Automatic Single Droplet Generator with Control over Droplet Size and Detachment Frequency - MDPI

colloids
 and interfaces

Article
Automatic Single Droplet Generator with Control
over Droplet Size and Detachment Frequency
Dorota Gawel and Jan Zawala *
 Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, 00-901 Krakow, Poland
 * Correspondence: nczawala@cyfronet.pl; Tel.: +48-6395-133
 



 Received: 7 August 2019; Accepted: 20 August 2019; Published: 22 August 2019 


 Abstract: This paper presents a quite simple, fully automatized single droplet generator, which can be
 an alternative for more expensive and complicated microfluidic devices. The simple generation nozzle
 connected to the pressure cells and cheap peristaltic pumps, synchronized via developed software
 with simple GUI (graphical user interface) implemented into the Raspberry Pi microcomputer allows
 precise control over the single droplet diameter and detachment frequency. The generator allows
 the formation of droplets of quite wide range of diameters without the need of orifice diameter
 replacements. Free control over time available for adsorption of surface active-substances over
 the surface of immobilized droplet, before its detachment from the orifice, is an advantage of the
 developed device.

 Keywords: dispersion; droplet; velocity; generator

1. Introduction
 The behavior of droplets dispersed in the liquid phase has great importance in many diverse
industrial and technological applications, such as extraction of liquid-liquid mixtures or emulsions
formation, waste-water treatment or hydrometallurgy. Efficiency of many of these applications can
be investigated and partially predicted on the basis of results of fundamental studies related to the
hydrodynamics of single droplets in a liquid phase. In laboratory practice, for proper experimental
investigations of rising droplet hydrodynamics as well as collisions between droplets and formation of
single emulsion films under dynamic conditions, control over the formation of a single droplet with
the desired diameter and detachment frequency is of crucial importance.
 Nowadays, microfluidic devices, for which fabrication and modification has been extensively
studied and described [1–6], are the most widely used for single droplet generation in a liquid
phase. In such devices two immiscible liquids are pumped through microchannels, where one phase is
dispersed in the other. For the sake of efficient device design and control on the size and monodispersity
of generated microdroplets, many researchers have investigated various microfluidic methods of droplet
generation both experimentally and numerically [1–16]. According to the mechanism of dispersion,
microfluidics devices can be divided into three groups: devices where a droplet is formed as a result of
(i) breakup in co-flowing streams [4,5,7,10]; (ii) breakup in cross-flowing streams (T-junction) [9–12];
and (iii) breakup in elongational strained flows (flow-focusing) [14–16]. The undoubted advantage of
microfluidic devices (called microchips) is their ability to generate single droplets with micrometer
size. Furthermore, such devices are usually small and handy. However, the size range of generated
droplets depends on the microchannels diameter. Moreover, the microchannels diameter affects a
single droplet velocity inside the channel as a result of droplet-wall interactions [1,5,9,17–19]. Droplet
size strictly depends on microchannel geometry (for example channel length, nozzle to orifice distance
etc.) [5], which should be precisely controlled for sufficient repeatability of the results. In addition,
the microchips have limited mechanical durability and defined shape, which limits its applicability

Colloids Interfaces 2019, 3, 57; doi:10.3390/colloids3030057 www.mdpi.com/journal/colloids

Colloids Interfaces 2019, 3, 57 2 of 9
 Colloids Interfaces 2019, 3, x FOR PEER REVIEW 2 of 9

