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solution systems. II. The process of attachment
Dynamic Contact Angles in Mercury/Carbon Tetrachloride/Solution Systems II. The Processof Attachment M.
C. WILKINSON
~ AND
T. A. ELLIOTT
2
Trent Polytechnic,Nottingham, England Received August 31, 1972; accepted November 5, 1973 The process of attachment of carbon tetrachloride drops at mercury/solution of surfaceactive agent interfaces has been studied by means of high-speed cine photography. At smooth interfaces drops do not apparantly make single point attachments, but attach over relatively large basal areas. The contact angle on attachment depends on the difference in the works of adhesion of the two liquids for the mercury surface. In the present system the initial stages of spreading were mainly controlled by the mercury/ solution and mercury/carbon tetrachloride interfacial tensions. The contact angle during this stage was almost of 90°, and remained at this value as the base area increased. The drop maintained its ellipsoidal, preattachment shape until the base area equalled the maximum cross-sectional area, when drop distortion occurred and the contact angle (measured through the aqueous phase) exceeded 90° . The rapid processes of attachment and initial spreading (0.1 sec) and the manner in which the contact angle changed with time, have been shown to be very reproducible. The passage of the drop from an ellipsoidalshape to that of the cap of a sphere was marked by the induction of oscillations in the vertical plane, which produced vibrations in the spreading plane. For a given system, the effect of induced oscillations on the dynamic contact angle depended on the magnitude of the interfacial driving force causing spreading. INTRODUCTION Several workers have observed the a t t a c h m e n t and d e t a c h m e n t of drops and bubbles at interfaces (1-5). Schulman and L e j a (1) studied d y n a m i c contact angles in the detachm e n t of air bubbles from m e r c u r y / s o l u t i o n interfaces, where escape times were in the region of several minutes. T h e y also obtained high speed p h o t o g r a p h s (4000 frames per second) of the processes of impact, and adhesion, of gas bubbles at a solid surface. However, no measurements of r a p i d contact angle vs. time variations were obtained, and no a t t e m p t was m a d e to describe the processes. x Present address: Chemical Defense Establishment, Porton Down, Salisbury, Wiltshire, England. 2 Deceased.
T h e s t u d y of d y n a m i c contact angles b y Elliott and co-workers (3-5) did n o t yield a n y information as to the mechanism of drop (or bubble) a t t a c h m e n t due to the slow filming speeds employed (53 frames per second). M o s t of the process was completed in less t h a n 0.05 sec. I n the present system a high-speed cine camera has enables the process of a t t a c h m e n t of carbon tetrachloride drops at m e r c u r y / water interfaces to be studied. Although a great deal of information was obtained at relatively slow filming (400 frames per second), speeds up to 5000 f.p.s, h a d to be used to record the initial processes occurring on attachment. The unavoidable difficulty with the highspeed cine p h o t o g r a p h y of a t t a c h m e n t processes is the fact t h a t no control on the length
209 Copyright ~) 1974 by Academic Press, Inc. All rights of reproduction in any form reserved.
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WILKINSON AND ELLIOTT
of the induction period can be achieved. Processes with zero or very small induction periods can be recorded readily (5) but in these systems the drop (or bubble) is not in a steady state when attachment occurs. In the present system the induction periods were sufficiently long so that all oscillations due to drop impact had ceased before attachment occurred. Thus, the processes causing attachment were entirely interfacial ones. EXPERIMENTAL The apparatus, procedure and purification of materials have been described (6). RESULTS The attachment of drops of carbon tetrachloride to mercury/water, and mercury/solution interfaces was extremely rapid. Velocities of spreading at different time stages have been plotted in Fig. 1. The examples chosen are typical of results observed for drops spreading in different concentrations of the ionic and nonionic surface-active agents. The highest velocities of about 12,000 to 14,000 ram~rain were calculated assuming the drops made point contacts at the mercury surface on attachment. In all cases, the velocity of spreading fell rapidly during the first 0.05 sec; after this, the rate of fall was much slower. The manner in which the contact angle changed during the first 0.20 sec for drops in distilled water, and in a range of concentrations of different surface-active agents, is illustrated in Figs. 2-5. The corresponding variations in the drop height and base diameter, during the first 0.10 sec, are illustrated in Figs. 6 and 7. The values of the drop heights recorded in these figures were always measured to the uppermost part of the drop (see Plates VII and VIII). With filming speeds as high as 400 f.p.s, the first contact angles recorded were invariably greater than 50 ° . The contact angles in most cases were in excess of 90 ° in less than 0.01 sec. After these rapid rises in the contact angle very much slower increases were recorded, e.g.,
90-105 ° in 0.20 sec for distilled water (Fig. 2). During the initial periods of spreading, contact angles were taken as the average of the measured left- and right-hand angles (i.e., 0L and 0R). It is apparent from the results of Fig. 8 that there was very little difference in these values after the first 0.02 sec. It is surprising that during the rapid process of attachment very good reproducibility of the contact angle changes was recorded. This point is illustrated by the three repeat runs in distilled water (Fig. 2). The reproducibility was almost as good for drops in solutions of surfaceactive agents; the relative positions of the curves (Figs. 3-5) did not alter (6). As long as drop oscillations had ceased before attachment, variations in the magnitude of the induction periods had no effect on reproducibility. The difference in spreading behavior for drops which attached while still in their oscillatory mode due to impact, was marked. This type of impact spreading has been considered by Ford and Furmidge (7) and was found to be the exception in the present system, rather than the rule.
Induced Drop Vibrations on Attachment Violent drop oscillations were set up when the drop attached at the mercury/solution interface. These vibrations are reflected in the contact angle vs. time plots of Figs. 2-5, and the drop base diameter and height vs. time plots of Figs. 6 and 7. There was a large peak in the contact angle at ~0.05 sec, and smaller ones at multiples of this, i.e., 0.10, 0.15 sec; these were recorded at the same times for drops in different concentrations and types of surface-active agent. The primary vibrations were rapidly damped, and successive ones, which were more predominant in the case of drops in distilled water and sodium dodecyl sulfate solutions, were of much smaller magnitude. The characteristic peak in the contact angle at about 0.02 sec, which in some cases was more of a plateau region, was probably caused by the "braking" action of the drop/solution
Journal of Colloid and Interface Science, Vol. 48, No. 2, August 1974
DYNAMIC CONTACT ANGLES. II
211
14000
I0000
600C
200C
t IO(DC
A
I 60C
20C i
I
r6c
-
I000
-2000
-300C
0
0 " 0I 5
0.110
0.115
0 12 0
z 0.25
0 " 3+ 0
0.35;
TIME (seconds)
FIG. 1. Rapidity of attachment at the interface. Curve
Conch. (moles/liter)
Surface-active a g e n t
A B C
0.0040 0.0020 --
Sodium dodecyl sulfate Sodium dodecyl sulfate Distilled water
interfacial tension as the contact angle exceeded 90 °. Up to 90 °, this interfacial force acted to aid spreading, but in excess of 90 ° it opposed spreading (as a cosine function). This phenomenon has been recorded by Elliott and Leese (3) and Elliott and Ford (5). Elliott and Morgan (4) however, did not note this since in their system the contact angle did not reach 90 ° . If the characteristic frequency of natural oscillations of the drop can be approximated by the Rayleigh equation (8), which describes the eigenvibrations of spherical drops, then
the frequency, v, of the drop, should be given by: t, .
.
