Tikalon Blog by Dev Gualtieri

Droplets

May 20, 2024

As famed inventor and entrepreneur, Thomas Edison (1847-1931) so famously stated, "Genius is one percent inspiration, ninety-nine percent perspiration."[1] This was one of the principle lessons I learned when doing experiments in support of my thesis in graduate school. My research was on the heats of formation of intermetallic compounds of iron and aluminum. These were measured using a high temperature differential scanning calorimeter designed and built in collaboration with another graduate student.

Differential scanning calorimeter data for the reaction of iron and aluminum powder to form FeAl3

Differential scanning calorimeter data for the reaction of iron and aluminum powders to form FeAl₃.

The reaction initiates at the melting point of aluminum, and the heat of formation of this intermetallic compound is related to the area under the curve.

In the early 1970s, when these data were taken, the data analysis that gave the calorie value was done by an APL program on an IBM 370 mainframe computer using keyboard input of data.

(Author's data. Click for larger image.)

Since the calorimeter could only accommodate quarter gram sized specimens, an analytical balance was needed to weigh the metal powders. Not only that, but this weighing operation and subsequent mixing needed to be done in a nitrogen-purged glovebox to prevent oxidation of the fine metal powders. This was a difficult operation, but the precision of the analytical balance assured a proper product after all that work. Such measurement precision was not available to scientists in the past.

From antiquity, small quantities of solid materials were measured as grains, the weight of a single grain of barley or wheat. Eventually, the grain as a unit was quantified as 64.79891 milligrams. Drops were used as a measure of small liquid quantities, but the volume of a drop depends on the mechanics of the dropper, and the viscosity, density, and surface tension of the liquid. Pharmacists have defined a drop to be exactly 0.05 milliliters to give 20 drops per milliliter of medication.

As inkjet printing has demonstrated, very small liquid droplets can be made. An inkjet droplet is about 100 micrometers in diameter, but droplets a quarter that size with 8 picoliter volume can be made.[2] Inkjet technology has been applied to things other than printing. Inkjets have been used to create not only conductive interconnects between transistors, but the transistors themselves.

Inkjets are designed to produce droplets of a fixed volume, and this limits their application in cases for which a variable droplet size would be more appropriate. Chemists from the Institute of Physical Chemistry, the Polish Academy of Sciences (Warsaw, Poland) and the Broad Institute of MIT and Harvard (Cambridge, Massachusetts) have recently published research on a technique for production of droplets varied in diameter over three orders of magnitude.[3-4] Their objective was a means to control the porosity and composition of soft materials such as hydrogels.[4]

Production of varied hexadecane droplet sizes in a fibrinogen solution.

(Fig. 4a of ref. 4.[4] Click for larger image.)

Material porosity is important in such applications as catalysis, adsorption of gases, and absorption of light.[4] Some of the usual methods to create porosity are foaming, spray drying, and emulsification.[4] A limitation of these methods is the inability to control pore size and shape.[4] The research team of the present study combined the existing technology of step emulsification with a method to change the spray nozzle geometry by means of a pressure controlled flexible membrane.[3-4] That, combined with changes in the spray nozzle height, controlled droplet diameter in oil-in-water and water-in-oil emulsions over three orders of magnitude.[4] They call their process, tuna-step, short for tunable step[3-4]

To increase efficiency, the researchers arranged 14 such nozzles in parallel, thereby increasing the throughput by a factor of about fourteen.[3-4] They attached their device on a custom 3-D printing stage to produce unique structures in a hydrogel by extruding an oil-in-water emulsion in agarose gel, followed by polymerization.[3-4]

Droplets

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