infor
 laboratory
 sufficientexperiments
 repeatabilityconcerning the motion
 of the results. of a single
 In addition, droplet from
 the microchips thelimited
 have very beginning
 mechanical of its
formation and acceleration in liquid column.
 durability and defined shape, which limits its applicability in laboratory experiments concerning the
 The of
 motion paper presents
 a single dropletanfrom
 alternative
 the verymethodology
 beginning of its offormation
 single droplet generation in
 and acceleration inliquid
 the liquid
 column.phase.
We have developed a quite simple, fully automatic single droplet generator,
 The paper presents an alternative methodology of single droplet generation in the liquid phase. allowing precise control
over
 We the
 havesingle droplet
 developed diameter
 a quite simple,andfullydetachment
 automatic single frequency. Our automatized
 droplet generator, allowing generator allows
 precise control
formation
 over the of droplets
 single in quite
 droplet wide range
 diameter of diametersfrequency.
 and detachment with very Our goodautomatized
 precision without
 generatorneedallows
 of orifice
 formation
diameter of droplets in
 replacements. quite wide
 Moreover, therange
 time of diameters
 available for with very good
 adsorption precision
 of surface without need of
 active-substances over
 orifice diameter replacements. Moreover, the time available for adsorption
the immobilized droplet, before its detachment from the orifice, can be precisely and easily controlled, of surface active-
assubstances
 well as the over the immobilized
 degree of adsorption droplet, beforeat
 coverage itsliquid/liquid
 detachment from the orifice,
 interface. Thiscan be precisely
 feature and
 is important
 easily controlled,
especially as well as the
 for investigations degree ofofadsorption
 of stability coverage
 single emulsion at liquid/liquid
 films formed under interface.
 dynamic Thisconditions
 feature
 is important
(kinetics especially
 of droplet for investigations of stability of single emulsion films formed under dynamic
 coalescence).
 conditions (kinetics of droplet coalescence).
2. Materials and Methods
 2. Materials and Methods
2.1. Experimental Set-Up
 2.1. Experimental Set-Up
 All the experiments were carried out using a single-droplet generator, which was fully developed
and builtAll in
 theour
 experiments
 laboratory. were
 Thecarried out using
 experiments werea single-droplet
 aimed to testgenerator, whichprecision
 the generator was fully anddeveloped
 potential
 and built in our laboratory. The experiments were aimed to test the
in a single droplet generation of desired size. The schematic illustration of the main parts generator precision and potential
 of the
 in a single
device droplet in
 is presented generation
 Figure 1a.of It
 desired
 consists size.
 of The schematic
 (i) two illustration
 identical, of the main
 programmable, lowparts of theperistaltic
 pressure device
 is presented
pumps (DC 12 inV,
 Figure
 flow1(a).
 rateIt20-60
 consists of (i) two
 mL/min identical,
 with siliconeprogrammable,
 tubes of 2 mm low pressure
 inner peristaltic
 diameter) pumps to
 connected
 (DC 12 V, flow rate 20-60 mL/min with silicone tubes of 2 mm inner diameter) connected to (ii) two
(ii) two glass pressure cells (height 70 mm and width 35 mm, round cross-section) and controlled
 glass pressure cells (height 70 mm and width 35 mm, round cross-section) and controlled via dual
via dual stepper motor driver (L298N, Induino ST1112); (iii) three pressure sensors (GY-68 BMP180,
 stepper motor driver (L298N, Induino ST1112); (iii) three pressure sensors (GY-68 BMP180, I2C); (iv)
I2C); (iv) two polytetrafluoroethylene (PTFE) two-way valves, automatized using the servomechanism
 two polytetrafluoroethylene (PTFE) two-way valves, automatized using the servomechanism (Giant
(Giant Servo HD-1235MG); (v) two pressure stabilizers; (vi) glass tube with side-tube (cross-Section
 Servo HD-1235MG); (v) two pressure stabilizers; (vi) glass tube with side-tube (cross-Section 8 mm,)
8 mm,) andsteel
 and thin thin needle
 steel needle
 (outer(outer diameter
 diameter 0.3 mm) 0.3 sealed
 mm) sealed concentrically,
 concentrically, referred
 referred furtherfurther
 in the in theastext
 text
asgeneration
 generationnozzle,
 nozzle, as well as (vii) Raspberry Pi 3 microcomputer for control and synchronization of all of
 as well as (vii) Raspberry Pi 3 microcomputer for control and synchronization
allelectronic
 electronicparts of thethe
 parts of generator
 generator by means
 by means of developed
 of developed software
 software with user-friendly
 with user-friendly GUI (graphical
 GUI (graphical user
user interface),
 interface), presented
 presented in more
 in more detaildetail in Appendix
 in Appendix A. A.