. . F++T ,, L praJ 27r
[1]
where T is the interfacial tension ; p the density (for this case p = Pwater "~- Pcarbon tetrachloride 2.6); and r the radius of the drop. Inserting into Eq. [-11 values for a carbon tetrachloride drop in distilled water yields a frequency of 21.6 Hz, i.e., a time interval between vibrations of 0.047 sec, a value that is in good agreement with ~ 0 . 0 5 sec recorded experimentally.
do2*rnal of Colloid and Interface Science, Vol. 48, No. 2, A u g u s t 1976
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WILKINSON AND ELLIOT]7
'20I
ot
I00
~,
~
9O
I-
u
"70
o.o~
o.',o
0/,5
0.20
TIME (seconds)
FIo. 2. Reproducibility of dynamic contact angles (distilled water). A, B, and C are repeat runs.
Calculations using the data for drops in surface-active agent solutions yielded values between 20 and 22 Hz, which are in good agreement with the observation that the characteristic period of vibrations were similar for all the systems (Fig. 2-5). This conclusion might
be expected, since the drops all fell from the same dropping tip, and, therefore, from the theory of dropping (9) : ~ , a r 3. From Eq. [-17 it would be expected that all
180
Iso
~
B
§ 6o 3O
0
0
0.05
0"I0
O.IS
O' 0
TIME ( s e c o n d s ) FIO.
3. Dynamic contact angles during initial stages of spreading (solutions of sodium dodecyl sulfate). Curve
A B C D E Journal of Colloid and Interface Science, Vol. 48, No. 2, August 1974
Collon.
(moles/liter) 0.0040 0.0030 0.0020 0.0010 0.0001
DYNAMIC CONTACT ANGLES. II
213
18o A C
~3--
150
'~
12o
z <
90
~
__
0-----
D
b 60 u 30
o
0.6 s
o~ls
o.'~o
0.20
TIME {seconds]
FIG. 4, Dynamic contact angles during initial stages of spreading (solutions of N-decylamine hydrochloride). Curve
Conch. (moles/liter)
A B C D
0.0100 0.0200 0.0400 0.0010
iBO
150 ~
~
,
~ x ' ~ C
J2C
II..--
D
90
z
oo 30
0
0
Of'OS
O-IlO TiME
0'115
0"20
{ se(:on ds}
FIG. 5. Dynamic contact angles during initial stages of spreading (solutions of nonionic). CuI've A B C D
Conch.
(moles/liter) 6.48 3.89 1.30 3.24
X X X X
10 -6 10 -G 10 -6 10 -7
Journal of Colloid and Interface S~ience,
Vol. 48, No. 2, August 1974
214
WILKINSON AND ELLIOTT 0.7 0"6
A
0,5
~ 0.4
¢¢ O 0'2
0.1
!
~
t
~
f
o.'o4
o!o6
-J~
tl
o~2
TIME (seconds]
o~8
040
FIc. 6. Induced drop oscillations on attachment at the interface. Base diameters are represented by open symbols, and drop heights by closed ones. Curve
Concn. (moles/liter)
Surface-active agent
A B C
-0.0040 0.0010
Distilled water Sodium dodecyl sulfate Sodium dodecyl sulfate
A
0.7
0.6
"~
0'5
I~ ..t~_.~_C~
--
o:
0'4 O-E 0.2 0.1 .I i
,/,J/
0
----,
0 . 0I 4 0"106 TIME (seconds}
0.02
0.~08
0.10
FIO. 7. Induced drop oscillations on attachment at the interface. Base diameters are represented by open symbols~ and drop heights by closed ones. Curve
Conch. (moles/liter)
Surface-active agent
A B C
0.0100 0.0010 0.0400
N-decylamine hydrochloride N-decylamine hydrochloride N-decylamine hydrochloride
Journal of Colloid and Inlerface Science, Vol. 48, No. 2, August 1974
"°[
DYNAMIC CONTACT ANGLES. II
215
170
160L
140
130 Z U
.
5
12C
I10
• lOG
//1 / /
/
90
8C
70
0
0"01
0.02
0"03
0"04 - - - 0 . 0 6 0"I00 TIME [secon4s )
0"140
0"180
0.220
FIG. 8. Drop symmetry during attchment and spreading. The two symbols given for each curve represent values of 0LEFTand 0RmHT. Curve
Concn. (moles/liter)
Surface-actlve a g e n t
A B C
0.0400 0.0020 --
N-decylamine hydrochloride Sodium dodecyI sulfate Distilled water
drops falling from the same dropping tip would have the same characteristic vibrational frequency. The frequency will vary with the size of the dropping tip but not with the induced energy in the drop (5). For example, Elliott and Morgan (4) used a larger dropping tip (0.300.cm as opposed to 0.200 cm) and recorded a vibrational frequency for carbon tetrachloride drops attaching at a solid/solution interface of 17.9 Hz; this value agreed well with the calculated value of 16.5 Hz. The plots of drop height and drop base diameter against time show that the height decreased dramatically at 0.02 sec, and the
base diameter peaked (or levelled out) at about 0.045 sec. These values are in good agreement with the times at which maxima in the contact angles were recorded. Furthermore, the minima in the drop height plots (Fig. 6 and 7) occurred about 0.01 sec prior to the maxima in the drop base diameters. Occassionally it was observed that one side of the carbon tetrachloride drop attached at the mercury surface before the other. This led to an initial dissymmetry of the two measured contact angles (0L and OR). Two examples of this type of dissymmetry are illustrated in Fig. 8, together with the normal type of attach-
Journal of Colloid and Inter/ace Science, Vol. 48, No. 2, A u g u s t 1974
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WILKINSON AND ELLIOTT
ment usually recorded (curve B) where 0L and 0n agreed very well after 0.005 sec. Where dissymmetry occurred on attachment it was never found to persist beyond 0.03-0.04 sec. Two types of vibration were obviously occurring simultaneously, one in the vertical plane, as reflected in the values of h (drop height), and the other in the horizontal plane, as reflected in the values of b (drop base diameter) and 0 (contact angle). Although drops in all three types of surface-active agent exhibited oscillations in 0 and h, only those in solutions below the "critical concentrations" of the ionic surface-active agents exhibited oscillations in the base diameter. For solutions above the "critical concentrations" no retraction in the base diameters of the drops occurred, continual increases with time being recorded (curve B of Fig. 6, and curves A and C of Fig. 7). At a nominal speed of 400 f.p.s, the region in the area of the carbon tetrachloride/mercury interface was blurred for the first couple of frames. Thus, the dotted portions of the base diameters against time curves (Figs. 6, 7, and 9) have been inserted on the assumption that point contacts occurred on attachment. Although films had been run at speeds of approximately 400 f.p.s., careful study showed that in all cases attachment occurred in less than one frame, i.e., less than 0.0025 sec. In order to study this short period in more detail, films were taken at 5000 f.p.s, of the attachment of drops at a mercury/water interface. For this three-phase system the induction periods were only of the order of zero to several seconds. This meant that there was a reasonable chance of obtaining a film of the phenomenon during the total filming time of less than 1 sec. Plates I - I X show in detail the attachment of a carbon tetrachloride drop to a mercury/ water interface. Plate I shows the drop before any visible contact was made; all frames previous to this were identical, i.e., measured contact angle (apparent) and drop dimensions remained constant. Observations of the film
by eye did not reveal any difference between the frames depicted in Plates I, II, and III, but under a magnification of 17 diam the drops appeared as shown. Plate II was of the second frame after that shown in Plate I, the time interval being 0.0006 sec. No difference was observed between Plates of frames one and two, even under a higher magnification. The limiting difficulty was the resolution by the camera. Using the method of measurement described the contact angles of the drops depicted in Plates I - I I I were measured as about 20 ° (Part I). The magnification of the negatives using this technique was of the order of 8 diam. The true contact angle was in fact almost 90 ° for Plates II and III, and remained at this value during the first 0.003 sec. Successive plates show that the points at which the attachment was made moved outwards; the interfacial area of contact between the mercury and the carbon tetrachloride increasing. The main body of the drop, however, remained stationary during this time. This was confirmed by measurements on the height of the drop during the stage depicted by Plates I-IV (Figs. 6 and 7). Only after 0.02 sec was any significant alteration in the drop height observed. After 0.003 sec "crimpling" of the drop profile occurred (Plate V). This effect increased in magnitude until after 0.0056 sec (Plate VI) it was marked. It is suggested that this phenomenon was due to ripples in the surface of the drop. These were probably produced by the hydrodynamic forces set up on attachment. They ceased within 0.005 sec. At 0.0056 sec (Plate VI) the base diameter of the drop had increased until it was equal to the maximum diameter. Up to this stage the maximum diameter, and height, had remained almost constant. Further extension of the base area, however, beyond this point, resulted in the contact angle exceeding 90 ° and the original drop shape being destroyed. As the spreading continued the main body of the drop was pulled down by cohesive forces. This process started with the sides of the drop
Journal of Colloid and Interface Science, Vol. 48, No. 2, August 1974
DYNAMIC CONTACT ANGLES. II
217
PLATE I. P r i o r to attachment.
