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Role of Self-Assembled Surfactant Structure on the Spreading of Oil on FlatSolid Surfaces

Bingquan Li, Ponisseril Somasundaran, Partha Patra

PII: S0001-8686(14)00140-7DOI: doi: 10.1016/j.cis.2014.04.004Reference: CIS 1435

To appear in: Advances in Colloid and Interface Science

Received date: 25 November 2013Revised date: 10 April 2014Accepted date: 10 April 2014

Please cite this article as: Li Bingquan, Somasundaran Ponisseril, Patra Partha, Roleof Self-Assembled Surfactant Structure on the Spreading of Oil on Flat Solid Surfaces,Advances in Colloid and Interface Science (2014), doi: 10.1016/j.cis.2014.04.004

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Role of Self-Assembled Surfactant Structure on the Spreading of Oil on

Flat Solid Surfaces

Bingquan Li1, Ponisseril Somasundaran

1 and Partha Patra

1*

1Langmuir Center for Colloids and Interfaces, Columbia University, New York, 10027

ABSTRACT

Uniform spreading of oil on solid surfaces is important in many processes where

proper lubrication is required and this can be controlled using surfactants. The role of oil-

solid interfacial self-assembled surfactant structure (SASS) in oil spreading is examined

in this study for the case of hexadecane-surfactant droplet spreading on flat horizontal

copper surface, with triphenyl phosphorothionate surfactants having varying chain

lengths (0 to 9). It is shown that the frictional forces (FSASS) as determined by the SASS

regulate droplet spreading rate according to surfactant chain length; surfactants with

longer chains led to higher reduction in the spreading rate. The extent of such forces,

FSASS, depend on the surfactant density of the evolving SASS, and specific configuration

the evolving SASS exhibit as per the orientations of the surfactant chains therein. Thus,

FSASS = [k1 + k2(t)] Γδ(t), where Γδ(t) is the surfactant adsorption density of SASS at time 't'

during evolution, and, k1 and k2(t) are the force coefficients for Γδ(t) and orientations (as a

function of spreading time) of the surfactant chains respectively. As a SASS

evolves/grows along with adsorption of surfactants at the spreading induced fresh

interface, the k1Γδ(t) component of FSASS increases and contributes to reduction in the net

spreading force (S). With decrease in the net spreading force, the existence of a cross-

over period, during which the transition of the spatial dynamics of the chains from

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disordered to realignment/packing induced ordered orientation occur, has been inferred

from the FSASS vs. chain length relationships. Such relationships also suggested that the

rate of realignment/packing is increased progressively particularly due the

realignment/packing induced decrease in the net spreading force. Therefore, the

realignment process is a self-induced process, which spans a measurable period of time

(several minutes), the cross-over period, during which the net spreading force decreases

essentially due to such self-induced process.

Key Words: Oil spreading, surfactants, self-assembled structures, Tanner’s law,

frictional forces

*Corresponding Author

Partha Patra

Email: pp2295@columbia.edu

Phone: +1 212 854 2925

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1. INTRODUCTION

An understanding of the mechanisms by which oil-solid interfacial self-

assembled surfactant structures (SASS) – surfactant film - impart specific oil spreading

behavior is fundamentally important to many applications including engine oil

lubrication, coating, painting, oil recovery, micro-fluidics, and drug delivery.1-6

Depending on the concentration of surfactants and their molecular architecture (polarity

of the head group, and chain length and branching), interfacial surfactant films attain

form such as uniform monolayer or irregular hemicelles.7, 8

SASS formation is not

necessarily a spontaneous event but structurally evolves along with adsorption of

surfactants at the fresh oil-solid interface generated during spreading and,

alignment/packing of the surfactants in the SASS. In spite of interference from structural

and configurational dynamics associated with evolution of SASS, possibly spanning the

entire spreading duration, the spreading behavior has been seen to follow Tanner's power

law.9

(1)

where, ‘R’ is the radius of a droplet on a surface, ‘t’ is the spreading time, ‘σ’ represents

surface tension, ‘η’ refers to the droplet viscosity, and ‘Ω’ is the droplet volume. Here,

spreading behavior of hexadecane droplet (having triphenyl phosphorothionate (TPPT)

type surfactants of varying chain lengths) on a flat horizontal copper surface was studied

to determine the role of surfactant structure on spreading behavior. Self-assembly of the

surfactants upon their adsorption at the solid-oil (s/o) interface and along the solid/oil/air