 (a) (b)
 Figure1.1.Schemes
 Figure Schemesof:of: (a)
 (a) Single
 Single droplet
 droplet generator;
 generator;(b)
 (b)Experimental
 Experimentalset-up used
 set-up to to
 used study thethe
 study single
 single
 droplet size and its motion parameters.
 droplet size and its motion parameters.

 Thefollowing
 The followingprocedure
 procedurewas
 wasapplied
 appliedtotogenerate
 generateaasingle
 singleoil
 oildroplet
 dropletininwater.
 water.First,
 First,the
 thetwo-way
 two-
 way valves of the pressure cells (Cell 1 and Cell 2) were closed and the cells were filled with
valves of the pressure cells (Cell 1 and Cell 2) were closed and the cells were filled with corresponding,

Colloids Interfaces 2019, 3, 57 3 of 9

immiscible liquids directly from the beakers. Next, the cells were pressurized independently by two
peristaltic pumps according to the precisely adjusted (using software) pressure values (P1 and P2 ).
These pressures were in fact overpressures, normalized to zero at the beginning of the procedure,
according to the ambient pressure measured by the third, independent sensor (P3 ). Tubes used for cells
pressurization were made of silicone (inner diameter 2 mm). Chemically resistant PEEK (polyether
ether ketone) tubes (inner diameter 0.7 mm) were used for oil transport from the cell, through the
PTFE valves to the generation nozzle. In the case of water, a PTFE tube with inner diameter 2 mm was
applied. When the desired pressure values were reached independently in each cell, two-way valves
of the both cells were open to fill the tubes with respective liquids, which were pumped thanks to the
overpressure inside the cells. Correct, adjusted overpressure in the cells were continuously controlled
and supplied, if necessary, by the peristaltic pumps. The procedure of system filling was monitored
visually by observation of the air bubbles appearing inside the generation nozzle at the needle tip and
continued till all air was pushed out and the first oil droplet appeared and detached from the needle
tip, which indicated that the needle is filled with oil and both PTFE valves can be closed. To generate
the single droplet the PTFE valve of the oil cell (Cell 2) was opened and then immediately closed.
Time between valve opening and closure could be precisely adjusted. This was a very important
parameter, determining the size of the droplet formed at the needle tip. After formation under the
impulse of the oil phase flow of adjusted amplitude (pressure), the droplet was immobilized at the
needle, as capillary force related to the needle/droplet attachment area exceeded buoyancy. To detach
the droplet, water co-flow was applied. The droplet detachment was forced using water flow impulse
generated from the water cell (Cell 1) to the side tube of generation nozzle. Magnitude of this impulse
(and hence water flow rate and, consequently, shear force exerted at the droplet surface) depended on
P1 value. In practice, the water impulse was adjusted carefully to be as small as possible, to avoid
significant droplet deformation during too violent forced detachment. After each oil or water impulse,
the peristaltic pumps supplied the pressure to the desired (adjusted) level. The pressure inside the cell
could be freely modified, either up or down, by peristaltic pumps operating in pumping (forward
pump rotation) or withdrawing (backward pump rotation) modes.
 To measure the droplet size and rising velocity, the generation nozzle was connected to the square
glass column, according to the scheme presented in the Figure 1b. Each water impulse generated to
detach a droplet caused an increase in the water level inside the liquid column, which modified the
hydrostatic pressure. Therefore, a small side silicone tube was connected to the bottom of the column
to keep the water level constant and controllable. Pictures of a single droplet detaching from the needle
tip and rising in water were recorded using high-speed camera (with relatively low frequency applied,
equal to 100 Hz). To extract the droplet motion parameters (rising velocity, shape deformation) and
its size, well-known procedures, described in details elsewhere [20], were utilized. In this procedure
the Python (programing language) script for automatic and fast image analysis was used. Terminal
velocity was calculated as an average from five independent runs (five different droplets) for data
collected at the distance when a droplet velocity was constant. Equivalent droplet diameter was
calculated assuming an ellipsoidal droplet shape, as:
 q
 3
 d= dh 2 dv (1)