PLATE I I . 0 . 0 0 0 6 sec a f t e r a t t a c h m e n t . Journal of Colloid and Interface Science, Vol. 48, No. 2, August 1974
2 18
WILKINSON AND ELLIOTT
PLATE I l L 0.0010 sec after attachment.
PLATE IV. 0.0016 sec after attachment. Journal of Colloid and Interface Science, Vol. 48, No. 2, August 1974
D Y N A M I C C O N T A C T ANGLES. 11
219
PLATE V. 0,0030 se¢ after attachment.
]PLATE VI. 0.0056 sec after attachment. Journal of Colloid and Interface Science, VoL 48, No, 2, August 1974
220
WILKINSON AND ELLIOTT
PLATE VII. 0.0116 sec after attachment.
PLATE V I I I . 0.0176 sec after a t t a c h m e n t . Journal of Colloid and Interface Science, Vol. 48, No. 2, August 1974
DYNAMIC CONTACT ANGLES. II
221
ii~~iiii~!i! !
i!iiii!iii~ii!!i~
PLAT~IX. 0.2080 sec after attachment. (Plate VII) and continued until eventually the top of the drop collapsed (after 0.025 sec) and a dish-like structure was formed. The drop then rapidly recovered a more hemispherical shape (Plate IX). Further vibrations in the vertical plane still existed after this stage but were of very reduced magnitude. The entire process depicted in Plates I - I X required only 0.20 sec and the contact angle, and the interfacial base area of contact, were within 90% of their equilibrium values at this time. Although similar high-speed photographs of the attachment of carbon tetrachloride drops to the mercury/solution of surfaceactive agent interface were not obtained, it is probable that a similar process of attachment occurred. After attachment, the time taken for equilibrium to be reached depended on the surface-active agent present. Thus, although the drop in distilled water was within 90% of its equilibrium state after 0.20 sec, drops in solutions of the surface-active agents were far from equilibrium at this time (6). In order to examine the theory that the occurrence of characteristic vibrations during attachment was due mainly to hydrodynamic and/or gravitational effects, the spreading of much smaller drops than those used above was followed. The results for drops in several concentrations of sodium dodecyl sulfate solution are illustrated in Fig. 9. Reducing the size of the drop reduced the time at which the first
vibration occurred. The relative magnitude of the change, however, was not affected. DISCUSSION
Process of Attachment Although the drops employed in this work were small on a macro scale, they were quite large on a micro scale. The base of the drop could therefore be considered as completely flat and parallel to the atomically-smooth mercury surface. In this case there was no reason why any particular localized area of the drop should make contact in preference to another area. The asperities associated with solid surfaces were completely absent. In fact, as far as could be ascertained from the photographs, attachment occurred over most of the visible pseudo-contact area (Plate II). The size of the actual attachment area [-or point (s)~ could not be determined. Attachment probably occurred at a series of points, where the time differences between the attachment points were of the order 10-4 sec. I t is generally considered that when a drop attaches at a substrate surface, the initial contact angle (as measured through the drop) is 180 ° . This attachment angle decreases until an equilibrium contact angle is achieved. In the present system the attachment contact angle was closer to 90 ° , in which case the interfacial driving force was simply the difference
Journal of Colloid and Interface Science, Vol. 48, N o . 2, A u g u s t 1974
222
WILKINSON AND ELLIOTT O~
0-5
7u
0.4
o.3 ~
~_
.~
~
~,-
0.2
g Oq
,
O
O.;2
O.'O4 O';6 TIME [seconds)
O.;,
O'lO
Fro. 9. Re]ationship between drop size and magnitude of induced drop oscillations on attachment. Base diameters a r e represented by open symbols, and drop heights b y closed ones. Curve
Concn. (moles/liter)
Surface-active agent
Drop volume (ml)
A B
0.0010 0.0010
Sodium dodecyl sulfate Sodium dodecyl sulfate
0.0189 0.0043
C
0.004
Sodium dodecyl sulfate
0.0017
between the mercury/water and mercury/ carbon tetrachloride interracial tensions. The 90 ° attachment contact angle may not be too surprising when it is considered that the works of adhesion of water and carbon tetrachloride to mercury are very similar, i.e., 152 and 145 ergs/cm 2, respectively. As the base area of the drop extended beyond that of the maximum cross-sectional area, the ellipsoidal shape was destroyed. Carbon tetrachloride was forced upwards, resulting in a slight peak in the plot of drop height against time (curves A and C) of Fig. 6, and A and B of Fig. 7). The advancing front of the drop was fed with carbon tetrachloride from the drop bulk, until the situation as depicted in Plate VIII resulted. The remaining "cap" then collapsed rapidly--a dish-like structure being formed. This process resulted in excess carbon tetrachloride being forced into the advancing drop front, and a consequent reduction in contact angle. Reference to Figs. 3 and 6, and 4 and 7, illustrates this phenomenon. Good agreement on the time scale between hm~n and 0mln was recorded.
As the drop recoiled under the action of the induced excess surface energy, h increased due to excess carbon tetrachloride being forced upwards. This caused 0 to increase and, in some cases, d to decrease. Further vibrations were of much reduced amplitude, and all vibrations had usually ceased within 0.25 sec. For drops spreading in solutions above the respective "critical concentrations," no base retraction occurred. In some cases the increase in b was momentarily halted (curve B of Fig. 6) while in others it was only slowed. Here, the interracial driving force (F = "gmercury/solution --'Ymercury/carbon tetrachloride (adsorbed surface-active --~solution/carbon tetrachloride) must be greater, or equal to, the sum of the cohesive energy of the carbon tetrachloride plgs the resident energy in the drop produced by attachment. The energy induced in the drop is caused by the rapid collapse of the cap of the drop (Plate II). Although the mass of this cap is only of the order 0.01 g, it is pulled into the main body of the drop at about 20 cm/sec-agent)
Journal of Colloid and Interface Scle~ce, Vol. 48, No. 2, August 1974
DYNAMIC CONTACT ANGLES. II sufficiently high velocity to produce a kinetic energy of several ergs. Induced vibrations of the drop in the vertical plane produced corresponding vibrations in the spreading plane (horizontal). These vibrations were superimposed on the general spreading curve, which, in the absence of oscillations would appear as depicted by the broken line in Fig. 2. The magnitude of the induced oscillations decreased as the interfacial driving force increased. This is shown by reference to Figs. 6 and 7, where the interracial driving force causing spreading increased as the concentration of surface-active agent increased. For high concentrations of surface-active agent no retraction in drop base area was observed, although vertical oscillations (producing variations in h and 0) were still present.
