Protein and ionic surfactants - promoters and inhibitors of contact line
  pinning
RSC Communication Template (Version 2.1)
Protein and ionic surfactants - promoters and inhibitors of contact line 
pinning 
Viatcheslav V. Berejnova
We report a new effect of surfactants in pinning a drop contact 
line, specifically that lysozyme promotes while lauryl sulfate 
inhibits pinning. We explain the pinning disparity assuming 
difference in wetting: the protein-laden drop wets a "clean" 
surface and the surfactant-laden drop wets an auto-precursored 
surface.  
 To date, the effect of surfactants on a drop's wetting and 
spreading has been well established 1-12. It was observed that low 
molecular weight surfactants extend spreading and decrease the 
contact line stability 13, 14. However, the effect of surfactants and 
proteins on pinning a drop contact line has not yet been thoroughly 
appreciated despite being a primary issue for several important 
methods in life science. Optimization of the pinning conditions 
could benefit the crystallization of globular and membrane proteins 
15, 16, formulation of the pesticide sprays for protecting the plants 1, 
17, and the influence of the pulmonary surfactants on physiological 
wettability of alveoli in the lungs 18.  
 In this letter, we first study the quasi-static pinning of the protein 
and surfactant drops pinned by siliconized glass slides. We 
demonstrate that lysozyme (Lys) increases the hysteresis effect and 
stabilizes the drop contact line, enhancing the size of the 
completely pinned drops by a factor of 4-5 (compared to water). 
Conversely, the sodium dodecyl sulfate (SDS) reduces the apparent 
contact angles, decreasing the size of the completely pinned drops 
by a similar factor. We found the protein pinning to be similar to 
pinning induced by the geometrical and chemical corrugations of 
the contact line 15.  
 In our experiments, DI water was purified by NANOpure II 
(Barnstead, Boston, MA). Lys protein, 6-x times crystallized hen 
egg white, was purchased from Seikagaku America (Mr~14 kD, 
lot: LF1121, Falmouth, MA). Lys was dissolved in a 50 mM 
sodium acetate buffer with pH = 5. pH of the SDS (0.29 kD, 99%, 
Sigma) solutions was 9. All solutions were filtered.  
 Flat, siliconized 22 mm glass slides HR3-231 were purchased 
from Hampton Research (Laguna Niguel, CA). On a new 
siliconized slide, a water drop with a volume of ~20 μL formed a 
reproducible contact angle of ~(92±1)o. The siliconized material of 
the HR3-231 slides is similar to the organosilane-composed 
solution AquaSil (Hampton Research). We inspected the surface 
topography for some of the HR3-231 slides using a contact mode 
AFM (DI MultiMode III, Santa Barbara, CA) with NSC 1215 tips 
from MikroMasch. We found the manufacturer's coating to be 
homogeneous. We treated some hydrophobic slides chemically 15 
in order to obtain the highly hydrophilic (contact angle <2°) 
circular area. Both the edge and hydrophilic-hydrophobic gradient 
provide a high threshold of pinning 15 that was taken as a reference 
in our experiments.  
Fig.1 Dimensionless diagram of the stability of inclined drops.  The 
symbols: ■, □, and ∆ correspond to Lys (5 mg/mL), buffer (Bf), and SDS 
(33.3 mg/mL) solutions, respectively. Insert (a) is a geometrical sketch of 
an inclined drop.  Insert (b) subtracts Vc from the raw-data diagrams 
(sin(α), V). The images 1, 2, and 3 represent three pinning zones: 1 - an 
absolutely stable drop; 2 - a drop is stable up to certain inclination < 90o, 
depending on V; and 3 - a drop is unstable and moves continuously.  
 Each drop was dispensed manually onto initially horizontal glass 
slides, which were then slowly rotated in 2-4° steps (Fig. 1a). For 
the Lys drops the relaxation time between rotations was ~1 min to 
allow the transient disturbances of the drop to dissipate. For the 
SDS drops the initial spreading was similar to that described earlier 
7-10, 12 and the relaxation time interval was different depending on 
concentration. We did not observe the autophobic 14 and 
Marangoni-induced 13 contact line displacements. The apparent 
advancing θa and receding θr contact angles were measured for 
some drops at quasi-static equilibrium using a horizontal 
goniometer 19. Dispensed drop volumes were accurate to 0.1-0.5%, 
and tilt and contact angle measurements were accurate to 1-2°. The 
surface tension γ for the Lys solutions was measured using the 
pendant drop counting method 15. 