where dh and dv is horizontal and vertical diameter.

2.2. Materials
 Dodecane oil was used for testing the generator potential and precision. We used contaminated
dodecane available in our laboratory, with water/oil interfacial tension equal to 33 mN/m. We decided
to do so, because, for preliminary tests of the generator capability, the system purity was an insignificant
parameter. Milli-Q water with surface tension 72.4 mN/m was used in all the experiments, which were
carried out at a room temperature equal to 22 ± 1 ◦ C.

Colloids Interfaces 2019, 3, 57 4 of 9

 Colloids Interfaces 2019, 3, x FOR PEER REVIEW 4 of 9
3. Results and Discussion
 Colloids Interfaces 2019, 3, x FOR PEER REVIEW 4 of 9
 3. Results and Discussion
 3. A sequence
 Results and of photos showing moment of a single droplet detachment from the needle tip are
 Discussion
presented A sequence
 in Figureof2.photos showing
 The first photo moment
 presents of thea elongated
 single droplet detachment
 droplet shape, which from theis aneedle tip are of
 consequence
 A sequence
 presented in Figure of photos
 2. The showing
 first photo moment
 presents ofthe
 a single droplet
 elongated detachment
 droplet shape, from the
 which is a needle tip are
 consequence
the shear flow of water (generated impulse) parallel to the needle. The detached droplet starts to rise
 presented inflow
 Figure 2. The(generated
 first photo presents parallel the elongated droplet shape, which is a consequence
as of the shear
 a result of buoyancy. of water
 Please note thatimpulse)
 due to the carefully to the needle.magnitude
 adjusted The detached droplet
 of the water starts to
 shearing
 of the shear
 rise as the
 a result flow of water
 of buoyancy. (generated
 Please impulse)
 notedisturbed, parallel
 that due towhich to the
 the carefully needle. The
 adjusted detached droplet starts to
impulse, droplet shape is only slightly is positive featuremagnitude
 of the method, of theespecially
 water
 rise as
 shearing a result of
 impulse,ofthe buoyancy. Please note that due to the carefully adjusted magnitude of the water
when the influence thedroplet
 adsorptionshapelayer’s
 is onlyexistence
 slightly at disturbed,
 the oil/water whichinterface
 is positive feature of the
 is investigated. It is
 shearingespecially
 method, impulse, when the droplet
 the shape of
 influence is the
 only slightly disturbed,
 adsorption layer’s whichatisthe
 existence positive
 oil/water feature of the
 interface
worth mentioning here that such methodology of single droplet generation allows precise and is free
 method, especially
 investigated. It isof whenmentioning
 worth the influence here ofthat
 the adsorption
 such methodologylayer’s existence
 of single at the oil/water
 droplet generation interface
 allows is
control over
 investigated. time a
 It iscontroldroplet
 worth over residue
 mentioning at the needle tip. Therefore, this method is a great tool for
 precise and free time of here that such
 a droplet methodology
 residue at the needle of single droplet generation
 tip. Therefore, this method allows
 is a
investigation
 precise and of influence
 control of time available for adsorption of
 thesurface-active substances thisatmethod
 liquid/liquid
 great tool forfree
 investigation over time of a droplet
 of influence residue atfor
 of time available needle tip.ofTherefore,
 adsorption surface-active substances is a
interface
 great on
 tool droplet
 for motion
 investigation parameters,
 of influence as
 of well
 time as stability
 available for of single
 adsorption thin offilms formed,
 surface-active when the oil
 substances
 at liquid/liquid interface on droplet motion parameters, as well as stability of single thin films formed,
droplet reaches theinterface
 at liquid/liquid upper liquid surface
 on droplet (liquid/air, liquid/liquid asorstability
 compound interfaces). Informed,
 addition,
 when the oil droplet reaches the motion
 upper parameters,
 liquid surface as well(liquid/air, of single
 liquid/liquid thinorfilms
 compound
thiswhen
 method the canoil be easily reaches
 droplet adjustedthe to produce
 upper water surface
 liquid drops falling in oil as
 (liquid/air, continuous or
 liquid/liquid phase for study
 compound
 interfaces). In addition, this method can be easily adjusted to produce water drops falling in oil as
inversed
 continuousemulsion
 interfaces). phase systems.
 In addition, thisinversed
 for study method emulsion
 can be easily adjusted to produce water drops falling in oil as
 systems.
 continuous phase for study inversed emulsion systems.