Fate of Adsorbed Surface-Active Molecules During Attachment The attachment of carbon tetrachloride drops to a mercury/solution interface occurred in less than 0.0002 sec, and there was an approximately 20% decrease in the surface area of the drop within ~0.20 sec of attachment (Plates I - I X ) . Several mechanisms can be proposed for the fate of the surface-active molecules during this period. 1. The surface-active molecules were driven into the carbon tetrachloride phase. EIIiott and Morgan (4) showed that this was not the case for sodium octyl sulfate molecules. 2. The trapped film, as it drained from beneath the drop, dragged the surface-active ions with it. In this case the energy of desorption of the molecules from the carbon tetrachloride/water interface had to be overcome. The energies of adsorption of sodium alkyl sulfates to the oil/solution interface have been given (10) and are of the order 7000-11,000 cal/mole (chain lengths of 8-12 carbon atoms). This quantity of energy was required to remove a mole of surface-active agent from the trapped film/drop interface. Since, however, the molecules were in a gaseous state in this
223
interface, less energy would be required for these molecules to be swept around the surface of the drop. This has been suggested by Elliott and Morgan as the fate of surfaceactive molecules in the carbon tetrachloride drop when contact with a resin occurred. 3. The surface-active molecules at the carbon tetrachloride/water interface were adsorbed onto the mercury surface since the anionic surface-active molecules were oriented in the right sense for direct adsorption to the positively charged mercury surface. (Unpolarized mercury in water assumes a positive charge of about 0.5 V). This possibility decreased with increase in concentration due to the closer packing of the surface-active molecules already adsorbed on the mercury surface, and hence an increase in the repulsion. 4. The surface-active molecules were retained in the carbon tetrachloride/water interface, and were compressed on drop attachment. I t was considered that at low surface concentrations the surface-active molecules at both the interfaces were removed from the immediate vicinity of the drop/mercury region by the magnitude of the viscosity forces, the forces of repulsion between the surface active molecules at these large surface areas per molecule being-small. Surface-active molecules at the water/carbon tetrachloride interface were swept around the drop, and those at the mercury/water interface compressed along the mercury surface. As the surface concentration of molecules at the interfaces built up, the forces resisting further compression increased. I t is known that ionic surface-active molecules at the carbon tetrachloride/solution interface can be compressed until a minimum surface area of about 22 A2 per molecule is reached (11). Thus, assuming on the average a 200-/o decrease in the carbon tetrachloride/solution interface on drop attachment (Plate VI) then, at the maximum concentration of surfaceactive agent used, the surface area available per molecule would still be greater than this minimum (11). Thus, it is suggested that in all cases, and at all concentrations used in this
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WILKINSON AND ELLIOTT
work, surface-active molecules were retained in the carbon tetrachloride/solution interface on attachment. It is not known what the limiting surface areas per molecule of ionic surface-active agents at mercury/solution interfaces were. The molecules were probably not steeply oriented to the mercury surface, and hydroxyl ions probably competed for sites on the surface. Thus, the minimum surface areas into which these molecules could be compressed at this type of interface were probably much larger than 25 A2. As the surface concentration of molecules on the mercury surface increased, a position would be reached where the attaching drop could not compress the molecules further to enable it to spread on a "clean" mercury surface. At this particular concentration (referred to as the "critical" concentration) the drop would then spread over the adsorbed layer of surface-active molecules on the mercury surface.
True and Apparent Areas of Contact Many workers have observed that the true area of wetting may not be the same as the apparent area. Wark (12) found that drops of carbon tetrachloride at a cerrusite/water interface did not spread immediately if the surface was freshly prepared. Contact occurred at a series of small isolated areas, whose extent gradually increased. The water was gradually forced out from between the spread carbon tetrachloride and the cerrusite surface. Frumkin and co-workers (13, 14) have observed similar phenomena for hydrogen bubbles making contact at a mercury/solution interface. If the drop initially makes a point contact with the surface and spreads from this point, no solution should be trapped between the spreading drop and the substrate. Frumkin has suggested that this type of attachment occurred in some cases. Smolders (15) found that on attachment of a hydrogen bubble to a mercury/solution interface all the water was expelled by the spreading bubble, a dehy-
drated monolayer of the surface-active agent remaining adsorbed at the mercury/gas interface. In this work the attachment of drops to the mercury/solution interface was extremely rapid. If attachment occurred over a large area then it was possible that small droplets of solution would be trapped at the mercury/ carbon tetrachloride interface. These droplets being less dense would gradually diffuse through the main drop when they were displaced from the mercury surface. This effect was observed on numerous occasions. Sometimes only one escaping droplet was observed but occasionally a large number of very small droplets were seen. This phenomenon, however, did not affect the spreading of the drop or the way the contact angle changed with time. REFERENCES 1. SCI~IULMAN, J. FI., AND LEJA, J., "Surface Phenomena in Chemistry and Biology," p. 236. Pergamon Press, London, 1958. 2. HARTLE'Z, G. S., AND BI~IJNSKILL, R. T., ibid., p. 214. 3. ELLIOTr, T. A., AND LEESE, L., J. Chem. Soc. 22 (1957); 1466 (1959). 4. ELLIOTT,T. A., AND MORGAN, M., J. Chem. Soc. Part 5a, 558 (1966). 5. ELLIOTT, T. A., and FO~D, O. M., Trans. Faraday Soc. I 1814 (1972). 6. WILKINSON,M. C., AND ELLIOT, T. A., J. Colloid Interface Sci. 48, 187 (1974). 7. FORD, R. E., AND FURM/DGE,C. G. L., "Wetting," S.C.I. Monograph No. 25, p. 417, 1967. 8. LEVlCH,V. G., "Physicochemical Hydrodynamics," p. 655. Prentice-Hall, Englewood Cliffs, NJ, 1962. 9. HARKINS, W. D., AND BROWN, F. E., J. Amer. Chem. Soo. 41, 499 (1919). 10. HAx~I)OSl,D. A., AND TAYLOR, F. H., Phil. Trans. 252, 225 (1960). 11. WILKINSON,M. C., AND ELLIOTT,T. A., J. Colloid Interface Sci. 48, 225 (1974). 12. WAaK, I. W., J. Phys. Chem. 37, 637 (1933). 13. FRU~I~IN,A., ANDGORODETZKAJA,A., A cta Physicochlmica U.S.S.R. 9, 327 (1938). 14. FRUMKIN, A., GORODETZKAJA,A., KABANOV,B., AND NEKRASSOW,N., Sow. Phys. 1, 255 (1932). 15. SM[OLDERS,C. A., AND DUYVIS, E. M., Rec. Tray. Chim. 80, 635, 699 (1961).