(s/o/a) contact line can be viewed microscopically as a flexible soft SASS "tray"

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electrochemically glued to the interface and having wedges at the s/o/a contact line. (Fig.

1). The forces (FSASS) – frictional forces at the SASS-oil interface – which vary with

structural evolution are governed by surfactant density and alignment/packing of

surfactants. In order to determine the progressive effects of an evolving SASS on the

spreading rate (determined as n = ln (normalized base area of a droplet)/ln t, area/time),

particularly with emphasis on structural evolution being unique as per surfactant

structures that constitute a SASS, this study focuses on the how variations in surfactant

chain lengths from 0 to 9 regulate spreading rate. Typically, for surfactants having similar

head groups, adsorption density and alignment of the chains in the SASS are dependent

on the surfactant chain length. 10

Fig. 1. Illustration of the forces contributing to spreading of a hexadecane-

surfactant droplet on a flat horizontal copper substrate. SASS - resembling a

flexible tray (blue in color) - can be seen at the ‘substrate Cu’-droplet interface;

viscous forces (FVISC) are in the spreading direction and FSASS towards the droplet

center.

2. EXPERIMENTAL

FVISC

FVISC

FSASS

Hexadecane + Surfactants SASS - A Flexible Tray

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Materials

The non-aqueous solvent used was hexadecane (>99%, Sigma). The copper metal

surface of 100 nm thickness was prepared by thermal evaporation (Edwards BOC Auto

306) of 99.99% pure copper (Kurt J. Lesker co.) from a tungsten boat followed by

deposition on a silicon wafer (University Silicon) at a rate of 2 Å/sec and at 5 x 10-7

Torr

pressure. The surface tension of solid copper is about 1300 mN/m.11

Triphenylphosphorothionate type surfactants having different chain lengths [triphenyl

phosphorothionate (TPPT), butylated triphenyl phosphorothionate (butylated TPPT),

nonylated triphenyl phosphorothionate (nonylated TPPT)] were obtained from Ciba

(Fig.2). Solutions of these surfactants were prepared in hexadecane at desired

concentrations.

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Fig. 2. Molecular structures of triphenyl phosphorothionate (TPPT), butylated

triphenyl phosphorothionate (Butylated TPPT), nonylated triphenyl

phosphorothionate (Nonylated TPPT) surfactants

Imaging and analysis of oil droplet spreading

The substrate was placed on the stage of a microscope (Nikon) and a 2 μL

droplet of the surfactant solution was gently placed onto a stage using a syringe and

taking care to avoid any effect due to the loading impact. The ambient temperature was

controlled at 25 °C. The images of the droplet during the spreading process were captured

by a Hitachi CCD camera from top and recorded by a Labview program at preset

intervals. The base contact area values of the droplets were analyzed from the images

with software ImageJ (National Institutes of Health). The droplet area was measured as

soon as a pure hexadecane droplet was placed on the Cu metal surface. This area was

accounted for in the estimation of the normalized areas of the droplets during spreading.

Rheological measurements

Kinematic viscosity values of hexadecane-surfactant solutions were measured using an

Anton Paar DSR rheometer. 100 mL of solution was poured into a glass cylinder and

measurements were taken using a vane type probe, and at 25 0C. Hexadecane density was

considered to be 0.77 gm/cm3. Kinematic viscosity values were determined within the

shear rate range of 0.1-100 s-1

, and using Cannon viscosity standards.

