Last Minute Tests

Question 1

Define a Flow Reaction

Answer 1

• a chemical reaction that is run in continuous flow as opposed to a batch reaction.
• Typically performed in a channel or tube where reagents are pumped in at constant flow rate.
• The dimensions of the channels or the tubes are often less that 1 mm, thus making these microreactors.
• The reaction time in a flow reactor is determined by the residence time of the flow reactor,

Question 2

Define Microfluidics

Answer 2

• Microfluidics is a broader term that describes also mechanical flow control devices like pumps and valves or sensors like flowmeters and viscometers.

Question 3

How are nanofluidics and microfluidics compared?

Answer 3

• looks at fluid flow in nanochannels

- here the phenomena are different as there are considerable surface effects

Question 4

How did flow chemistry develop historically?

Answer 4

• advances in fabrication technology
• first gas chromatogram was formed in silicon using photolithography and chemical etching techniques
• poor resolution but set the benchmark for minaturisation

Question 5

What are the disadvantages of batch reactions?

Answer 5

• Large ( expensive due to heat and light)
• Dangerous due to large quantities of stored chemical energyy
• heat transfer is bad, causes exothermic reactions to be dangerous as heat is contained within the system
• storage is expensive and dangerous
• mass transport changes as you scale up, affecting yield, selectivity and product quality

Question 6

Give the key characteristics of Flow reactions

Answer 6

• chemistry is largely unchanged, yield and product distribution may change due to different flow profiles/ heat exchange
• consider surface area to volume ratio effects
• good mass and heat transport
• different flow regime
• no significant interaction on a molecular level

Question 7

Advantages of the microscale (13)

Answer 7

• Reduced sample/reagent consumption (for synthesis and analysis)
• Portability (eg. PoC diagnostics, on-site analytics)
• Greater efficiencies of heat and mass transfer
• Easier integration of technology and automation (detection systems for example) - Reduced cost (time, space, energy etc. savings)
• Improved performance (throughput, control, power consumption, improved chemical yields and selectivities)
• Less space, material, energy, shorter response times, more info./space/time,
• low costs of devices,
• integration of small functional elements, enhancing system performance
• low fluid volume consumption, because of the low internal chip volumes, which is beneficial for e.g. environmental pollution (less waste), lower costs of expensive reagents and less sample fluid is used for diagnostics.
• higher analysis and control speed of the chip and better efficiency due to short mixing times (short diffusion distances), fast heating (short distances, high wall surface to fluid volume ratios, small heat capacities).
• better process control because of a faster response of the system
• compactness of the systems, due to large integration of functionality and small volumes.

Question 8

Why are linear dimensions important?

Answer 8

• important for temperature, concentration, density or pressure
• driving forces for heat transfer, mass transport or diffuisonal flux per unit volume or area increase when we use microreactors because the length scales (linear dimensions) are typically much smaller.
• Microreactors can handle and process small volumes and much larger number of samples.

Question 9

What is a space time yield?

Answer 9

• Quantity of product per volume of reactor per unit of time,
• simple way of quantifying efficiency of reactors

Question 10

Define Shear Stress

Answer 10

the stress parallel to a material surface, where the force component is perpendicular to the material surface

Question 11

Define Cohesive

Answer 11

attractive forces between two like particles

Question 12

Define Adhesive

Answer 12

attractive forces between two different particles

Question 13

How is the viscosity of a newtonian fluid measured?

Answer 13

•During fluid flow, the fluid close to a wall doesn’t move as it acts as a boundary layer, but further from the wall the fluid is flowing, so a velocity gradient exists, due to adhesive, cohesive and frictional forces.

• The magnitude of the velocity gradient is known as viscosity and is characteristic of a material
• • The greater the force required to move the plate at velocity v, the greater the viscosity of the fluid

Question 14

How does velocity vary when measuring viscosity?

Answer 14

• The stationary layer of fluid in contact with the stationary wall will inhibit the flow of the layer just above it. This layer will then inhibit the layer above and so on. Thus the velocity will vary linearly with distance above the stationary wall.

Question 15

Define Turbulent flow

Answer 15

• Fluid flow where nearby regions flow in different directions and velocities.
• This creates eddys and vertexes.
• This flow is typical near a fluid boundary.
• No precise mathematical definition

Question 16

Define Eddy

Answer 16

the swirling of a fluid and the reverse current created when a fluid flows past an obstacle.