Journal of Colloid and Interface Science, Vol. 48, No. 2, August 1974
C. WILKINSON
~ AND
T. A. ELLIOTT
2
Trent Polytechnic,Nottingham, England Received August 31, 1972; accepted November 5, 1973 The process of attachment of carbon tetrachloride drops at mercury/solution of surfaceactive agent interfaces has been studied by means of high-speed cine photography. At smooth interfaces drops do not apparantly make single point attachments, but attach over relatively large basal areas. The contact angle on attachment depends on the difference in the works of adhesion of the two liquids for the mercury surface. In the present system the initial stages of spreading were mainly controlled by the mercury/ solution and mercury/carbon tetrachloride interfacial tensions. The contact angle during this stage was almost of 90°, and remained at this value as the base area increased. The drop maintained its ellipsoidal, preattachment shape until the base area equalled the maximum cross-sectional area, when drop distortion occurred and the contact angle (measured through the aqueous phase) exceeded 90° . The rapid processes of attachment and initial spreading (0.1 sec) and the manner in which the contact angle changed with time, have been shown to be very reproducible. The passage of the drop from an ellipsoidalshape to that of the cap of a sphere was marked by the induction of oscillations in the vertical plane, which produced vibrations in the spreading plane. For a given system, the effect of induced oscillations on the dynamic contact angle depended on the magnitude of the interfacial driving force causing spreading. INTRODUCTION Several workers have observed the a t t a c h m e n t and d e t a c h m e n t of drops and bubbles at interfaces (1-5). Schulman and L e j a (1) studied d y n a m i c contact angles in the detachm e n t of air bubbles from m e r c u r y / s o l u t i o n interfaces, where escape times were in the region of several minutes. T h e y also obtained high speed p h o t o g r a p h s (4000 frames per second) of the processes of impact, and adhesion, of gas bubbles at a solid surface. However, no measurements of r a p i d contact angle vs. time variations were obtained, and no a t t e m p t was m a d e to describe the processes. x Present address: Chemical Defense Establishment, Porton Down, Salisbury, Wiltshire, England. 2 Deceased.
T h e s t u d y of d y n a m i c contact angles b y Elliott and co-workers (3-5) did n o t yield a n y information as to the mechanism of drop (or bubble) a t t a c h m e n t due to the slow filming speeds employed (53 frames per second). M o s t of the process was completed in less t h a n 0.05 sec. I n the present system a high-speed cine camera has enables the process of a t t a c h m e n t of carbon tetrachloride drops at m e r c u r y / water interfaces to be studied. Although a great deal of information was obtained at relatively slow filming (400 frames per second), speeds up to 5000 f.p.s, h a d to be used to record the initial processes occurring on attachment. The unavoidable difficulty with the highspeed cine p h o t o g r a p h y of a t t a c h m e n t processes is the fact t h a t no control on the length
209 Copyright ~) 1974 by Academic Press, Inc. All rights of reproduction in any form reserved.
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WILKINSON AND ELLIOTT
of the induction period can be achieved. Processes with zero or very small induction periods can be recorded readily (5) but in these systems the drop (or bubble) is not in a steady state when attachment occurs. In the present system the induction periods were sufficiently long so that all oscillations due to drop impact had ceased before attachment occurred. Thus, the processes causing attachment were entirely interfacial ones. EXPERIMENTAL The apparatus, procedure and purification of materials have been described (6). RESULTS The attachment of drops of carbon tetrachloride to mercury/water, and mercury/solution interfaces was extremely rapid. Velocities of spreading at different time stages have been plotted in Fig. 1. The examples chosen are typical of results observed for drops spreading in different concentrations of the ionic and nonionic surface-active agents. The highest velocities of about 12,000 to 14,000 ram~rain were calculated assuming the drops made point contacts at the mercury surface on attachment. In all cases, the velocity of spreading fell rapidly during the first 0.05 sec; after this, the rate of fall was much slower. The manner in which the contact angle changed during the first 0.20 sec for drops in distilled water, and in a range of concentrations of different surface-active agents, is illustrated in Figs. 2-5. The corresponding variations in the drop height and base diameter, during the first 0.10 sec, are illustrated in Figs. 6 and 7. The values of the drop heights recorded in these figures were always measured to the uppermost part of the drop (see Plates VII and VIII). With filming speeds as high as 400 f.p.s, the first contact angles recorded were invariably greater than 50 ° . The contact angles in most cases were in excess of 90 ° in less than 0.01 sec. After these rapid rises in the contact angle very much slower increases were recorded, e.g.,
90-105 ° in 0.20 sec for distilled water (Fig. 2). During the initial periods of spreading, contact angles were taken as the average of the measured left- and right-hand angles (i.e., 0L and 0R). It is apparent from the results of Fig. 8 that there was very little difference in these values after the first 0.02 sec. It is surprising that during the rapid process of attachment very good reproducibility of the contact angle changes was recorded. This point is illustrated by the three repeat runs in distilled water (Fig. 2). The reproducibility was almost as good for drops in solutions of surfaceactive agents; the relative positions of the curves (Figs. 3-5) did not alter (6). As long as drop oscillations had ceased before attachment, variations in the magnitude of the induction periods had no effect on reproducibility. The difference in spreading behavior for drops which attached while still in their oscillatory mode due to impact, was marked. This type of impact spreading has been considered by Ford and Furmidge (7) and was found to be the exception in the present system, rather than the rule.
Induced Drop Vibrations on Attachment Violent drop oscillations were set up when the drop attached at the mercury/solution interface. These vibrations are reflected in the contact angle vs. time plots of Figs. 2-5, and the drop base diameter and height vs. time plots of Figs. 6 and 7. There was a large peak in the contact angle at ~0.05 sec, and smaller ones at multiples of this, i.e., 0.10, 0.15 sec; these were recorded at the same times for drops in different concentrations and types of surface-active agent. The primary vibrations were rapidly damped, and successive ones, which were more predominant in the case of drops in distilled water and sodium dodecyl sulfate solutions, were of much smaller magnitude. The characteristic peak in the contact angle at about 0.02 sec, which in some cases was more of a plateau region, was probably caused by the "braking" action of the drop/solution
Journal of Colloid and Interface Science, Vol. 48, No. 2, August 1974
DYNAMIC CONTACT ANGLES. II
211
14000
I0000
600C
200C
t IO(DC
A
I 60C
20C i
I
r6c
-
I000
-2000
-300C
0
0 " 0I 5
0.110
0.115
0 12 0
z 0.25
0 " 3+ 0
0.35;
TIME (seconds)
FIG. 1. Rapidity of attachment at the interface. Curve
Conch. (moles/liter)
Surface-active a g e n t
A B C
0.0040 0.0020 --
Sodium dodecyl sulfate Sodium dodecyl sulfate Distilled water
interfacial tension as the contact angle exceeded 90 °. Up to 90 °, this interfacial force acted to aid spreading, but in excess of 90 ° it opposed spreading (as a cosine function). This phenomenon has been recorded by Elliott and Leese (3) and Elliott and Ford (5). Elliott and Morgan (4) however, did not note this since in their system the contact angle did not reach 90 ° . If the characteristic frequency of natural oscillations of the drop can be approximated by the Rayleigh equation (8), which describes the eigenvibrations of spherical drops, then
the frequency, v, of the drop, should be given by: t, .
.
