Droplet pinned

Kimura, M., Misner, M.J. et al. (2003) Long-range ordering of diblock copolymers induced by droplet pinning. Langmuir, 19(23), 9910-9913. 

Several strategies have been employed in order to reduce the number or defect found in the microstruc-tured BC thin fdms [143], These include the use of electric fields [152], graphoepitaxy to cylindrical thin films [153], photoinduced alignment [154], droplet pinning [155], directional crystallization [156] or simply temperature or solvent annealing [157]. 

This first experiment consists in making a bubble with air saturated with heptane. The gaseous mixture is brought to the center of a surfactant droplet pinned between two horizontal glass plates in this way, a "flat bubble" is inflated (radius of such a circular bubble 4 cm, distance between the plates 1.5 mm, and mean thickness of the film 0.56 mm). Then the concentration of heptane inside the bubble is measured as a function of time gas samples of 100 pi (i.e. about 1% of the initial volume of the bubble) are periodically taken with a syringe, and analysed by gas chromatography. 

Figure 8 shows H2SO4 droplets on mica at very low humidity (<5%). The contact line aronnd the drops is smooth and circnlar, revealing that no pinning has occurred [49]. Although the area between drops is flat, it does not correspond to clean mica but to a liquid film covering it of a few monolayers thickness. This is deduced from the hysteresis in the force versns distance experiments, where the tip is brought into contact with the surface and then pulled off. 

The rate of the droplet motion was limited by the rate of the viscous flow of the liquid because electrochemical reaction took place much faster (in less than 1 s) than the deformation of the droplet ( 10 s). Pinning of the droplet sometimes occurred at a defect on the surface. In most cases, the pinned droplet could be moved again by... 

Assume that you are now ready to create your first array on a previously selected substrate. How many elements (spots) do you wish to print This number will determine what kind of pin you will need and while it appears to be a rather fundamental question to ask, it may not be simple to answer. As noted, spot density is directly related to spot size and pitch (Figure 4.25). The pitch will determine how many spots can actually be printed on a slide (Figure 4.26). The pin will deliver a specific droplet volume that will spread to a certain diameter largely based upon the tip s diameter and the print buffer used (Figure 4.27). The larger the spot diameter, the fewer the spots that can be printed (Figure 4.28). 

Stamping Time — This is the amount of time fhaf fhe pin resides on the substrate surface. The more time spent on the surface, the greater ink volume deposited on the substrate. The greater volume has a tendency to cause spreading of the fluid, thereby increasing spot diameter. However, other factors such as the contact angle of the substrate and the capillary hydrostatic head also influence the size and spread of droplets. 

Z Acceleration — Certain printing mechanisms involve striking the surface with the quill pin to dispense a droplet onto a substrate. Control of the acceleration rate can be useful in ejecting the droplet without crashing the pin into the substrate (sometimes called overdriving Figure 4.30). Not all arrayers have this feature. 

Opposing surface tension force is the pinning force. Essentially, pinning forces promoted by surface features tend to fix the contact line of the droplet and drive the DNA toward the contact line (solvent perimeter). Solutes such as salts and probes spread to the perimeter by convection as the droplet evaporates (Figure 4.36). Uneven evaporation leads to differences in spot uniformity. 

Figure 14.17 Printing mechanism based on surface tension and adhesion. The sample is first loaded into a sample channel by capillary action. The horizontal terminus of the tip ensures that a thin layer of a sample solution is accumulated at the end of the tip. The loaded tip contacts the printing surface depositing a droplet between the substrate and the pin. After the contact time (0.05 s), the pin is retracted and the droplet is held by strong adhesive forces to the substrate.

In the case of contact printing the surface is contacted for probe deposition. Various types of pin tools have been developed to facilitate reproducible droplet release within a volume from 50 pi-100 nl (Fig. 2). The typical feature size resulting from this procedure is in a range from 100-300 im. The main drawback of this application is the lack of durability owing to the tapping force and possible damage to the surface coating. 

The evaporation of a droplet on a rough surface behaved similarly except that the second regime was not entered the diameter stayed constant due to the pinning of the droplet s contact line by the rough surface. 

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