
microfluidic set-ups. The best example is introduction of air bubbles from under-
neath of the static layer of liquid. The air bubbles have lower density than liquid and
will move towards the top of the channel thus causing disruption in the solvent
layers and introducing mixing Fig.1.3. We will discuss other details later in this
chapter.

2.4 Hydrodynamics: Physics of the Flows

Fluids at motion are governed by a set of variables and these are crucial in
understanding the phenomena taking place within confined boundaries in
microfluidics. There are few properties we will first acquaint with before looking
into other aspects.

Fig. 1.2 An illustration of Archimedes principle of buoyancy on a body of density‘ρo’dipped in a
fluid of density‘ρf’. The body of massmoexperiences two opposite forces on it, gravity acting
downwards and buoyancy or thrust acting upwards. Denser the body will be with respect to the
fluid, the greater the gravity force will be on it. Thus the body will drown. On the contrary, if it is
lighter than fluid, then it will float on the surface

1 Fundamentals of Fluidics 9