. . F++T ,, L praJ 27r
[1]
where T is the interfacial tension ; p the density (for this case p = Pwater "~- Pcarbon tetrachloride 2.6); and r the radius of the drop. Inserting into Eq. [-11 values for a carbon tetrachloride drop in distilled water yields a frequency of 21.6 Hz, i.e., a time interval between vibrations of 0.047 sec, a value that is in good agreement with ~ 0 . 0 5 sec recorded experimentally.
do2*rnal of Colloid and Interface Science, Vol. 48, No. 2, A u g u s t 1976
212
WILKINSON AND ELLIOT]7
'20I
ot
I00
~,
~
9O
I-
u
"70
o.o~
o.',o
0/,5
0.20
TIME (seconds)
FIo. 2. Reproducibility of dynamic contact angles (distilled water). A, B, and C are repeat runs.
Calculations using the data for drops in surface-active agent solutions yielded values between 20 and 22 Hz, which are in good agreement with the observation that the characteristic period of vibrations were similar for all the systems (Fig. 2-5). This conclusion might
be expected, since the drops all fell from the same dropping tip, and, therefore, from the theory of dropping (9) : ~ , a r 3. From Eq. [-17 it would be expected that all
180
Iso
~
B
§ 6o 3O
0
0
0.05
0"I0
O.IS
O' 0
TIME ( s e c o n d s ) FIO.
3. Dynamic contact angles during initial stages of spreading (solutions of sodium dodecyl sulfate). Curve
A B C D E Journal of Colloid and Interface Science, Vol. 48, No. 2, August 1974
Collon.
(moles/liter) 0.0040 0.0030 0.0020 0.0010 0.0001
DYNAMIC CONTACT ANGLES. II
213
18o A C
~3--
150
'~
12o
z <
90
~
__
0-----
D
b 60 u 30
o
0.6 s
o~ls
o.'~o
0.20
TIME {seconds]
FIG. 4, Dynamic contact angles during initial stages of spreading (solutions of N-decylamine hydrochloride). Curve
Conch. (moles/liter)
A B C D
0.0100 0.0200 0.0400 0.0010
iBO
150 ~
~
,
~ x ' ~ C
J2C
II..--
D
90
z
oo 30
0
0
Of'OS
O-IlO TiME
0'115
0"20
{ se(:on ds}
FIG. 5. Dynamic contact angles during initial stages of spreading (solutions of nonionic). CuI've A B C D
Conch.
(moles/liter) 6.48 3.89 1.30 3.24
X X X X
10 -6 10 -G 10 -6 10 -7
Journal of Colloid and Interface S~ience,
Vol. 48, No. 2, August 1974
214
WILKINSON AND ELLIOTT 0.7 0"6
A
0,5
~ 0.4
¢¢ O 0'2
0.1
!
~
t
~
f
o.'o4
o!o6
-J~
tl
o~2
TIME (seconds]
o~8
040
FIc. 6. Induced drop oscillations on attachment at the interface. Base diameters are represented by open symbols, and drop heights by closed ones. Curve
Concn. (moles/liter)
Surface-active agent
A B C
-0.0040 0.0010
Distilled water Sodium dodecyl sulfate Sodium dodecyl sulfate
A
0.7
0.6
"~
0'5
I~ ..t~_.~_C~
--
o:
0'4 O-E 0.2 0.1 .I i
,/,J/
0
----,
0 . 0I 4 0"106 TIME (seconds}
0.02
0.~08
0.10
FIO. 7. Induced drop oscillations on attachment at the interface. Base diameters are represented by open symbols~ and drop heights by closed ones. Curve
Conch. (moles/liter)
Surface-active agent
A B C
0.0100 0.0010 0.0400
N-decylamine hydrochloride N-decylamine hydrochloride N-decylamine hydrochloride
Journal of Colloid and Inlerface Science, Vol. 48, No. 2, August 1974
"°[
DYNAMIC CONTACT ANGLES. II
215
170
160L
140
130 Z U
.
5
12C
I10
• lOG
//1 / /
/
90
8C
70
0
0"01
0.02
0"03
0"04 - - - 0 . 0 6 0"I00 TIME [secon4s )
0"140
0"180
0.220
FIG. 8. Drop symmetry during attchment and spreading. The two symbols given for each curve represent values of 0LEFTand 0RmHT. Curve
Concn. (moles/liter)
Surface-actlve a g e n t
A B C
0.0400 0.0020 --
N-decylamine hydrochloride Sodium dodecyI sulfate Distilled water
drops falling from the same dropping tip would have the same characteristic vibrational frequency. The frequency will vary with the size of the dropping tip but not with the induced energy in the drop (5). For example, Elliott and Morgan (4) used a larger dropping tip (0.300.cm as opposed to 0.200 cm) and recorded a vibrational frequency for carbon tetrachloride drops attaching at a solid/solution interface of 17.9 Hz; this value agreed well with the calculated value of 16.5 Hz. The plots of drop height and drop base diameter against time show that the height decreased dramatically at 0.02 sec, and the
base diameter peaked (or levelled out) at about 0.045 sec. These values are in good agreement with the times at which maxima in the contact angles were recorded. Furthermore, the minima in the drop height plots (Fig. 6 and 7) occurred about 0.01 sec prior to the maxima in the drop base diameters. Occassionally it was observed that one side of the carbon tetrachloride drop attached at the mercury surface before the other. This led to an initial dissymmetry of the two measured contact angles (0L and OR). Two examples of this type of dissymmetry are illustrated in Fig. 8, together with the normal type of attach-
Journal of Colloid and Inter/ace Science, Vol. 48, No. 2, A u g u s t 1974
216
WILKINSON AND ELLIOTT
ment usually recorded (curve B) where 0L and 0n agreed very well after 0.005 sec. Where dissymmetry occurred on attachment it was never found to persist beyond 0.03-0.04 sec. Two types of vibration were obviously occurring simultaneously, one in the vertical plane, as reflected in the values of h (drop height), and the other in the horizontal plane, as reflected in the values of b (drop base diameter) and 0 (contact angle). Although drops in all three types of surface-active agent exhibited oscillations in 0 and h, only those in solutions below the "critical concentrations" of the ionic surface-active agents exhibited oscillations in the base diameter. For solutions above the "critical concentrations" no retraction in the base diameters of the drops occurred, continual increases with time being recorded (curve B of Fig. 6, and curves A and C of Fig. 7). At a nominal speed of 400 f.p.s, the region in the area of the carbon tetrachloride/mercury interface was blurred for the first couple of frames. Thus, the dotted portions of the base diameters against time curves (Figs. 6, 7, and 9) have been inserted on the assumption that point contacts occurred on attachment. Although films had been run at speeds of approximately 400 f.p.s., careful study showed that in all cases attachment occurred in less than one frame, i.e., less than 0.0025 sec. In order to study this short period in more detail, films were taken at 5000 f.p.s, of the attachment of drops at a mercury/water interface. For this three-phase system the induction periods were only of the order of zero to several seconds. This meant that there was a reasonable chance of obtaining a film of the phenomenon during the total filming time of less than 1 sec. Plates I - I X show in detail the attachment of a carbon tetrachloride drop to a mercury/ water interface. Plate I shows the drop before any visible contact was made; all frames previous to this were identical, i.e., measured contact angle (apparent) and drop dimensions remained constant. Observations of the film
by eye did not reveal any difference between the frames depicted in Plates I, II, and III, but under a magnification of 17 diam the drops appeared as shown. Plate II was of the second frame after that shown in Plate I, the time interval being 0.0006 sec. No difference was observed between Plates of frames one and two, even under a higher magnification. The limiting difficulty was the resolution by the camera. Using the method of measurement described the contact angles of the drops depicted in Plates I - I I I were measured as about 20 ° (Part I). The magnification of the negatives using this technique was of the order of 8 diam. The true contact angle was in fact almost 90 ° for Plates II and III, and remained at this value during the first 0.003 sec. Successive plates show that the points at which the attachment was made moved outwards; the interfacial area of contact between the mercury and the carbon tetrachloride increasing. The main body of the drop, however, remained stationary during this time. This was confirmed by measurements on the height of the drop during the stage depicted by Plates I-IV (Figs. 6 and 7). Only after 0.02 sec was any significant alteration in the drop height observed. After 0.003 sec "crimpling" of the drop profile occurred (Plate V). This effect increased in magnitude until after 0.0056 sec (Plate VI) it was marked. It is suggested that this phenomenon was due to ripples in the surface of the drop. These were probably produced by the hydrodynamic forces set up on attachment. They ceased within 0.005 sec. At 0.0056 sec (Plate VI) the base diameter of the drop had increased until it was equal to the maximum diameter. Up to this stage the maximum diameter, and height, had remained almost constant. Further extension of the base area, however, beyond this point, resulted in the contact angle exceeding 90 ° and the original drop shape being destroyed. As the spreading continued the main body of the drop was pulled down by cohesive forces. This process started with the sides of the drop
Journal of Colloid and Interface Science, Vol. 48, No. 2, August 1974
DYNAMIC CONTACT ANGLES. II
217
PLATE I. P r i o r to attachment.
