Application Report
Application report:

AR278

Industry section:

All industries

Authors:

M. Jin, R. Sanedrin, D. Frese, C.
Scheithauer, and T. Willers

Date:

03/2016

Liquid Needle as external unit
or integrated in the MSA

Method:Keywords:

Drop Shape Analyzer –DSA100

Contact angle, liquid needle, drop deposition, super-hydrophobic surface

Replacing the solid needle by a liquid one when measuring
static contact angles
Comparison of the traditional solid needle dosing system and a novel pressure-based dosingsystem, a so-called Liquid Needle
Needle-deposition of sessile drops is a well-established method in contact angle analysis. However, users regularly reportuser-dependent variations of contact angles as well as problems to deposit liquid drops on super-hydrophobic surfaces,or they demand higher deposition speeds. Here we present a new technique for drop deposition that is based on a Liquid
Needle in contrast to a solid one. Further, we highlight the main findings of a scientific study published in the Journal ofColloid and Polymer Science (DOI 10.1007/s00396-015-3823-1) [1] where we thoroughly compared both techniques using
14 different sample surfaces. We show that the resulting contact angles of droplets dosed by either technique areidentical. In addition, we explain how potential pitfalls of needle-dosing – when not carried out very carefully – areeliminated by the alternative Liquid Needle dosing technique.

BackgroundContact angle measurements are widely used as afundamental technique to investigate and describe
surface and interface phenomena. In multiple fields ofindustry such measurements are carried out to optimize
adhesion of coatings and understand wetting of liquidson solid substrates, or as part of quality control processes
to check surface activation as well as surface cleaning
steps.

This way of dosing runs into limitations when superhydrophobic surfaces or fast deposition speeds are anissue [1]. In addition, erroneous results may be obtained
if the drop deposition is not performed very carefully.
Variations in measured contact angles between different
users can occur (see Fig. 1), while it is of high interest to
render the experiment as user-independent as possible.

The basic way how liquid drops are deposited onto solid
sample surfaces has not changed ever since opticalcontact angle measurements had been established: Adrop of liquid is generated at the tip of a needle, which isthen carefully brought in contact with and subsequently
transferred to the sample surface.
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In order to show that the new technique leads to results
equal to those obtained with (thoroughly applied) needle
dosing, we have performed an extensive comparative
study between both dosing techniques.

Experimental section

Fig. 1: Deposition of a drop of water onto a PDMS substrate.
Even small changes in needle position can cause the drop tobe squeezed and thus lead to different (smaller) contact
angles. This effect is particularly important for surfaces
exhibiting large contact angle hysteresis.

To avoid these potential pitfalls, we have developed andimplemented an alternative pressure-based dosing
technique which builds the drop with a well-defined
liquid jet (referred to as Liquid Needle) directly on thesubstrate (see Fig. 2). The Liquid Needle is optimized todeposit a drop as carefully as possible, while otherneedle-less techniques reported in literature introduce aconsiderable amount of kinetic energy into the drop,causing the drop to “splash” onto the surface. Contact
angles obtained from e. g. falling drops therefore
describe the result of a de-wetting process and are mostoften significantly smaller than contact angles obtained
from needle dosing systems.

Static contact angle measurements were performed witha Drop Shape Analyzer – DSA100 and evaluated using
the software ADVANCE. To assure a repeatable and userindependent drop deposition, an automatic procedure
was defined in the software for both dosing systems.
Each different sample was analyzed with the same lightsettings and fitting algorithms. Measurements werecarried out under ambient conditions.
Contact angle measurements were carried out on 14 very
different samples: Hydrophilic surfaces (glass, smartphone display) Hydrophobic surfaces (P2i super-hydrophobic paper,polydimethylsiloxane PDMS)
 Technical polymers (polyethylene PE, polypropylene PP,polyamide PA6)
 Silicon wafers (wafer 1, wafer 2)
 Self-assembled monolayers on SiO2 (dichlorodimethylsilane DDMS, aminopropyltrimethoxysilaneAPTMS)
 Rough surfaces (DDMS-silanized sand papers of grain
sizes 500, 1000, and 1500)
With the samples we covered a broad range of watercontact angles from very low (smartphone display) tovery high (P2i hydrophobic paper) and also different
degrees of surface homogeneity (from homogeneous
SAMs to sand paper samples with a macroscopic surface
roughness).

