Flow Chemistry

Question 1

Lab on a chip devices

Answer 1

LoC devices use microchannels to manipulate and process fluids. The flow in the micro channels are characteristically different to that in bulk/batch synthesis.

They are made from glass, Si, stainless steel, polymeric chip devices, and even paper.

Question 2

Benefits of micro fluidics

Answer 2

Portable
Minimal waste
Safer amounts of toxic chemicals
Integration of analytical technology
Ease of use/ automation
Faster processes
Improved yield
Reduced cost
Shorter mixing times
Reduced reagent and solvent consumption
Micro reactors do not interact significantly on a molecular level, leaving the kinetics unchanged from what is well-known

Question 3

Scale of microfluidics

Answer 3

Between micro and nano scale (nano fluidics are different because the size of the channels starts to influence kinetics etc).

Question 4

Disadvantages of batch reactors

Answer 4

Scaling up of batch reactors can be quite difficult
Storage and use of large amounts of chemicals inherently risky
Exothermic reactions can produce dangerous amounts of heat
How to mix things changes with reactor design, impacting yield and purity
Greater space demands for scaling up produce their own hazards

Question 5

Advantages of microreactors

Answer 5

Mass and heat transport are different- mixing CAN be made more efficient, and due to the greater surface area to volume ratio, heat transport is more efficient.
To scale up the reaction you simply need to increase the number of units rather than changing any mechanics of the process.
Reaction screening can be fast due to running many reactions in parallel.
Space time yield is increased.

Question 6

Space time yield

Answer 6

Quantity of product per volume of reactor per unit time. Convert ml/time to g/dm3/h.

For flow reactors this tends to be much higher than for batch reactors.

Question 7

Fluid flow

Answer 7

Newtonian fluids deform continuously under applied shear stress. A linear velocity gradient forms with flow between a stationary wall and a moving plate applying shear stress.
The more viscous a fluid the more force is required to move the liquid at a velocity.

Question 8

Turbulent vs. laminar flow

Answer 8

Turbulent flow is characterized by chaotic behavior of the fluid, where adjacent regions in the liquid flow in different directions with different velocities. This tends to be how we mix things in normal chemistry.

Laminar flow is smooth, well-characterized streams of liquid where the region is characterized by flow in the same direction with (roughly) the same velocity. Easier to understand. What we tend to find in microfluidics.

Question 9

Laminar flow in microchannels

Answer 9

Microchannels have such well- defined laminar flow that when many branches of fluid are joined together in a single tube, the different fluids, even if they have very similar viscosities, flow smoothly side by side. This limits mixing, which can be unfortunate.
The reynolds number that characterizes laminar flow is Re<1000
Mixing in laminar flow is diffusion-controlled.

Question 10

Reynolds Number

Answer 10

Re= (rho v D_h)/(eta) = inertial forces / viscous forces
rho= density of fluid (kg/m^3)
v= velocity of flowing fluid (m/s)
D_h = hydraulic diameter (diameter of a round tube, 4*Area/perimeter for anything else)
Low Re is when viscosity dominates over inertial forces. This is when either the viscosity is very high or diameter of the pipe is small.
Re <2000= laminar
Re >4000= turbulent

Question 11

Diffusion and mixing

Answer 11

When laminar flow dominates a system, the main mechanism for mixing is through diffusion. The tie required for diffusion is t_d ~ x^2/2D where D= RT/(6 pi r_o eta N_A) where r_o is the radius of the molecule, NA is avagadros number and the rest is obvious.
Diffusion mixing is very fast on a small scale (micro m) but VERY long on a larger scale (cm)

Question 12

Lab on a chip devices

Answer 12

LoC devices use microchannels to manipulate and process fluids. The flow in the micro channels are characteristically different to that in bulk/batch synthesis.

They are made from glass, Si, stainless steel, polymeric chip devices, and even paper.

Question 13

Benefits of micro fluidics

Answer 13

Portable
Minimal waste
Safer amounts of toxic chemicals
Integration of analytical technology
Ease of use/ automation
Faster processes
Improved yield
Reduced cost
Shorter mixing times
Reduced reagent and solvent consumption
Micro reactors do not interact significantly on a molecular level, leaving the kinetics unchanged from what is well-known

Question 14

Scale of microfluidics

Answer 14

Between micro and nano scale (nano fluidics are different because the size of the channels starts to influence kinetics etc).

Question 15

Disadvantages of batch reactors

Answer 15

Scaling up of batch reactors can be quite difficult
Storage and use of large amounts of chemicals inherently risky
Exothermic reactions can produce dangerous amounts of heat
How to mix things changes with reactor design, impacting yield and purity
Greater space demands for scaling up produce their own hazards

Question 16

Advantages of microreactors

Answer 16

Mass and heat transport are different- mixing CAN be made more efficient, and due to the greater surface area to volume ratio, heat transport is more efficient.
To scale up the reaction you simply need to increase the number of units rather than changing any mechanics of the process.
Reaction screening can be fast due to running many reactions in parallel.
Space time yield is increased.

Question 17

Space time yield

Answer 17

Quantity of product per volume of reactor per unit time. Convert ml/time to g/dm3/h.

For flow reactors this tends to be much higher than for batch reactors.

