Lecture 8 Flashcards

Question

What is microfluidics? (Important)

Answer 1

Microfluidics is the science and technology of the manipulation of fluids at the micrometre scale. One reason why it is relevant is that it allows one to miniaturize fluid-based processes. Another reason is that the fluids behave differently at micrometre length scales than in macroscopic equivalents; the flow has no turbulence (laminar) and this means that only mixing which occurs has to happen through diffusion. Microfluidics allows you to minimize the use of expensive reagents by downscaling the reaction size, while also allowing you to perform combinational chemistry, where hundreds or thousands of different reactions can be performed in parallel thus drastically reducing the time invested into screening processes, where hundreds or thousands of compounds need to be evaluated for some property like, for example, their toxicity. It also raises the idea that instead of producing chemicals in conventional macro reactors, it would maybe be better to make the same chemicals in similar quantities, but in plants built from millions of integrated microreactors. PDMS is the most useful platform material for microfluidics. (This is depicted in digital notes from the slides)

Answer 2

A microfluidic device is often fabricated by rapid prototyping: This process was developed after the realization that the feature size needed for microfluidics could be achieved by using an inkjet printer. The circuit is thus designed on a computer and printed in actual size on a transparency. such transparency will work as a mask for the fabrication of the master, against which the PDMS is going to be cast. In order to do so, a glass substrate is coated with a layer of SU-8 (a commercial photoresist widely used in laboratory settings). The printed transparency is then put on top of the SU-8 layer and the whole film is irradiated with light; the areas that are exposed to the light will polymerize and will resist further washing, which will instead remove material from all unexposed areas. Now that your glass substrate is patterned with SU-8 features resembling the channels you need to add glass capillaries where you want to have inlets for fluids; you can then fabricate the PDMS mold out of this construct, peel off the master, and remove the capillaries the PDMS mold can now be welded against another PDMS flat surface to yield the microfluidic device; this delicate step is performed by treating the two opposite faces of PDMS with oxygen plasma. this will create silanols on each face which will then react upon contact of the two surfaces, leading to the cold-welding of the two PDMS pieces. (This is depicted in digital notes from the slides)

Answer 3

Rapid prototyping - building solid forms by delivery of material and/or energy to predefined regions in space. The enormous practical advantage of rapid prototyping is that it does not require the use of sophisticated facilities and that the whole process can be carried out in a few hours instead of a few days and for a minute fraction of the cost. This has obvious positive repercussions when it comes to the often trail-and-error protocols that sometimes need to be used in nanochemistry research.

Answer 4

The case we will consider is about the flow-focussing device that is shown in the central diagram in Figure 4.3. three different fluids are focused towards a narrow opening, Liquid A is a polymerizable monomer, while Liquid B is a carrier liquid, The flow-focussing device produces exceedingly monodisperse droplets from liquid A; such droplets are carried away by the carrier liquid and their reciprocal distance ensures they will not coalesce. In this specific case, the droplet of liquid A can be polymerized within the channels by two diffrent mechanisms: i) IUv irradiation or by temp. ii) initiator. If the droplet is small enough to fit into the channel it would form a spherical colloid, if the channel is shallower than the diameter of the droplet then you would obtain disks. If the channel is instead both shallower and thinner than the diameter of the droplet, you can form rod-like colloids. (If confused about the job that PDMS plays I think it is the nuzzle itself, look at the description written for Figure 4.3)

Answer 5

Besides the control of shape, you are also fairly free to choose composition. You are not limited to polymers in the formation of monodisperse colloids by microfluidics. you can use any material that can be consolidated in conditions compatible with a microfluidic device. Another twist to this methodology was obtained by changing the liquid A for a gas (See Figure 4.3). In this case, you can obtain the same monodisperse colloids but, instead of being made of a liquid monomer, they are made of gas.

Answer 6

When we talk about controlling the properties of PDMS we refer to the study of millimeter-scale self assembly. Not the nanometer scale. Now we ask are self this is nanochemi why are we talking in terms in of nanoscale??? This is becasue what self-assembly can teach us at the millimeter scale will be useful aslo at the nanoscale and vice versa!

Answer 7

They are forces that are caused by a meniscus, for example at the interface between a particle and another particle or a surface

Answer 8

Background (just read and try to understand): The study involves fabricating hexagonal PDMS disks with a hydrophobic bottom surface and a hydrophilic top surface (achieved by oxygen plasma treatment). These disks are placed at the interface between a denser, hydrophobic liquid (perfluorodecalin, PFD) and water. The disks stay at the interface due to the contrasting surface properties—hydrophilic tops contact water, and hydrophobic bottoms avoid it. The hexagonal shape was chosen to introduce directional interactions between disks. If all sides were hydrophobic, PFD would bend the interface to wet the sides, while fully hydrophilic sides would cause water to bend the interface downward. When opposite sides have opposite wettability, the disk tilts, as water and PFD pull on the hydrophilic and hydrophobic edges, respectively.