Fig. 2: Formation/deposition of a drop of water on a PDMS
surface. Left: classical needle dosing system (NDS). Right:
new pressure-based Liquid Needle dosing system (PDS).
Figure from reference [1] with kind permission of Springer.

All samples were cleaned and dried prior to themeasurements. A detailed description of theexperimental setup and the sample preparation can befound in the cited reference [1].

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Results
The results of contact angle measurements on the 14 samples are shown in Fig. 3a and Fig. 3b, with water anddiiodomethane as test liquids, respectively.

abFig. 3: Mean static water (top, 3a) and diiodomethane (bottom, 3b) contact angle as measured after dosing with the NDS (opencolumns) and PDS (filled columns). The error bars reflect the standard deviation as obtained from a minimum of 20 different
drops measured. Figures adapted from reference [1] with kind permission of Springer.

For all samples, both ways of dosing give comparable
contact angles and small standard deviations.
Concerning the effect of the needle deposition on thecontact angle on surfaces with a high contact angle
hysteresis such as PDMS (see fig. 1), one can conclude
that the liquid needle is an even more careful way ofdrop deposition than the solid needle, as the resulting
contact angle on PDMS is even higher for PDS compared
to NDS [1].
For diiodomethane as a second, non-polar test liquid wealso obtained well-comparable contact angles for both
dosing types (Fig. 3b). For glass, the contact angles of
1 µL drops dosed by PDS were significantly smaller thanthose dosed by NDS. This can be explained by the higher
density of diiodomethane compared to water, whichbrings more kinetic energy into the drop [1]. A simple
way to rule out the influence of this additional kinetic

energy on contact angles is to dose a recommended
drop volume of 2 to 2.5 µL instead of 1 µL. Then thekinetic energy of the liquid jet dissipates over a larger
drop volume and both dosing systems yield identicalcontact angles (dark blue columns).
On super-hydrophobic surfaces like the P2i paper, it was
not possible to transfer drops of volumes as small as 3 µL
to the surface with the NDS. This is due to the very lowsurface free energy (very low adhesive interactions) of
this sample. Therefore drops of 6 µL had to be used for
NDS (Fig. 3a, green bar). In contrast, drops of virtually
any volume can be dosed on such samples with the PDS.
This is something we consider a strong point of the PDS.
The fully automatic measurement of 20 drops using theNDS and the PDS usually took around 220 s and 54 s,
respectively, which makes the PDS a much fastertechnique for analyzing a surface.

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Summary
We present a new way of dosing called Liquid Needle
which is based on a liquid jet building up a drop on asolid substrate for static optical contact angle
measurements. This new technique is compared with theclassical technique of using a solid needle for dosing in astudy of two common test liquids on 14 very different
solid substrates.
The obtained contact angles are basically identical forboth drop deposition methods, while the Liquid Needle
method offers some advantages over the classicalmethod: experiments are much faster and dropdeposition on super-hydrophobic surfaces is easilypossible.

Most importantly, any possible influence by the user andresulting issues with reproducibility of results are virtuallyruled out for this novel drop deposition technique. This
will help to further increase the comparability of contact
angles measured by various groups. In conclusion, webelieve that using the Liquid Needle is advantageous formany contact angle studies and may become anotherstandard method in optical contact angle measurements.
A detailed description of all experimental conditions anddiscussion of all results can be found in the article
published in the Journal of Colloid and Polymer Science[1].

Literature[1]

M. Jin, R. Sanedrin, D. Frese, C. Scheithauer and T.
Willers, “Replacing the solid needle by a liquid one
when measuring static and advancing contact
angles”, Colloid Polym. Sci. 294(4), 657-665,
DOI 10.1007/s00396-015-3823-1 (2016).

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