 Figure
 Figure 2. 2. Sequence
 Sequence ofof photos
 photos illustratingsingle
 illustrating singledroplet
 dropletdetachment
 detachmentfrom
 fromthe
 thesteel
 steelneedle
 needletip
 tipunder
 underthe
 Figure
 the shear 2.flow
 Sequence
 of of photos
 generated illustrating
 water impulse. single droplet detachment from the steel needle tip under
 shear flow of generated water impulse.
 the shear flow of generated water impulse.
 Figure3a3apresents
 Figure presents photos
 photos of
 ofsingle
 singledroplets
 droplets of of
 various sizes,
 various afterafter
 sizes, automatic imageimage
 automatic analysis was
 analysis
 Figure
 applied. Each3acolumn
 presents photos different
 presents of single subsequent
 droplets of various
 droplets sizes, after automatic
 positions. As seen, image analysis
 elaborated was
 software
was applied. Each column presents different subsequent droplets positions. As seen, elaborated
 applied.
 determinesEach column
 the positionpresents different
 of the geometric subsequent
 center of droplets positions.
 the droplet As
 as well as seen, elaborated
 dh and
 aswell software
software determines
 determines of
 the position
 thea shape
 position
 of the geometric
 of the geometric
 center of the droplet as ddvand
 droplet as well as dh and hdv values,
 values, for
 dv values,
 calculations deformation ratio (dhcenter
 \dv) and of dthe
 values. for
forcalculations
 calculationsofofa ashape
 shapedeformation
 deformation ratio
 ratio (dh(d
 \dh \d v ) and
 v) and
 d values.
 d values.

 (a) (b)
 (a) (b)
 Figure 3. (a) Sequences of photos presenting rising droplet of different sizes after automatic analysis
 Figure
 Figure
 in 3. (a)
 3. (a)
 developed Sequences
 Sequences of
 softwareof(time photos
 photos presenting
 presenting
 interval rising
 rising
 between dropletofofdroplet
 droplet
 subsequent different
 different sizesafter
 sizes
 positionsafter automatic
 forautomatic analysis
 analysis
 each sequence is in
 different); (b) Single droplet diameter as a function of overpressure inside the oil cell (servo-valveis
 in developed
 developed softwaresoftware
 (time (time
 intervalinterval
 betweenbetween
 subsequentsubsequent
 droplet droplet positions
 positions for each for each
 sequence sequence
 is different);
 different);
 (b)opening (b)0.3
 Single droplet
 time Single droplet
 diameter
 s). asdiameter
 a functionas aoffunction of overpressure
 overpressure inside theinside the(servo-valve
 oil cell oil cell (servo-valve
 opening
 opening
 time 0.3 s). time 0.3 s).