Answer 9

Now the actual self-assembly: If two disks with all-hydrophobic or all-hydrophilic sides would get close to each other, the best way for the system to minimize surface energy is to get the building blocks attached. In this way, from four menisci you are down to two and the sides of the disks have similar metallicities which make them compatible with each other. The force exerted on the building blocks in such cases will thus be attractive allowing for self-assembly. In the case in which oppositely wettable disks are approached, a repulsive force will ensure an opposite energetic consideration would be valid. So, here you have a controllable system subject to two opposing forces of similar kind, which govern the interaction between the building blocks. PDMS hexagonal disks with different combinations of hydrophilic/hydrophobic edges were placed at the interface and then gently stirred in order to allow the system to find the equilibrium conditions (to self-assemble) faster (Figure 4.4). In the first case, only one edge of the disk was hydrophobic so that the disks autonomously formed stable couples, in the case in which two neighbouring edges were hydrophobic, triplets were observed. You can see in Figure 4.4 how other edge functionalizations gave rise to different self-assembled architectures. The important message of this example is that we can control self-assembly if we can control the characteristics and directionality of the interaction between building blocks.

Answer 10

The interface between three phases (Liquid, solid and vapor) is defined by an angle, called the contact angle, which is determined by the interface energies γ between each couple of phases. Each surface-energy component will act on the line of contact between the three phases trying to find the best compromise in terms of minimum free energy. The surface energy can be expressed as a vector representing the force that derives from it and it measures the amount of energy necessary to increase the surface area by a surface unit. Such surface energy will generate a force called surface tension, which will have a direction tangential to the surface and be directed in a way to reduce the interface. If you see Figure 4.5 the γsv vector has the magnitude of the solid-vapour surface energy and its direction indicates how the force will try to increase the spreading of the droplet on the surface, thus decreasing the area of the solid surface exposed to the vapour phase. Oppositely, the γsl vector representing the surface energy of the solid-liquid interface is pointing towards the bulk of the droplet trying to decrease the contact between the solid substrate and the liquid. All these opposing forces will have to equilibrate and this can be expressed mathematically with the century-old Young's equation: γsv = γslcos(a)

Answer 11

In the first case (in ref with fig 4.5), of superhydrophilicity, the surface energy of the solid-liquid interface will be smaller than γsv - ylv; the contact angle will approach zero, representing the maximum spreading of the liquid on the solid surface. This can be achieved by decreasing the surface energy of the solid-vapor interface or by increasing the surface energy of the solid-vapor interface. In the case of PDMS, increasing the hydrophilicity can be accomplished by exposing the surface to oxygen plasma this will produce some temporary silanols on the surface, increasing the surface charge, and decreasing the PDMS-water interface energy. The reason for such a decrease in energy is due to the good solvation that water will provide to the exposed silanol.

Answer 12

when γsl is greater than γvl the contact angle will be higher than 90, thus leading to the condition defined as hydrophobic.

Answer 13

In order to further increase the contact angle increasing the γsl does not seem to be enough, at least in general. Even using perfluorinated molecules grafted to teh surface of PDMS hardly brings you to contact angles higher than 120. What is necessary is the introduction of specific nanoscale roughness in the surface. The droplet will transition from a so-called wentzel state, in which the droplet uniformly wets all the recesses of the surface, to a Cassie-Baxter state, in which the droplet sits on top of the protrusions without wetting the recesses in the surface, In such conditions, when high γsl is combined with the right roughness of the surface the contact angle approaching 180 can be approached.

Answer 14

Superhydrophilic - really water-loving; when a liquid spreads as much as possible on a surface, with a contact angle approaching 0. Superhydrophobic - really water-fearing; where a droplet sits wholly on top of a surface, with a contact angle approaching 180

Answer 15

The leaf of the lotus plant; it repels water almost completely (i.e., exhibits superhydrophobicity)

Answer 16

1. Antifouling 2. drug delivery 3. Anti-fogging

Answer 17

One of the most common problems when using materials in biology is that proteins tend to stick to almost any material via non-specific adsorption, thus complicating the design and the reliability of any device. It was thus very important to discover that protein would not stick as much if the surface was covered with a specific polymer called poly(ethylene oxide) PEO.

Answer 18

Surfaces of PEO are called 'stealth surfaces' for their ability to go mostly unnoticed in vivo for prolonged periods of time. This is of fundamental importance in the design of the surfaces of drug delivery vehicles, since you want them to go unnoticed till they hit the region of interest. Without stealth surfaces, most drug delivery systems would be immediately attacked by proteins and sequestered.

Answer 19

PEO grafted into PDMS surfaces in water solution will extend into the solvent in the attempt to maximize its interaction with it: The oxygen atoms within the backbone form hydrogen bonds with water, greatly increasing the affinity of the polymer for this solvent. A PEO monolayer will be compressed by approaching proteins (Figure 4.6), the response to such compression can be compared to the response of a mattress. The PEO monolayer will resist compressions due to two parallel factors. On the one hand, the compression of the chain will reduce their conformational freedom thus reducing their entropy; a reduction of entropy, with no significant change in enthalpy leads to an increase in free energy, and thus to a non-spontanous process. This will lead the chains to spring back into their extended conformation. PEO is special in this regard in the sense that its ether groups are very flexible - particularly when swollen with water - and thus confer large entropy to the molecule. The other force involved in this process is osmosis. When a part of a solution gets artificially concentrated in a certain solute, the outlying liquid will develop osmotic pressure to enter that part of the solution in order to equilibrate the concentration across the whole system. This is what happens to our PEO monolayer when it gets compressed by a protein. The local concentration of PEO will increase and this will generate an osmotic pressure from the rest of the solution. The resulting flow of water molecules in the spaces of the PEO monolayer will lead to its springing back and repelling the protein. This process is dependent on the surface denisty of PEO, since the tighter the PEO is packed, the more unlikely it will be for the proteins to find an unprotected spot of surface to stick to

Answer 20

Found in digital notes