# 6.021/Notes/2006-09-15

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## Microfluidics

- small channels about width of hair
- supports experiments not possible macroscopically
- small size inhibits mixing by convection
- mixing primarily due to diffusion
- no turbulence, laminar flow

## Diffusion into cells

- Dissolve and diffuse model
- Fick's law for membranes [math]\displaystyle{ \phi_n(t)=P_n(c^i_n(t)-c^o_n(t), P_n=\frac{D_nk_n}{k} }[/math]
- Charles Overton: 1st studies of diffusion through cell membranes
- tested 100s of solutes, dozens of cell types
- Surprising results: everyone thought membranes were impermeant to essentially everything but water

- Overton's rules
- Cell membranes are semi-permeable
- plant and animal cells are similar
- permeabilities correlate with solubility of solute in organic solvents
- hypothesized that membrane is lipid (cholesterol and phospholipis) (was correct)
- some cells concentrate solute (active transport)
- Meyer-Overton theory of narcosis: potentcy of anesthetics correlated with lipid solubility
- muscles don't contract in sodium free media

- Collander: first quantitative studies
- find permeabilities and solubilities
- enormous range of [math]\displaystyle{ P_n }[/math] and [math]\displaystyle{ k_n }[/math]
- clear correlation between [math]\displaystyle{ P_n }[/math] and [math]\displaystyle{ k_n }[/math] but large scatter (many orders of magnitude)
- can be attributed to differences in diffusivity (due to molecular weight)

**Transport of many substances consistent with dissolve and diffuse mechanism**