Question 17

Define Laminar Flow

Answer 17

when fluids of one layer hardly mix with fluids of another layer, characterised by parallel streams of moving fluids.
- As this flow gets faster it becomes turbulent

Question 18

How does flow differ in Bulk and Flow systems?

Answer 18

• Fluid flow in microchannels is different from in bulk, providing a unique environment for chemical reactions
• Viscous forces dominate over inertial forces, generally resulting in laminar flow
• In bulk systems turbulent flow dominates and this has a significant effect on the mixing systems of microfluidic systems

Question 19

What is a Reynolds number and how does it affect Flow?

Answer 19

• Mixing is defined by a Reynolds number, which is the ratio of inertial forces to viscous forces for a specific flow region
• Laminar flow occurs at a low Reynolds number, where viscous forces cause a smooth and constant fluid motion
• Turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce chaotic eddies, vortices and other flow instabilities.

Question 20

How does microscale and macro scale mixing occur?

Answer 20

• macroscale mixing is achieved by turbulence

- In the microscale the laminar regime means diffusion is the main cause of mixing, as defined by ficks law

Question 21

What is diffusion?

Answer 21

• diffusion is considered to be the result of a random walk of diffusing particles
• theories of Brownian motion and Einstein’s theory of atomistic diffusion being used to explain the motion.

Question 22

When is the Stokes- Einstein equation used?

Answer 22

• in the limit of a low Reynolds number

- used to describe the motion of spherical particles

Question 23

When is Einsteins approximation equation used?

Answer 23

• to estimate the mean diffusion distance for a group of particles diffusing in 3 dimensions.

Question 24

How can diffusion limit reactions?

Answer 24

• in microchannels, where there are low Re, we can expect mixing to take place only by diffusive processes
• if the diffusional distance is too far then this will be a slow process
• limitations on reactions processes that are limited by diffusion.

Question 25

What is the inter diffusion zone?

Answer 25

• In a T sensor, shown below different fluids come together at a t junction to flow down a channel.
• Interdiffusion zone spreads diffusively with the square root of time (breaks down near edges of reactor)
• Microscopy reveals the 3D nature of the interface spreading
• To achieve complete mixing the retention time of the channel must be at least equal to the diffusion time

Question 26

Describe the H-Filter

Answer 26

• Microfluidic device that allows the separation of molecular analytes from fluids containing interfering particles without the need for cleaning or replacement

• Designed to accommodate for small fluid volumes although can be scaled up
• Exploits the different diffusion constants and how this affects the spreading across a channel
• The length is selected so that large (red) particles don’t have time to diffuse across the channel, but the small (blue) particles of interest do have a chance to

Question 27

What is dispersive mixing?

Answer 27

• reduction in size of the cohesive minor component

Question 28

What is distributive mixing?

Answer 28

reorganisation of the minor component through the matrix

Question 29

What are the two types of micro mixer?

Answer 29

Passive and Active

Question 30

What is a passive micromixer?

Answer 30

• do not require external energy
• the mixing process relies entirely on diffusion or chaotic advection.
• They are categorised by the arrangement of their mixed phases

Question 31

Give named examples of passive micromixers

Answer 31

• parallel lamination
• serial lamination
• injection
• chaotic advection
• droplet

Question 32

What is an active micro mixer?

Answer 32

• Active micromixers use the disturbance generated by an external field for the mixing process.
• Thus, active mixers can be categorized by the types of external disturbance effects such as pressure, temperature, electrohydrodynamics, dielectrophoretics, electrokinetics, magnetohydrodynamics and acoustics.
• With external fields and the corresponding integrated components, the structures of active micromixers are often complicated and require complex fabrication processes.
• Furthermore, external power sources are needed for the operation of active micromixers.
• active mixers are harder to integrate into microfluidic devices
• expensive

Question 33

Give examples of active micro mixers

Answer 33

• serial segmentation
• pressure disturbance along mixing channel
• integrated microstirrer into the mixing channel
• electrohydrodynamic disturbance

Question 34

What is parallel lamination? What are its effects?