PLATE I I . 0 . 0 0 0 6 sec a f t e r a t t a c h m e n t . Journal of Colloid and Interface Science, Vol. 48, No. 2, August 1974
2 18
WILKINSON AND ELLIOTT
PLATE I l L 0.0010 sec after attachment.
PLATE IV. 0.0016 sec after attachment. Journal of Colloid and Interface Science, Vol. 48, No. 2, August 1974
D Y N A M I C C O N T A C T ANGLES. 11
219
PLATE V. 0,0030 se¢ after attachment.
]PLATE VI. 0.0056 sec after attachment. Journal of Colloid and Interface Science, VoL 48, No, 2, August 1974
220
WILKINSON AND ELLIOTT
PLATE VII. 0.0116 sec after attachment.
PLATE V I I I . 0.0176 sec after a t t a c h m e n t . Journal of Colloid and Interface Science, Vol. 48, No. 2, August 1974
DYNAMIC CONTACT ANGLES. II
221
ii~~iiii~!i! !
i!iiii!iii~ii!!i~
PLAT~IX. 0.2080 sec after attachment. (Plate VII) and continued until eventually the top of the drop collapsed (after 0.025 sec) and a dish-like structure was formed. The drop then rapidly recovered a more hemispherical shape (Plate IX). Further vibrations in the vertical plane still existed after this stage but were of very reduced magnitude. The entire process depicted in Plates I - I X required only 0.20 sec and the contact angle, and the interfacial base area of contact, were within 90% of their equilibrium values at this time. Although similar high-speed photographs of the attachment of carbon tetrachloride drops to the mercury/solution of surfaceactive agent interface were not obtained, it is probable that a similar process of attachment occurred. After attachment, the time taken for equilibrium to be reached depended on the surface-active agent present. Thus, although the drop in distilled water was within 90% of its equilibrium state after 0.20 sec, drops in solutions of the surface-active agents were far from equilibrium at this time (6). In order to examine the theory that the occurrence of characteristic vibrations during attachment was due mainly to hydrodynamic and/or gravitational effects, the spreading of much smaller drops than those used above was followed. The results for drops in several concentrations of sodium dodecyl sulfate solution are illustrated in Fig. 9. Reducing the size of the drop reduced the time at which the first
vibration occurred. The relative magnitude of the change, however, was not affected. DISCUSSION
Process of Attachment Although the drops employed in this work were small on a macro scale, they were quite large on a micro scale. The base of the drop could therefore be considered as completely flat and parallel to the atomically-smooth mercury surface. In this case there was no reason why any particular localized area of the drop should make contact in preference to another area. The asperities associated with solid surfaces were completely absent. In fact, as far as could be ascertained from the photographs, attachment occurred over most of the visible pseudo-contact area (Plate II). The size of the actual attachment area [-or point (s)~ could not be determined. Attachment probably occurred at a series of points, where the time differences between the attachment points were of the order 10-4 sec. I t is generally considered that when a drop attaches at a substrate surface, the initial contact angle (as measured through the drop) is 180 ° . This attachment angle decreases until an equilibrium contact angle is achieved. In the present system the attachment contact angle was closer to 90 ° , in which case the interfacial driving force was simply the difference
Journal of Colloid and Interface Science, Vol. 48, N o . 2, A u g u s t 1974
222
WILKINSON AND ELLIOTT O~
0-5
7u
0.4
o.3 ~
~_
.~
~
~,-
0.2
g Oq
,
O
O.;2
O.'O4 O';6 TIME [seconds)
O.;,
O'lO
Fro. 9. Re]ationship between drop size and magnitude of induced drop oscillations on attachment. Base diameters a r e represented by open symbols, and drop heights b y closed ones. Curve
Concn. (moles/liter)
Surface-active agent
Drop volume (ml)
A B
0.0010 0.0010
Sodium dodecyl sulfate Sodium dodecyl sulfate
0.0189 0.0043
C
0.004
Sodium dodecyl sulfate
0.0017
between the mercury/water and mercury/ carbon tetrachloride interracial tensions. The 90 ° attachment contact angle may not be too surprising when it is considered that the works of adhesion of water and carbon tetrachloride to mercury are very similar, i.e., 152 and 145 ergs/cm 2, respectively. As the base area of the drop extended beyond that of the maximum cross-sectional area, the ellipsoidal shape was destroyed. Carbon tetrachloride was forced upwards, resulting in a slight peak in the plot of drop height against time (curves A and C) of Fig. 6, and A and B of Fig. 7). The advancing front of the drop was fed with carbon tetrachloride from the drop bulk, until the situation as depicted in Plate VIII resulted. The remaining "cap" then collapsed rapidly--a dish-like structure being formed. This process resulted in excess carbon tetrachloride being forced into the advancing drop front, and a consequent reduction in contact angle. Reference to Figs. 3 and 6, and 4 and 7, illustrates this phenomenon. Good agreement on the time scale between hm~n and 0mln was recorded.
As the drop recoiled under the action of the induced excess surface energy, h increased due to excess carbon tetrachloride being forced upwards. This caused 0 to increase and, in some cases, d to decrease. Further vibrations were of much reduced amplitude, and all vibrations had usually ceased within 0.25 sec. For drops spreading in solutions above the respective "critical concentrations," no base retraction occurred. In some cases the increase in b was momentarily halted (curve B of Fig. 6) while in others it was only slowed. Here, the interracial driving force (F = "gmercury/solution --'Ymercury/carbon tetrachloride (adsorbed surface-active --~solution/carbon tetrachloride) must be greater, or equal to, the sum of the cohesive energy of the carbon tetrachloride plgs the resident energy in the drop produced by attachment. The energy induced in the drop is caused by the rapid collapse of the cap of the drop (Plate II). Although the mass of this cap is only of the order 0.01 g, it is pulled into the main body of the drop at about 20 cm/sec-agent)
Journal of Colloid and Interface Scle~ce, Vol. 48, No. 2, August 1974
DYNAMIC CONTACT ANGLES. II sufficiently high velocity to produce a kinetic energy of several ergs. Induced vibrations of the drop in the vertical plane produced corresponding vibrations in the spreading plane (horizontal). These vibrations were superimposed on the general spreading curve, which, in the absence of oscillations would appear as depicted by the broken line in Fig. 2. The magnitude of the induced oscillations decreased as the interfacial driving force increased. This is shown by reference to Figs. 6 and 7, where the interracial driving force causing spreading increased as the concentration of surface-active agent increased. For high concentrations of surface-active agent no retraction in drop base area was observed, although vertical oscillations (producing variations in h and 0) were still present.