Colloids Interfaces 2019, 3, 57 5 of 9

 The pressure characteristics of the generator under experimental conditions, i.e., dependence of
droplet diameter as a function of overpressure inside the oil cell (Cell 2), are presented in Figure 3b.
The points represent experimentally obtained data, while the solid line is a fitted polynomial.
Using fitted equation, all parameters of the generator can be easily adjusted to produce a droplet of
the desired size. No error bars for diameter values are shown in the Figure for clarity. In practice,
the relative standard deviation (RSD) was less than 4%.
 It has to be underlined that, obviously, the pressure characteristics presented in Figure 3b are valid
only under certain experimental conditions, and will be different for different physical parameters of
the dispersed (oil) phase, various values of interfacial tension and hydrostatic pressures (height of the
liquid column). Such characteristics however, can be easily determined and catalogue for different
substances as well as experimental set-up geometry, and used as a guideline (kind of “calibration
curves”) during experiments performed under reproduced conditions. This is a big advantage of the
method, allowing a simple way of repeating the experiments using “calibrated” systems, without
blind tests.
 Figure 4 presents terminal velocities of a single droplet (Figure 4a) and corresponding Reynolds
numbers (Figure 4b) as a function of the droplet size. Terminal velocity of every object rising (or falling)
in a liquid phase is reached when the buoyant force and drag force of a continuous medium are equal.
Buoyant (FB ) and drag (FD ) forces can be expressed as:

 FB = Vb ∆ρg (2)

 FD = 0.5ACD ρc u2 (3)

where Vb is an object volume, ∆ρ is difference between density of dispersed (ρd ) and continuous (ρc )
phases, g is gravitational acceleration, A is object projected area, CD is drag coefficient and u is terminal
velocity. In our case, due to the fact that the oil was contaminated with surface-active substances, rising
droplet surface should be immobile (as a result of motion induced surface tension gradients) and can
be treated as a surface of a rigid sphere. Drag coefficient of a rigid sphere can be calculated from the
empirical correlation given by Schiller–Naumann [21]:

 24  
 CD = 1 + 0.15Re0.687 (4)
 Re
where Re is the Reynolds number, which, for the droplet of diameter d rising in continuous medium of
viscosity µc can be expressed as:
 dρc u
 Re = (5)
 µc
After rearrangement of Equations (1)–(4), assuming that the Vb is equal to 1/6πd3 and A is equal to
1/4πd2 , the theoretical droplet terminal velocity can be calculated as:
 1/3
  4 ρd − ρc
 
 
 u =  gReµc
  (6)
 3 ρc 2 CD 

 Theoretical dependences calculated from Equations (3)–(5), according to the physical parameters
of dispersed (oil) and continuous (water) phases given in Table 1, are shown in the Figure 4, as solid
lines. As seen, very good agreement between experimental data and theoretical predictions was
obtained. It indicates that indeed, the droplet surface was fully no-slip (immobilized). Moreover,
the presented results confirm the reliability of the droplet generation method and generator itself. It is
seen that using our automatic generator quite wide range of droplet size can be obtained. In practice,
by using a steel needle of an outer diameter of 0.3 mm, droplets with size ranging between 600 µm to
3.5 mm can be easily produced. In addition, droplet size can be changed smoothly and continuously.
The RSD of determined velocity values shown in Figure 4a was less than 5%.

be treated as a surface of a rigid sphere. Drag coefficient of a rigid sphere can be calculated from the
 empirical correlation given by Schiller–Naumann [21]:
 24 .
 = 1 + 0.15 (4)
 
 where Re is the Reynolds number, which, for the droplet of diameter d rising in continuous
Colloids Interfaces 2019, 3, 57 6 of 9
 medium of viscosity μc can be expressed as:

 Colloids Interfaces 2019, 3, x FOR PEER REVIEW 6 of 9

 = (5)
 