Answer 34

• Streams of fluid are split, made smaller and interdigitalised
• Length scales for diffusion are shorter and mixing is much faster

Question 35

Describe the KI, KIO3, NaAc, HCl microractor examples

Answer 35

• Two different outcomes depending on mixing efficiency.
• For the fast mixing process (in the microreactor) the acid is quick neutralised and consumed, thus it cannot react in order for the second slower reaction to occur and no I2 is formed.
• For the batch system, mixing is less efficient and not all the acid is consumed by the NaAc and is thus able to react with KI and KIO3 to form Iodine
• Thus, local deviations of the average concentration due to an imperfect mixing are detected by iodine formation and can be used to characterize the mixing process.
• The iodine concentration is measured by UV-vis absorption. The higher the absorption due to iodine formation, the worse is the quality of mixing.

Question 36

What is hydrodynamic focussing?

Answer 36

• Reduction of the fluid layer down to nanometres in diameter, increasing the speed of diffusive mixing
• Reduced size of planar jets and rapid mixing transverse to the jet
• The stream width of the sample from the central inlet is controlled by the flow rates of the samples from the side inlets

Question 37

What is chaotic advection?

Answer 37

• The process by which flow tracers develop into complex fractals by obliquely oriented ridges on a wall
• The width of the channel is w and principal wavevector of the ridges is q. The red and green lines represent trajectories in the flow.
• Optical micrograph showing a top view of a red stream and a green stream flowing on either side of a clear stream in a channel

Question 38

What is a staggered herringbone mixture?

Answer 38

• two regions of asymmetric herringbones which switch direction after each section of the mixer, with streams churning into one another
• After around 15 cycles full mixing is achieved. Weak separation of streamlines occurs in the rectangular grooves even at low Re.
• an example of chaotic advection

Question 39

How can droplets show chaotic advection?

Answer 39

• by the bakers method of stretching and folding, reducing the scale of diffusion.
• By reorienting droplets using bends in the channels, moving and stationary walls mixing within a droplet can occur.

Question 40

What is electroosmotic flow?

Answer 40

Electroosmotic Flow (EOF) describes the movement of ions through a solute under the control of an applied potential.

Question 41

How does Chemical Electrophoresis work?

Answer 41

The capillary columns consist of silica with silanol groups exposed on the inner surface. The exposed silanol groups are ionized above pH 3, therefore creating a negatively charged inner capillary surface.

• Cations present in ionic solutions will migrate toward the negatively charged wall forming an electric double layer.
• Generation of an electrical potential across the column now causes cations to migrate towards the cathode.
• Electroosmotic flow results as the solvated cations clustered at the capillary walls drag the bulk solution in tow towards the cathode.

Question 42

How does flow rate cause a change in pressure within a micro channel?

Answer 42

• the velocity of the different streams within laminar flow are determined by their distance from the centre of the tube, where the liquid is moving fastest, and the boundary of the liquid and wall where the particles are essentially stationary.
• This causes a drop in pressure as defined by poiseuille law for the pipe

Question 43

What is Taylor- Aris Dispersion?

Answer 43

• Happens in a Poiseuille flow, describes the dispersion of anisolated bolus along a channel.
  • a): initial bolus of fluid. b): fluid after elapsed time t in the absence of diffusion. c): fluid after elapsed time t with a finite axial diffusion. A slight broadening of the distribution is seen, but the effect of diffusion is minor as compared to the dispersive effect of the flow. d): fluid after elapsed time t with finite radial/transverse diffusion. The radial diffusion reduces the dispersive effects by causing the scalar to sample both slow- and fast-moving regions of the flow.

Question 44

How can Taylor - Aris dispersion be used in a micro reactor?

Answer 44

Leads to circular micromixers with peristaltic pump that drives flow around a ring, mixing two spots a and b in a matter of hundreds of milliseconds.

Question 45

What is a syringe pump?

Answer 45

• gives consistent flow at lower flow rates.
• Operated by a screw mechanism that drives the base and plunger of the syringe.
• Can’t be efficiently refilled and can only handle lower pressures, typically < 20 bar

Question 46

What is a single piston?

Answer 46

• The piston expels liquid through a one-way valve (check valve).
• The pumping rate is usually adjusted by controlling the distance the piston retracts, thus limiting the amount of liquid pushed out by each stroke, or by the cam rotating speed.
• The main disadvantage of this type of pump is sinusoidal pressure pulsations which lead to the necessity of using pulse dampers.
• Higher pressures up to 350 bar.