3. RESULTS

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Effect of surfactant chain length on spreading rate

The effects of TPPT surfactants having 0, 4 and 9 alkyl groups on spreading

behavior of hexadecane droplets were studied in the concentration range of 0.01 to 2 wt.

%. Particle size measurement was carried out to investigate surfactant aggregation in the

oil phase and surfactant aggregation was not observed in the concentration range from

0.01-2 wt. %. Ideally, it can be inferred from numerous studies that a SASS exhibits a

configuration where the head groups are contact with the substrate and the chains being

buried in the oil phase (Fig. 1). 7 Fig. 3 indicated that irrespective of the surfactant

concentrations in the droplets the spreading rate decreased (smaller droplet areas) with

increase in the chain length: 0<4<9. Complete wetting of a pure hexadecane droplet on a

flat copper surface (surface tension 1300 mN/m)11

was observed with the droplets areas

'A' scaling with time, t, as A ~ t0.2

. For a droplet having TPPT at 0.01 wt. %, the spreading

rate was 'n'~0.2 (Fig 4a). The spreading rate decreased further at higher TPPT

concentration (0.1 wt. %) and exhibited two regimes, the first regimen being faster than

the second; in comparison to 'n'~0.2 at 0.01 wt. %, 'n' was 0.11 in the faster regime (0th

to 8th mins) and 'n'~0.019 in the slower regime (from 8th min to droplet pinning) at 0.1

wt. % TPPT (Fig. 4 b). At 2 wt. % of TPPT, droplet pinning was observed in less than 1

min (Figs 3 and 4c). The spreading rate with ‘butylated TPPT’ (4 alkyl groups) at 0.01

wt. % was similar to that of TPPT, i.e., 'n' ~ 0.2. With increase in the concentration of

butylated TPPT from 0.01 to 0.1 wt. % the spreading rate decreased with 'n' values as

0.08 and 0.015 (n~0.11/0.019 for TPPT) in faster and slower spreading regimes

respectively. As similar to the effect of TPPT surfactants at 2 wt. %, with butylated TPPT

type surfactant, droplet pinning was observed in less than 1 minute (Fig. 4c) at 2 wt. %.

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With nonlylated TPPT (9 alkyl groups), even at a lower (0.01 wt. %) concentration, the

spreading rate reduced considerably in comparison to that with TPPT or butylated-TPPT,

i.e., 'n' values were 0.13 and 0.04 (Fig. 4b) respectively in faster and slower spreading

regimes (compared to 'n'~0.2 for both TPPT

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1st min 26th min 51st min

TPPT

Butylated

TPPT

Nonlylated

TPPT

n = 0.2

n = 0.019

n = 0.009

n = 0.2

n = 0.015

n = 0.009

n = 0.04

n = 0.008

n ≈ 0

Fig. 3. Optical images of hexadecane-surfactant droplets at 1st, 26

th and 51

st min

during spreading, with droplets having varying (0.01 0.1and 2 wt. %)

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concentrations of TPPT, butylated TPPT, and nonylated TPPT. The spreading

rate, ‘n’, at 51 minutes are shown.

0.0 0.4 0.8 1.2 1.6 2.0-0.1

0.0

0.1

0.2

0.3

0.4

0.5

Lo

g(N

orm

aliz

ed

Ba

se

Are

a)

Log(Time), min.

TPPT 2%

Butylated TPPT 2%

Nonylated TPPT 2%

0.0 0.4 0.8 1.2 1.6 2.0-0.1

0.0

0.1

0.2

0.3

0.4

0.5 TPPT, 0.01%

Butylated TPPT, 0.01%

Nonylated TPPT, 0.01%

Lo

g(N

orm

aliz

ed

Ba

se

Are

a)

Log(Time), min.

t0.2

t0.13

t0.04

0.0 0.4 0.8 1.2 1.6 2.0-0.1

0.0

0.1

0.2

0.3

0.4

0.5

Log(Time), min.