 After rearrangement of Equations (1–4), assuming that the Vb is equal to 1/6πd3 and A is equal to
 1/4πd2, the theoretical droplet terminal velocity can be calculated as:
 ⁄
 4 | − |
 = (6)
 3 
 Theoretical dependences calculated from Equations (3–5), according to the physical parameters
 of dispersed (oil) and continuous (water) phases given in Table 1, are shown in the Figure 4, as solid
 lines. As seen, very good agreement between experimental data and theoretical predictions was
 obtained. It indicates that indeed, the droplet surface was fully no-slip (immobilized). Moreover, the
 (a) (b)and generator itself. It is
 presented results confirm the reliability of the droplet generation method
 seen that using our automatic generator quite wide range of droplet size can be obtained. In practice,
 Figure 4. Dependence of (a) single droplet terminal velocity; (b) Reynolds numbers on droplet diameter
 by using a steel needle of an outer diameter of 0.3 mm, droplets with size ranging between 600 μm
 (points–experimentally
 Figure 4. Dependencedetermined
 of (a) single values, line—theoretical
 droplet terminal predictions,
 velocity; (b) according
 Reynolds numbers onto Equations
 droplet (3)–(5)).
 diameter
 to 3.5 mm can be easily produced. In addition, droplet size can be changed smoothly and
 (points–experimentally determined values, line—theoretical predictions, according to Equations (3–5)).
 continuously. The RSDTableof 1.
 determined velocity values
 Physical parameters of theshown in Figureand
 oil (dodecane) 4a water.
 was less than 5%.

 Table Title 1
 1. Physical ρ, kg/m
 parameters µ, and
 of the3 oil (dodecane) Pa·swater.
 water
 Title 1 1000
 ρ, kg/m 3 μ, 10−3
 1 ×Pa⋅s
 dodecane 750 - −3
 water 1000 1 × 10
 dodecane 750 -
 As was mentioned above, the time of the opening of the PTFE valve of oil cell (Cell 2) could be
 As was Therefore,
freely adjusted. mentioned theabove,
 sizethe
 of time of the opening
 the formed droplet of the PTFE
 could valve of oil
 be controlled notcell (Cell
 only via2)control
 could beof the
overpressure in the cell, but it also depended on the valve opening time, when the overpressureofwas
 freely adjusted. Therefore, the size of the formed droplet could be controlled not only via control
 the overpressure in the cell, but it also depended on the valve opening time, when the overpressure
constant. Figure 5 presents measured droplet diameter values for five independent runs (detached
 was constant. Figure 5 presents measured droplet diameter values for five independent runs
droplet) for constant oil cell overpressure (15 kPa) and six different values of time of the oil cell
 (detached droplet) for constant oil cell overpressure (15 kPa) and six different values of time of the
(Cell 2) PTFE valve opening. As seen, indeed this method of droplet size control works very well.
 oil cell (Cell 2) PTFE valve opening. As seen, indeed this method of droplet size control works very
In practice,
 well. In the minimum
 practice, time of open/close
 the minimum cycle ofcycle
 time of open/close the valves was equal
 of the valves to 0.3tos, 0.3
 was equal so much smaller
 s, so much
droplet diameters
 smaller droplet could be obtained
 diameters using this
 could be obtained approach.
 using In addition,
 this approach. In addition,from
 fromthetheresults
 results presented
 presented in
Figure 5, the precision of our generator can be judged. Values of the droplet diameters
 in Figure 5, the precision of our generator can be judged. Values of the droplet diameters formed formedin in
five independent runs (under five subsequent oil and water impulses) are practically
 five independent runs (under five subsequent oil and water impulses) are practically identical, which identical, which
indicates thatthat
 indicates thethe
 developed
 developedgenerator
 generatorisisable
 able to
 to produce monodisperse
 produce monodisperse droplets
 droplets of repeatable
 of repeatable sizes.
 sizes.
This This
 is anisextremely
 an extremely important
 important feature
 feature in respect
 in respect to fundamental
 to the the fundamental studies
 studies on emulsion
 on emulsion stability,
 stability, single
 singlefilms
emulsion emulsion filmsand
 drainage drainage and coalescence
 coalescence phenomenon, phenomenon, where
 where liquid liquid
 film size film
 is ofsize is ofimportance.
 crucial crucial
 importance.

