208 6.2  Rheology and Hydrodynamics Tools

length scales of a few tens of microns, and different channels can be connected to generate a

complex flow-​based device. These systems are discussed fully in Chapter 7.

6.2.3  TOOLS THAT UTILIZE OSMOTIC FORCES

Dialysis, or ultrafiltration, has similar operating principles to chromatography in that the

sample mobility is characterized by similar factors, but the solvated sample is on one side of a

dialysis membrane that has a predefined pore size. This sets an upper molecular weight limit

for whether molecules can diffuse across the membrane.

This selectively permeable membrane (also referred to as a semipermeable mem­

brane) results in an osmotic force driven by entropy. On either side of the membrane,

there is a concentration gradient, that is, the concentration of solvated molecules on

one side of the membrane is different from that on the other side. The water molecules

in the solution that has a higher concentration have more overall order since there are

a greater relative number of available solute molecules to which they bond, usually

via electrostatic and/​or hydrogen bonding. This entropy difference between the two

solutions either side of the membrane is manifested as a statistical/​entropic driving

force when averaged over time scales are much larger than the individual water mol­

ecule collision time, which acts in a direction to force a net flux of water molecules

from the low-​to high-​concentration solutions (note that this is also the physical basis of

Raoult’s law, which states that the partial vapor pressure of each component of an ideal

mixture of liquids is equal to the vapor pressure of the pure component multiplied by

its mole fraction in that mixture; in other words, the components act independently of

each other). This process can be used to separate different populations of biomolecules

of the basis of molecular weight, often facilitated by a pressure gradient. The use of mul­

tiple dialysis stages using membranes with different pore sizes can be used to purify a

complex mix of different molecules.

Osmotic pressure can also be used in the study of live cells. Lipid membranes of cells and

subcellular cell organelles are selectively permeable. Although some ions undergo passage

diffusion through pores in the membrane, in general the passage of water, ions, and various

biomolecules is highly selective and often tightly regulated. Enclosure of solutes inside a cell

membrane, therefore, results in a strong osmotic pressure on cells, exerted from the inside of

the cell onto the membrane toward the outside.

As discussed previously (see Chapter 2), there are various mechanisms to prevent cells

from exploding due to this osmotic pressure depending on the cell type, for example, cell

walls in bacteria and plant cells, and/​or regulation of ion and water pumps that are especially

important in eukaryotic cells that in general have no stiff cell wall barrier. These mechanisms

can be explored in an osmotic chamber. This is a device that allows cells to be visualized using

light microscopy in their normal aqueous environment but allowing the external pressure

exerted through the liquid environment to be carefully controlled, up to pressures or sev­

eral tens of atmospheres. Combining cellular pressure control with fluorescence micros­

copy to probe cell wall proteins and ion channel components has proved informative in our

understanding of cellular osmoregulatory mechanisms.

6.2.4  DEFORMING BIOLOGICAL MATTER WITH FLOW

The scientific study of flow and deformation of matter is known as rheology. It addresses

the interesting observation that all ultimately matter “flows” and deforms under mechanical

stress, but the time scales over which this occurs, and how the matter responds after the

driving force is removed, varies widely across different materials, and this can tell us a great

deal about the underpinning physical properties of that material which are often more com­

plex than just considering how single molecules behave, but rather how molecules behave

when they cooperate together in a so-​called emergent way. This is particularly true for the soft

matter that makes up biological systems, which can have highly diverse rheological properties,