Log(N

orm

aliz

ed B

ase A

rea)

TPPT, 0.1%

Butylated TPPT, 0.1%

Nonylated TPPT, 0.1%

t0.11

t0.019

t0.015

t0.08

(a)

(b)

(c)

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Fig. 4. Spreading behavior (droplet area as a function of spreading time) of

hexadecane droplets having TPPT, butylated TPPT, and nonylated TPPT type

surfactants at different concentrations (a) 0.01 wt. %, (b) 0.1 wt. % and (c) 2 wt. %

and butylated TPPT). The marked contribution of the 9 alkyl groups to the reduction in

spreading rate was evident when the droplet exhibited significantly slower spreading rate

(n=0.008) - almost pinning - at lower concentration (0.1 wt. %).

4. DISCUSSION

Macroscopic force balance accounting for frictional forces at SASS-oil interface

The interfacial tension of either s/o (solid/oil) or o/a (oil/air) interface decreases with

increase in surfactant chain length; in particular, the s/o interfacial tension 'σs/o' decreases

significantly with increase in chain length. 12

The spreading power which is a parameter

for the measure of wetting is determined as: 13

(2)

Where, ‘S’ indicates spreading power, σs/a, σo/a and σs/o indicate solid-air, oil-air and

solid-oil interfacial tensions respectively. If and decreases with increase in

chain length, it suggests in accordance with Equation 1 that the spreading power 'S' is less

negative with increase in chain length, suggesting wetting, and increase in the spreading

rate. On the contrary, a reduction in the spreading rate with increase in chain length

indicated that their exist forces that contribute to the reduction in the spreading rate which

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are opposite in direction of the forces which promote spreading, namely the viscous

and/or capillary forces.14

As such forces led to reduction in the spreading rate with

increase in surfactant chain length, the extent of these forces owe to chain length

dependent structural forms of the SASS, and variations in and are assumed

negligible. It has been reported that self-assembled surfactant structures impart frictional

forces (FSASS), and thereof, modulate the oil spreading rate.13, 15-16

Generally, in

assessment of the contributions of the forces that govern droplet spreading, the capillary

forces balance with the viscous forces.18, 19

∞

+

= (3)

The left hand side of the equation includes capillary forces, where γ and θd indicates

interfacial tension and oil droplet contact angle respectively. The right hand side of

equation 2 indicated forces owing to viscous forces ( ). In corroborating this

relationship with the chain length dependent reduction in the spreading rate, balancing of

the contributions from the viscous and capillary forces theoretically predict faster

spreading with increase in chain length, and thus, these forces fail to account for the

observed reduction in the spreading rate. Hence, it is further clear that the contributions

of the frictional forces, FSASS, during structural evolution of SASS are measurable enough

to cause reduction in spreading, where the frictional forces vary according to chain length

dependent SASS types and their evolving forms in course of spreading.

∞

(4)

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Table 1. Kinematic viscosity of hexadecane as a function of chain-length (0-9) and

concentration (0.01 – 2 wt. %)

Chain

Length Kinematic viscosity (centistokes, cs) vs. Surfactant concentrations

0.01 wt. % 0.10 wt. % 2 wt. %

0 4.458 4.698 5.158

4 4.558 4.498 5.258

9 4.498 4.598 5.158

Here, Table 1 shows that with increase in the TPPT chain length the changes in the

kinematic viscosity values of hexadecane-surfactant solutions are negligible. Thus, the

contributions of the chain length dependent variations in the capillary forces to the

reduction in spreading is negligible, and therefore, the FVISC predominantly regulate the

droplet spreading behavior. Thus, the force balance for oil spreading can be written as:

