Topic 6: Micro/nanofluidics

Question 1

• Briefly, what is micro/nanofluidics?

Answer 1

• The manipulation of fluids in channels with dimension of tens of micro/nanometers

Question 2

• Equations to know
  
  • Reynolds number for flow in a channel
  • Mixing length for two streams under laminar conditions
  • Time for complete mixing

Answer 2

• 3/2 V_f/dv
• 3/2 h U_av/v
• U_avh­²/D
• h²/D

Question 3

• What is microfluidics? GIve an advantgae of its use

Answer 3

• Utilising scaling laws for new effects and better performance of moving solution through tiny channels
• Volumes used are very small (nL) as are amounts of reagents
• Making it ideal for testing expensive reagents

Question 4

• What are some applications of microfluidics?

Answer 4

• Inkjet printing
• Proteomics
• High through put screening

Question 5

• How are microchannels produced generally

Answer 5

• Photolithography generates +ve/-ve resists with a mask aligner
• Channel structure created, solution connected in to channels, integrated with electrodes via evaporation/spattering techniques to put Au/Pt onto an insulating surface.

Question 6

How are microchannel fabricated

Answer 6

Question 7

• How can we make a mould that can be used many times?

Answer 7

Question 8

• Sketch the fluid velocity profile in an infinitely wide channel under hydrostatic conditions

Answer 8

• Pressure driven: solution forced through channel
• Parabolic velocity profile forms
• Central line indicates greatest velocity

Question 9

• What is the Reynolds number and what does it define about a solution? Give a diagram to support your answer

Answer 9

• Defines whether solution flow if laminar or turbulent

Question 10

• What is the difference between laminar and turbulent flow and their corresponding Reynolds number?

Answer 10

• Laminar: Re < 2100
• In pump conditions (hydrostatic) parabolic flow profile is characteristic of micro/nanofluidic systems that are laminar
• Turbulent: Re >2100
• Achieved through microfabrication of obstruction points in a micro/nano fluidic channel

Question 11

• Using a sketch, describe the mixing of 2 reagents in a microchannel

Answer 11

• A/B each have their own velocity
• Must go to very long mixing lengths if two reagents happily coexist

Question 12

• Describe an experiment where a silver wire can be produced through microfluidics

Answer 12

• Flow of silver solution into a reducing agent causes homogeneous ET resulting in deposition of silver at interface between 2 systems due to laminar flow

Question 13

• What is the other form of micro/nanofluid flow technique? Give a diagram of the setup of this flow

Answer 13

• Electro-osmotic flow

Question 14

• Sketch and explain the velocity profile for the electro-osmotic flow technique

Answer 14

• Fluid flow pattern has changed from parabolic
• DL forms at any surface in contact with electrolyte that has a surface charge
• Opposite charged ions are attracted up
• Movement of ions due to electrostatic interactions cause DL water molecules to move entire solution
• Surface has functional groups that have charge associated with them despite no charge directly applied to surface

Question 15

• What are the equations defining electro-osmotic velocity/mobility?

Answer 15

Question 16

• What is an advantage of EO over pump flow?

Answer 16

• Blockages in pump cause leaks, must therefore filter foreign particles
• Not a concern in EO

Question 17

• What is a disadvantage of EO?

Answer 17

• Adsorption on wall changes charge and therefore velocity profile