Fate of Adsorbed Surface-Active Molecules During Attachment The attachment of carbon tetrachloride drops to a mercury/solution interface occurred in less than 0.0002 sec, and there was an approximately 20% decrease in the surface area of the drop within ~0.20 sec of attachment (Plates I - I X ) . Several mechanisms can be proposed for the fate of the surface-active molecules during this period. 1. The surface-active molecules were driven into the carbon tetrachloride phase. EIIiott and Morgan (4) showed that this was not the case for sodium octyl sulfate molecules. 2. The trapped film, as it drained from beneath the drop, dragged the surface-active ions with it. In this case the energy of desorption of the molecules from the carbon tetrachloride/water interface had to be overcome. The energies of adsorption of sodium alkyl sulfates to the oil/solution interface have been given (10) and are of the order 7000-11,000 cal/mole (chain lengths of 8-12 carbon atoms). This quantity of energy was required to remove a mole of surface-active agent from the trapped film/drop interface. Since, however, the molecules were in a gaseous state in this
223
interface, less energy would be required for these molecules to be swept around the surface of the drop. This has been suggested by Elliott and Morgan as the fate of surfaceactive molecules in the carbon tetrachloride drop when contact with a resin occurred. 3. The surface-active molecules at the carbon tetrachloride/water interface were adsorbed onto the mercury surface since the anionic surface-active molecules were oriented in the right sense for direct adsorption to the positively charged mercury surface. (Unpolarized mercury in water assumes a positive charge of about 0.5 V). This possibility decreased with increase in concentration due to the closer packing of the surface-active molecules already adsorbed on the mercury surface, and hence an increase in the repulsion. 4. The surface-active molecules were retained in the carbon tetrachloride/water interface, and were compressed on drop attachment. I t was considered that at low surface concentrations the surface-active molecules at both the interfaces were removed from the immediate vicinity of the drop/mercury region by the magnitude of the viscosity forces, the forces of repulsion between the surface active molecules at these large surface areas per molecule being-small. Surface-active molecules at the water/carbon tetrachloride interface were swept around the drop, and those at the mercury/water interface compressed along the mercury surface. As the surface concentration of molecules at the interfaces built up, the forces resisting further compression increased. I t is known that ionic surface-active molecules at the carbon tetrachloride/solution interface can be compressed until a minimum surface area of about 22 A2 per molecule is reached (11). Thus, assuming on the average a 200-/o decrease in the carbon tetrachloride/solution interface on drop attachment (Plate VI) then, at the maximum concentration of surfaceactive agent used, the surface area available per molecule would still be greater than this minimum (11). Thus, it is suggested that in all cases, and at all concentrations used in this
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work, surface-active molecules were retained in the carbon tetrachloride/solution interface on attachment. It is not known what the limiting surface areas per molecule of ionic surface-active agents at mercury/solution interfaces were. The molecules were probably not steeply oriented to the mercury surface, and hydroxyl ions probably competed for sites on the surface. Thus, the minimum surface areas into which these molecules could be compressed at this type of interface were probably much larger than 25 A2. As the surface concentration of molecules on the mercury surface increased, a position would be reached where the attaching drop could not compress the molecules further to enable it to spread on a "clean" mercury surface. At this particular concentration (referred to as the "critical" concentration) the drop would then spread over the adsorbed layer of surface-active molecules on the mercury surface.
True and Apparent Areas of Contact Many workers have observed that the true area of wetting may not be the same as the apparent area. Wark (12) found that drops of carbon tetrachloride at a cerrusite/water interface did not spread immediately if the surface was freshly prepared. Contact occurred at a series of small isolated areas, whose extent gradually increased. The water was gradually forced out from between the spread carbon tetrachloride and the cerrusite surface. Frumkin and co-workers (13, 14) have observed similar phenomena for hydrogen bubbles making contact at a mercury/solution interface. If the drop initially makes a point contact with the surface and spreads from this point, no solution should be trapped between the spreading drop and the substrate. Frumkin has suggested that this type of attachment occurred in some cases. Smolders (15) found that on attachment of a hydrogen bubble to a mercury/solution interface all the water was expelled by the spreading bubble, a dehy-
drated monolayer of the surface-active agent remaining adsorbed at the mercury/gas interface. In this work the attachment of drops to the mercury/solution interface was extremely rapid. If attachment occurred over a large area then it was possible that small droplets of solution would be trapped at the mercury/ carbon tetrachloride interface. These droplets being less dense would gradually diffuse through the main drop when they were displaced from the mercury surface. This effect was observed on numerous occasions. Sometimes only one escaping droplet was observed but occasionally a large number of very small droplets were seen. This phenomenon, however, did not affect the spreading of the drop or the way the contact angle changed with time. REFERENCES 1. SCI~IULMAN, J. FI., AND LEJA, J., "Surface Phenomena in Chemistry and Biology," p. 236. Pergamon Press, London, 1958. 2. HARTLE'Z, G. S., AND BI~IJNSKILL, R. T., ibid., p. 214. 3. ELLIOTr, T. A., AND LEESE, L., J. Chem. Soc. 22 (1957); 1466 (1959). 4. ELLIOTT,T. A., AND MORGAN, M., J. Chem. Soc. Part 5a, 558 (1966). 5. ELLIOTT, T. A., and FO~D, O. M., Trans. Faraday Soc. I 1814 (1972). 6. WILKINSON,M. C., AND ELLIOT, T. A., J. Colloid Interface Sci. 48, 187 (1974). 7. FORD, R. E., AND FURM/DGE,C. G. L., "Wetting," S.C.I. Monograph No. 25, p. 417, 1967. 8. LEVlCH,V. G., "Physicochemical Hydrodynamics," p. 655. Prentice-Hall, Englewood Cliffs, NJ, 1962. 9. HARKINS, W. D., AND BROWN, F. E., J. Amer. Chem. Soo. 41, 499 (1919). 10. HAx~I)OSl,D. A., AND TAYLOR, F. H., Phil. Trans. 252, 225 (1960). 11. WILKINSON,M. C., AND ELLIOTT,T. A., J. Colloid Interface Sci. 48, 225 (1974). 12. WAaK, I. W., J. Phys. Chem. 37, 637 (1933). 13. FRU~I~IN,A., ANDGORODETZKAJA,A., A cta Physicochlmica U.S.S.R. 9, 327 (1938). 14. FRUMKIN, A., GORODETZKAJA,A., KABANOV,B., AND NEKRASSOW,N., Sow. Phys. 1, 255 (1932). 15. SM[OLDERS,C. A., AND DUYVIS, E. M., Rec. Tray. Chim. 80, 635, 699 (1961).
Journal of Colloid and Interface Science, Vol. 48, No. 2, August 1974