S = FVISC + (− FSASS) (5),

where ‘S’ is the spreading power. The term FSASS is negative as the direction of

these forces are opposite to that of the spreading direction - viscous forces. The FSASS

depends on the SASS surfactant density at the solid-oil interface and is derived as:

(6),

12

where, ‘Γδ’ denotes adsorption density, ‘ ’ represents surfactant chain length and C0 as

the concentration of surfactants in an oil droplet. As the interfacial area A0 (spontaneous

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area as soon as a droplet is placed on the copper surface) increases during spreading, the

adsorption density, Γδ(t), depends on normalized (‘interfacial area’ wise) bulk surfactant

concentration at time ‘t’, Ct, where Ct changes with increase in interfacial area ΔA as [C0

A0]/ [(A0 + (ΔA)]. If ΔA is derived from Tanner's law, then

(7)

As ‘Ω’ and ‘η’ values can reasonably be considered as constant here for the

hexadecane-surfactant system, the ‘Ω3/10

(1/η) 1/10

’ term equates to a constant value K.

Thus, with initial surfactant concentration as C0 and droplet area A0 at time t = 0, the

adsorption density, Γδ (t), at any time ‘t’ is a function of ‘n’, σ and as:

(8)

With increase in chain length Γδ(t) is higher and correspondingly the frictional

forces FSASS. Due to significantly higher Γδ(t) at higher (2 wt. %) concentration of TPPT

type surfactants in a droplet, higher FSASS resulted in droplet pinning at an early stage

(Fig. 4c). A similar explanation applies for spreading of droplets having lower (0.01 wt.

%) surfactant concentrations, where the droplet spreading rate changed moderately due to

lower adsorption density. In the intermediate concentration (~ 0.1 wt. %) range, the

spreading rate, as shown through Figs. 4, 5 and 6, demonstrated two spreading regimes.

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Two spreading regimes exhibited different spreading rates, faster followed by slower.

Fig. 5 shows that in either of the two spreading regimens the FSASS (represented as -n =

(noil - nsurfactant)) from the evolving SASS are higher with increase in chain length. After a

certain period of time, from the time spreading commenced, the faster spreading rate

reduced markedly, and progressively thereafter and, until an equilibrium spreading rate

value had been attained. Such marked reduction in the spreading rate at certain time

during spreading is not due to spontaneous increase in the surfactant density in SASS,

but, we propose that the marked increase in the forces that led to such reduction in

spreading rate is predominantly due to changes in the structural configuration of SASS.

Such configurational changes occur through the process of realignment/packing of

surfactants in the SASS, where the outcome of this process was contributive to the

reduction in the spreading rate only under conducive oil spreading dynamics at end of

faster spreading regimen. While the realignment process features surfactants attaining

uniformity in terms of their spatial orientations (relative to the horizontal substrate base),

in packing, the intermolecular chains tend be at closer proximity. 17, 18

A few research

investigations reporting effect of SASS on oil spreading behavior infer that realignment

of chains could lead to changes in the spreading rate. 17

Here, accounting for the

realignment/packing of surfactant in the SASS, the frictional forces increase as: ‘k2(t)

Γδ(t)’, where k2(t) is the force coefficient owing to the realignment/packing of surfactants

in the SASS. Thus, under scenarios where chain length induced variations in the

interfacial tensions have a negligible effect on the spreading behavior, which is seen here,

the net spreading power at time ‘t’ can be written as:

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(9)

0 4 8

0.10

0.15

0.20

FS

AS

S, m

easure

d a

s: [-n

oil-n

su

rf.]

Chain (Alkyl) Length

Faster Spreading Regimen

Slower Spreading Regimen

Fig. 5. Realignment/packing induced frictional forces as a function of surfactant

chain length for hexadecane droplets having TPPT types surfactants in the

intermediate concentration range (0.1 wt. %).