Question 18

Fluid flow

Answer 18

Newtonian fluids deform continuously under applied shear stress. A linear velocity gradient forms with flow between a stationary wall and a moving plate applying shear stress.
The more viscous a fluid the more force is required to move the liquid at a velocity.

Question 19

Turbulent vs. laminar flow

Answer 19

Turbulent flow is characterized by chaotic behavior of the fluid, where adjacent regions in the liquid flow in different directions with different velocities. This tends to be how we mix things in normal chemistry.

Laminar flow is smooth, well-characterized streams of liquid where the region is characterized by flow in the same direction with (roughly) the same velocity. Easier to understand. What we tend to find in microfluidics.

Question 20

Laminar flow in microchannels

Answer 20

Microchannels have such well- defined laminar flow that when many branches of fluid are joined together in a single tube, the different fluids, even if they have very similar viscosities, flow smoothly side by side. This limits mixing, which can be unfortunate.
The reynolds number that characterizes laminar flow is Re<1000
Mixing in laminar flow is diffusion-controlled.

Question 21

Reynolds Number

Answer 21

Re= (rho v D_h)/(eta) = inertial forces / viscous forces
rho= density of fluid (kg/m^3)
v= velocity of flowing fluid (m/s)
D_h = hydraulic diameter (diameter of a round tube, 4*Area/perimeter for anything else)
Low Re is when viscosity dominates over inertial forces. This is when either the viscosity is very high or diameter of the pipe is small.
Re <2000= laminar
Re >4000= turbulent

Question 22

Diffusion and mixing

Answer 22

When laminar flow dominates a system, the main mechanism for mixing is through diffusion. The time required for diffusion is t_d ~ x^2/2D where D= RT/(6 pi r_o eta N_A) where r_o is the radius of the molecule, NA is avagadros number and the rest is obvious.
Diffusion mixing is very fast on a small scale (micro m) but VERY long on a larger scale (cm).
Due to distance varying to the squared power with relation to time, at the small scale of microfluidics diffusion becomes very important.

Question 23

Residence Time

Answer 23

The amount of time fluids spend in a channel. 
t_r = V/Q=L/U (s)
V= channel volume (m^3)
Q= volumetric flow rate (m^3/s)
L=channel length (m)
U= av linear velocity (m/s)

Question 24

Interdiffusion Zone

Answer 24

The length of time/distance it takes for two solutions coming together at a T piece to fully mix. This is when t_d = t_r. This may be physically seen using fluorescent dye and a quencher, and measuring the time/distance that it takes for the fluorescence to be quenched.
t_d = x^2/2D
t_r = V/Q = L/U

Question 25

The H Filter

Answer 25

Microfluidic device that allows the continuous extraction of particles in solution from other particles without the need for a membrane. It looks like a sideways H, and uses the fact that smaller particles diffuse faster than large ones, and hence uses a residence time in which the desireable particles fully mix between the streams, but the larger, unwanted particles do not. Then there is another T piece that separates them.
Downside: can only be used for Newtonian fluids, or fluids with relatively low viscosity.

Question 26

Dispersion and Distribution Mixing

Answer 26

Improve mixing in systems where viscous forces dominate
Dispersion is reduction in size of the cohesive minor component.
Distribution is the reorganization of the minor component through the matrix.
When the blocks are very small, diffusive mixing becomes very fast.

Question 27

Active vs Passive Micromixers

Answer 27

Active micromixers use the disturbance generated by an external field for mixing. This makes them very difficult to incorporate into the microfluidic systems. These include pressure, temperature, electrohydrodynamics, dielectrophoretics, electrokinetsics, magnetohydrodynamics, and acoustics.
Passive micromixers involve no external energy input, and generally involve different ways flows can be organized, or changing the structure of the tubes. This include injection, lamination, chaotic advocation, and droplet.

Question 28

Parallel lamination micromixing

Answer 28

Rather than a simple T piece flowing into a single pipe to give two parallel streams, more complex systems can be involved where you split the stream into more channels and feed them into a tube to have 4+ thin streams. This is called interdigitisation.
This changes the diffusion time to t_d= x^2/n^2 2D where n is the number of laminae pairs.

Question 29

Chaotic Advection

Answer 29

Physical mixing induced by obstacles on wall or in channel, or a zig-zag shaped channel rather than a straight channel. There can also be grooves on the base of the channel, either slanted linear, like / / / / , or > > > herringbone pattern.
The slanted linear pattern gives rise to a twisting flow along the axis of the flow.
Staggered herringbone mixer swirls the flow along the axis of the flow and perpendicular to it.

Question 30

Hydrodynamic focusing

Answer 30

rather than a T piece, a t piece is used- from the branches of the t a less viscous fluid flows and interacts with a more viscous fluid flowing down the center. This narrows the fluid layer down to nm in diameter, which leads to mixing in the µs scale. The stream width of the central flow is controlled by manipulating the flow rates of the side inlets.

Question 31

Chaotic advection in droplets.

Answer 31

“stretching” and “folding”, much like how bakers knead bread, also leads to enhanced mixing in droplets by producing narrow sheets. To induce this effect in droplets in µfluidic devices, droplets have to go through square zig-zag paths eg |_| , around the corners there is the folding effect, and the progress along the linear part of the path is the stretching part.

Question 32

Electroosmotic Flow

Answer 32

Inducing flow using an electric field. Ions in a solute respond to an applied potential, and as they move, they induce movement of the solution. add more