Question 47

What is a dual piston system?

Answer 47

• A more efficient way to provide a constant and almost pulse free flow is the use of dual-headed reciprocating pumps.
• Both pump chambers are driven by the same motor through a common eccentric cam; this common drive allows one piston to pump while the other is refilling.
• As a result, the two flow- profiles overlap each other significantly reducing the pulsation downstream of the pump;
• Since the acceleration/deceleration profile is somewhat non-linear, the more efficient types of these pumps use eccentricity- shaped cams to obtain the best overlapping of the pressure curves and to obtain smooth flow.

Question 48

What is a gas pumping system?

Answer 48

• pressure of the gas in the chamber forces the liquid out of a sample vial and into the tubing

Question 49

What is a peristaltic pump?

Answer 49

• a type of positive displacement pump used for pumping a variety of fluids.
• The fluid is contained within a flexible tube fitted inside a circular pump casing
• A rotor with a number of “rollers”, attached to the external circumference of the rotor compresses the flexible tube.
• As the rotor turns, the part of the tube under compression is pinched closed forcing the fluid to be pumped to move through the tube.
• Typically, there will be two or more rollers occluding the tube, trapping between them a body of fluid.
• Peristaltic pumps may run continuously.

Question 50

Give the names of 6 types of microfabrication technique

Answer 50

• Dry etching
• Combined lithograph and moulding
• Wet chemical etching
• Mechanical milling based on precision engineering
• Microelectrical discharge machining
• Laser ablation

Question 51

What are the advantages of glass devices?

Answer 51

Good for reusability, chemical compatibility, optical transparency and thermal characteristics are important

Question 52

What is the most common material used for glass etching? Why?

Answer 52

Optical glass (B270) is most commonly used due to easy processing, chemical compatibility, optical transparency and thermal characteristic

Question 53

Why is Quartz good for micro fabrication?

Answer 53

• The combination of low thermal expansion, high chemical resistance and excellent optical properties, such as homogeneity and high UV transmission
• excellent material for microfluidic flow cells, microreactors and devices for UV polymerization.
• Quartz glass is also more resistant to thermal shock than other types of glass.

Question 54

Why are some devices made with borosilicate glass?

Answer 54

• it can resist extreme temperatures as well as many strong chemicals including acids, saline solutions, chlorine, oxidizing and corrosive chemicals.
• Common applications include high precision lenses, laboratory equipment and pharmaceutical containers.

Question 55

How are etching profiles generated?

Answer 55

• isotropic etching
• dry anisotropic etching
• wet anisotropic etching

Question 56

What are the steps in glass etching?

Answer 56

1) Apply a chrome and photoresist layer to glass
- expose a photoresist to UV light through a mask
- Develop photoresist
- etch chrome layer
- wet etch glass with HF acid solution
- Remove chrome resist and clean
- Microdrill flid holes and dice chip
- thermally bond glass cover layer

Question 57

Why are injection mould devices useful?

Answer 57

• Used in applications such as clinical diagnostics where devices need to be disposable, polymers are more suitable than glass devices
• Durable lightweight and flexible

Question 58

What are the steps in the production of glass masters and polymer prototype devices?

Answer 58

• Glass with chrome and photoresist layer
• etching solution selectively removes glass
• drill holes though glass to define fluid ports
• mount pins in glass master
• injection mould using glass master as one side of the mould tool
• plastic chip is removed prom mould tool, bonded to cover slip and milled to size

Question 59

What are PMDS devices? What are their advantages?

Answer 59

• polydimethylsiloxane
• can be easily deformed, and moulded with high accuracy
• Durable,clear, chemically inert and non toxic
• Cheap and easy to replicate

Question 60

How are PMDS devices synthesised?

Answer 60

• photoresist is spin-coated on a silicon wafer.
• A mask is placed in contact with the layer of photoresist.
• The photoresist is illuminated with ultraviolet (UV) light through the mask.
• An organic solvent dissolves and removes photoresist that is not crosslinked. - The master consists of a silicon wafer with features of photoresist in bas-relief.
• PDMS is poured on the master, cured thermally and peeled away.
• The resulting layer of PDMS has microstructures embossed in its surface.