≈ [(k2) Γδ (t))]

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0.4 0.8 1.2 1.6 2.0

0.0

0.1

0.2

TPPT, 0.01wt%,

butyl-TPPT, 0.01wt%

nonly-TPPT, 0.01wt%

TPPT, 0.1wt%,

butyl TPPT, 0.1wt%

nonly-TPPT, 0.1wt%

Spre

adin

g r

ate

, n

Spreading time (min), Log scale

Fig. 6. Spreading rate coefficient vs. spreading time relationship demonstrates

time of initiation (vertical dotted lines) and time-span of the cross-over regimes

(vertical bars). The horizontal bars indicate cross-over periods for nonylated-

TPPT (blue) at 0.01 wt. %, and TPPT (green) and butylated TPPT (red) at 0.1 wt.

%.

Fig. 6 shows that there exists a measurable (several minutes) period of time in the

entire spreading duration during which the spreading rate, represented as ‘n’,

progressively reduces to an equilibrium value. The forces contributive to such reduction

in the spreading rate corroborates to the differences (Figs. 5 and 6) of the forces in the

faster and slower spreading regimes. These forces, k2(t)Γδ(t), measured as the differences

(Fig. 5) of the forces in the faster and lower spreading regimens are significantly high

enough and cannot be accounted to an increase in the surfactant adsorption density, as in

such a short period of time it is unlikely that there will be a flux of surfactants adsorbing

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onto the surface to consequence a significant increase in the frictional forces. Thus, the

transitional (faster-to-slower) period, during which the FSASS increases, attributes to

realignment/packing of the surfactants in SASS. For aqueous surfactant solutions, similar

spreading regimes have been demonstrated, where the transitional/‘cross-over period’ is

described as molecular kinetic regime followed by a hydrodynamic regime, 19-22

and

according to the power law shift from t1/7

to t1/10

, with an asymptotic regime

corresponding to a longer relaxation time to equilibrium. Furthermore, the characteristic

time at which the cross-over between the regimes occur is of the order of seconds, 19-22

much shorter than the time in this study, where, it is ~6 mins for 0.01 wt.% nonylated

TPPT solution and, 5 and 7 mins for 0.1 wt.% TPPT and butylated TPPT respectively.

The exponent 'n' in the power law is much smaller than either 1/7 or 1/10, i.e., 1/50. The

time period spans from the commencement of the realignment/packing process to until an

equilibrium state (spreading rate, ‘n’, in slower spreading regimen) is reached. During

this period, the realignment/packing induces increase in frictional forces, k2(t)Γδ(t), which

contribute to the reduction in the spreading rate until a steady value is reached. Fig. 6

shows that the extent of cross-over period has no particular relationship with the

surfactant chain length, where the extent spans several minutes (~ 6 minutes) and the rate

(slope of the line ‘n’ vs. spreading time in the cross-over period) at which k2(t)Γδ(t)

increases is similar. Such regularity also suggest that the reduction the spreading rate in

the cross-over period is less dependent on the net spreading power and, there exist other

self-induced forces that govern chain realignment/packing. By following the progression

of the changes in the spreading rate, which progressively decreases in the cross-over

period, it can be inferred that the realignment/packing event that occurred at time ‘t’

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promotes the event that occurs at time ‘t+∆t’. It is notable that the spreading rate at time

‘t+∆t’ is less that that at ‘t’. Thus, the realignment/packing event at time ‘t’, which

resulted an increase in the frictional forces, k2(t)Γδ(t), contributed towards an increase in

the rate of realignment/packing in the duration ∆t. Thus, increase in the rate of

realignment/packing is likely to increase during the cross-over period until an equilibrium

spreading rate is attained. Thus, realignment/packing event is a self-induced process.