 We characterize pinning by measuring the critical tilt α 
corresponding to continuous motion of an inclined drop of volume 
V 19. We observed that the scaling law sin(α)~V-2/3 for large α may 
fit the (V, α) data for the different surfactant concentration C over a 
broad range of the supercriticality ratio V/Vc (Fig.1). Vc is a result 
of fitting, depends on C, and denotes the critical volume 
corresponding to a vertically (sin(α)=1) pinned drop (Fig.1b). The 
function (C, Vc) characterizes the surfactant pinning (Fig.2).   
a IESVic, University of Victoria, Victoria, BC V8W 3P6, Canada  
E-mail: berejnov@gmail.com 
 Two different regimes of pinning can be seen in Fig.2 depending 
on the chemical nature of the used surfactants. For very low 
concentrations close to point A, pinning of the Lys and SDS-laden 
drops behaves similarly. In the moderate and high concentration 
regions, adding Lys (triangles) and SDS (squares) increases and 
decreases the critical volumes of the pinned drops, respectively. 
The white and black circles represent the Lys drops initially 
dispensed on the circularly treated hydrophilic patterns with 
different radii 
15 having edge profiles presented in Fig.2c. These 
patterns do not affect pinning of the SDS-laden drop. Both the 
similarities between the natural and pattern-induced pinning for the 
Lys drops and the differences in pinning between Lys and SDS are 
very remarkable.  
 Summarized below are our results corresponding to the different 
pinning regimes presented in Fig.2. First, pinning is associated with 
the appearance of a trace of liquid film behind the drop 20-22 that can 
be treated as a sign of high adhesion. Verifying this, we mark 
intervals on the concentration axis in Fig.3a and Fig.3b where the 
trace behind the drop body appears. The buffer and pure water do 
not exhibit any traces on the hydrophobic slide (Fig.4a), but the 
treated slides show unstable traces covering the treated area 
(Fig.4c). The Lys and SDS-laden drops show similar traces (Fig.4b 
and Fig.4d). However, high pinning was observed only for the Lys 
drops (Fig3a and Fig3b). The SDS decreases pinning in the trace-
region. Thus, the presence of the trace does not correlate with the 
strength of pinning for our choice of surfactants. 
 Second, an equilibrium of an inclined drop yields the formula 
ρVcgsin(α)~DγΔ(cosθ)r,a 
19, 20, 23, where ρ, g, and γ are the density 
of liquid, gravity, and the surface tension, respectively, and the 
contact angle difference Δ(cosθ)r,a equals cosθr-cosθa. Thus, the 
critical volume of an inclined drop (or pinning) is proportional to 
the wetted diameter D.  Roughly, we can treat D as the diameter of 
an initially dispensed drop onto a horizontal substrate. This 
approximation holds in small drops when deformation of the drop 
surface describing by the Bond number Bo=ρgD2/γ (d1 for our 
case) is not too large. For the concentration range presented in 
Fig.3a we did not find a noticeable variation in D for the naturally 
dispensed Lys drops for either inclination or concentration. For the 
treated slides presented in Fig.2, increasing D qualitatively agrees 
with the above formula for low deviations of D in the Lys drops. 
The patterns with larger radii (open circle) provide higher drop 
stability (larger pinning). The diameters of the SDS drops on 
hydrophobic slides grow monotonically with concentration 7. 
However, unlike the Lys, the SDS decreases the drop pinning with 
respect to the concentration. Therefore, the D variation does not 
affect pinning for the SDS case, but it does for the Lys.  
Fig.3 Combined diagrams of critical volumes, contact angles, their 
difference, and surface tension versus the component concentrations. 
Black circles are γ, (a): Lys and (b): SDS 24. ∆, ▲, □, and ■ denote Vc 
and Δ(cosθ)r,a for the Lys and SDS drops, respectively. Dotted triangles 
(Lys) and squares (SDS) depict the contact angles: white - maximal θa 
and black - minimal θr.  The zones of a liquid film trace appearing behind 
the quasi-statically displaced drop are marked on (a) and (b). Lines are 
not fits. 
