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 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]
References:
- Thomas Edison, Wikiquote
- Dong Yeol Shin, Yoon Jae Moon, Jun Young Kim;, and Kyung-Tae Kang, "Measurement of inkjet droplet size based on Fraunhofer diffraction,"Rev. Sci. Instrum., vol. 94, no. 10 (October, 2023), Article no. 105106, https://doi.org/10.1063/5.0159472.
- Francesco Nalin, Maria Celeste Tirelli, Piotr Garstecki, Witold Postek, and Marco Costantini, "Tuna-step: tunable parallelized step emulsification for the generation of droplets with dynamic volume control to 3D print functionally graded porous materials, "Lab Chip, vol. 24, no. 1 (January 7, 2024), pp. 113-126, https://doi.org/10.1039/D3LC00658A. This is an open access article with a PDF file here.
- Drop the drop, Press Release of the Institute of Physical Chemistry of the Polish Academy of Sciences, February 23, 2024.
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