5. CONCLUSIONS

For spreading of oil-surfactant droplets on flat surfaces, depending on surfactant

adsorption density, Γδ(t), at the oil-‘flat surface’ interface, the frictional forces (FSASS) at

the ‘interfacial self-assembled surfactant structure (SASS)’-oil interface can be

measurable enough over viscous (FVISC) and capillary forces to regulate the spreading

behavior. Such forces increase progressively as a SASS structurally evolves along with

adsorption of surfactants at the spreading induced fresh interface. The frictional forces act

in a direction that is opposite to the spreading direction, and the extent of such forces at

time ‘t’ during spreading can be accounted as: FSASS = [k1Γδ(t)]. In addition, the extent of

such forces at any particular time during spreading also depend on the surfactant

adsorption kinetics, where, for surfactants which adsorb spontaneously, e.g., amino acid

based surfactants, FSASS = [k1Γδ(t)] will be measurable enough early on in spreading.

Thus, the extent of frictional forces primarily depend on the adsorption density and

adsorption kinetics; accordingly, it is seen here that as the substrate affinity of surfactants

having longer chains are higher the frictional forces are higher. As spreading progresses,

the frictional forces which restrict spreading increase, and the effect of FVISC which

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promotes spreading decreases. Thus, in the entire spreading regime, depending on the

relative measure of both k1Γδ(t) and FVISC, the spreading behavior in term of spreading rate

can be categorized as: 1) faster spreading rate, where FVISC is significantly higher than

k1Γδ(t), which is due to lower adsorption density, 2) significantly slower spreading rate

from the beginning, almost pinning behavior, where k1Γδ(t) is significantly higher than

FVISC due to faster adsorption kinetics and higher adsorption density, and, 3) mixed

spreading behavior, where depending on relative values of FVISC and k1Γδ(t) the spreading

rate is higher at the beginning and slower in the later stage. Depending on the droplet

volume the spreading rate is likely to be regulated by the FVISC in the first regime, 23

and,

in the second regime, dewetting in separate regions across the s/o interface could be a

possibility due to formation of SASSs in patches. For droplets exhibiting mixed

spreading behavior, there exist a cross-over period during which progressive transition in

the spreading rate from faster to slower occur. At the beginning of the transition period St

(net spreading power) becomes significantly less to promote realignment/packing of the

surfactants/chains in the SASS. As seen here, realignment of chains is not a spontaneous

event and spans several minutes, particularly for spreading of oil-surfactant droplet,

during which there is a measurable increase in the frictional forces, especially owing to

the realignment/packing process. The process of realignment/packing, which is favorable

under quiescent spreading dynamics, would typically include uncoiling of the chains,

chains attaining similar orientations, and packing – reduction in the distances between the

surfactants. A better understanding of how realignment process occur and the steps

thereof, sought further investigation.

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6. ACKNOWLEDGEMENTS

The authors are thankful to NSF I/UCR Center for Particulate and Surfactant Systems for

supporting the research program.

7. REFERENCES

1. Ananthapadmanabhan KP, Goddard ED, Chandar P. Colloids and Surfaces 1990; 44:

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23. David R. Heine, Gary S. Grest, and Edmund B. Webb III, PHYSICAL REVIEW

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Table 1. Kinematic viscosity of hexadecane as a function of chain-length (0-9) and

concentration (0.01 – 2 wt. %)

Chain

Length Kinematic viscosity (centistokes, cs) vs. Surfactant concentrations

0.01 wt. % 0.10 wt. % 2 wt. %

0 4.458 4.698 5.158

4 4.558 4.498 5.258

9 4.498 4.598 5.158

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Graphical Abstract

GRAPHICAL ABSTRACT

Hexadecane + Surfactants SASS - A Flexible Tray

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Highlights

1. Self-assembled surfactant structures (SASS) at oil-‘flat solid’ interface regulate oil

spreading

2. SASS structurally evolves

3. During evolution, spreading is regulated as per surfactant density and orientations of

surfactants

4. Orientation of chains is seen to be an important process in the reduction of spreading

rate

5. For longer chained surfactants, the forces owing to adsorption density and orientations

are higher

6. Realignment is a self-induced process leading to increase in frictional forces in cross

over period