Fig.2 Diagram of critical volumes versus concentration for vertically 
pinned drops. The inserts (a) and (b) represent drops on the untreated ∆ - 
Lys and □ - SDS, and treated ○ - Lys 8.0-diam-mm and ● - Lys 7.0-diam-
mm glass slides, respectively. The point A depicts Vc for water and 
buffer. (c) is an AFM image of the etched-2 versus unetched-1 areas for 
the treated slide (bar is 1μ). Dashed lines are not fits. 
 Third, we found that high pinning corresponds to high θa and 
Δ(cosθ)r,a, (Fig.3a and Fig.3c). For high concentration regions these 
parameters are related because the liquid-trace minimizes θr (Fig.3c 
dotted black triangles and squares). Keeping θa large is the only 
way to keep the difference Δ(cosθ)r,a also large. Adding Lys to the 
drops does not affect the initially high values of θa, and, 
consequently, Δ(cosθ)r,a (Fig.3a and Fig.3c). Conversely, adding 
the SDS decreases θa, controlling the overall low value of Δ(cosθ)r,a 
adequately, and provides the low pinning effect. 
 Forth, the overall effect of concentration is noticeably different 
between Lys and SDS. Fig.3a demonstrates that pinning is tied to 
the Lys amount in the drop. Starting at 10-4 mM, the Lys-pinning 
increases as Δ(cosθ)r,a increases, reaches its maximum 
corresponding to the minimum of θr (appearance of the liquid-
2    
trace), and finally saturates. Next, the high concentration region 
pinning decreases as γ(C) decreases (dγ/dC<0). The SDS-pinning, 
however, is much less sensitive to concentration. For the SDS 
drops, V
c is close to the water drop, is stable up to 10mM, and 
decreases as θr decreases. Additional probing of the Lys and SDS 
solutions with the plain glass slides (HR3-227, Hampton Research) 
does not show a qualitatively different behavior of pinning. For 
plain glass, the Vc(C) curves generally follow the data presented in 
Fig.2, corresponding to the hydrophobic untreated slides for both 
the Lys and SDS surfactants.  
 We see that the appearance of a trace indicating the high 
adhesion between solution and glass surface does not provide high 
pinning unambiguously: Lys increases and SDS decreases pinning. 
Neither does extending the drop's wetted diameter correlate with a 
pinning increase: for Lys, buffer, and water, a variation in D due to 
patterning increases pinning while it does not for SDS. Pinning and 
contact angle difference are correlated as well: an increase in 
Δ(cosθ)r,a corresponds to a pinning increase and decrease for the 
Lys and SDS solutions, respectively. 
 We attribute such a dramatic difference in pinning to the 
different gas-solid surfaces near the contact line. We hypothesize 
that the Lys contact line advances the "clean" 10 solid surface, 
whereas the SDS contact line advances the pre-cursored zone 
existing in close proximity to the contact line. We expect that 
mobility of the SDS molecules through a liquid interface may 
affect the advancing zone. Indeed, such mobility of the small 
amphiphilic molecules is well documented 2-4, 7-10, 12. Though the 
mobility of proteins is not available, we argue it to be negligible 
compared to SDS. The globular proteins are weak surfactants with 
large, less amphiphilic, and therefore much less interfacially mobile 
molecules 25-27 and, in addition, they irreversibly adhere to the 
glass. We assume that the proteins adhere to the solid from the drop 
interior and provide high heterogeneity for the three-phase contact 
that results in the high pinning threshold similar to the corrugated 
case 15 (Fig.2, C>10-2mM). In contrast, SDS penetrates the drop-
solid-air interface, increases affinity between the solid and the drop, 
and thus decreases the pinning threshold.  
Fig.4 Different regimes of displacement; (a): 40 μL drop of 50mM 
acetate buffer at pH=5 (water acts similarly); (b): 40 μL drop of 5mg/mL 
Lys; (c): 40 μl drop of water (bar is 5.5 mm); (d): side view of the 20 μL 
SDS 100 mg/mL drop (bar is 3.4 mm). Numbers are inclination in 
degrees; the slides (a), (b), and (d) are untreated, (c) is treated with D=5.5 
 The author thanks N. S. Husseini for proofreading the 
manuscript.  
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