 Figure
 Figure 5. Diameter
 5. Diameter of single,
 of single, subsequentdroplets
 subsequent droplets generated
 generated with
 withconstant
 constantoverpressure butbut
 overpressure different
 different
 opening times of the servo-valve of the oil cell (Cell 2).
 opening times of the servo-valve of the oil cell (Cell 2).

Colloids Interfaces 2019, 3, 57 7 of 9

4. Conclusions
 We have developed quite a simple, fully automatized single droplet generator, which can be an
alternative for more expensive and complicated microfluidic devices. The simple generation nozzle
connected to the pressure cells and cheap peristaltic pumps, synchronized via developed software
with simple GUI allows precise control over the single droplet diameter and detachment frequency.
The generator allows the formation of droplets on demand, in quite a wide range of diameters with
very good precision and accuracy without need of orifice diameter replacements. Free control over
time available for adsorption of surface active-substances over the immobilized droplet (degree of
adsorption coverage at liquid/liquid interface), before its detachment from the orifice, is a great
advantage of the developed device. Obviously, any kind of liquids can be used as oil and aqueous
phases, i.e., various kind of oils and water solutions of various substances, including surfactants can be
examined. The geometry of the experimental set-up can be easily modified allowing investigations
on the dynamics of falling water (or water solution) droplets in the oil phase, mimicking dynamic
phenomena in the inversed emulsion systems.

Author Contributions: Conceptualization, D.G. and J.Z.; methodology, J.Z.; software, J.Z. and D.G.; validation,
D.G. and J.Z.; investigation, D.G.; data curation, J.Z. and D.G.; writing—original draft preparation, D.G. and J.Z.;
writing—review and editing, J.Z. and D.G.; visualization, J.Z. and D.G.; supervision, J.Z.; project administration,
J.Z.; funding acquisition, J.Z.
Funding: This research was funded by National Science Centre (NCN), grant number 2017/25/B/ST8/01247.
Conflicts of Interest: The authors declare no conflict of interest.

Appendix A
 Figure A1 presents a screenshot of the GUI of the developed software, used for controlling of
the droplet formation procedure and its final size after detachment. It was developed using the
Tkinter Python module. The GUI window is divided into six panels. The first is used only for pumps
testing and emergency stops. The second can be used for immediate pressure cells decompression.
The third controls the manual procedure of the droplet generation, in which direct overpressure
values for each cell can be adjusted and the oil and water flow impulses can be manually induced,
when desired. The fourth panel allows it to start the automatic procedure of droplet generation. Again,
cells overpressure can be adjusted here together with precise value of the time interval between oil and
water flow impulses (in the case presented in Figure A1 equal to 40 s), which correlates with the droplet
detachment interval. The algorithm was developed in such a way that the oil impulse is generated after
10 s from the automatic procedure initiation and the water impulses after the adjusted time interval.
After the water impulse, all procedures start automatically from the beginning. This approach is useful,
when good statistics are desired, especially in experiments aimed to measure single emulsion film
stability (droplet lifetime at upper liquid interface in the column). Currently, automatic detection of the
droplet lifetime at the water/oil interface is under development. The fifth panel controls the opening
times of the servo PTFE valves. The last one is used for manual opening and closure of the PTFE valves
and is usually used for system cleaning and filling.

Colloids Interfaces 2019, 3, 57 8 of 9
Colloids Interfaces 2019, 3, x FOR PEER REVIEW 8 of 9

 Figure A1.
 Figure A1. Graphical user interface
 Graphical user interface(GUI)
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 thesoftware
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 control over
 over thethe parameters
 parameters of
 of single
 single droplet
 droplet generation.
 generation.

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