different drop sizes
Susanna Laurén Sep 27, ’22 ~ 6 min

Effect of drop volume on contact angle

For most of us measuring contact angles, the question of which drop volume should I use has come up. Typically, the drop size used in contact angle measurements is between 1 and 10 microliters. However, there are situations where smaller or sometimes larger drop volumes are needed. Smaller drop volumes are used when the measurement area is so small that microliter drops wouldn’t fit into it. Bigger droplets are sometimes needed when advancing and receding contact angles are measured by sample tilting. But does the drop size affect the contact angle values?

Typical drop volumes used for contact angle measurement

The typical drop size for contact angle measurements is 1 to 10 μl. However, in recent years, the interest in picoliter droplets has increased due to the need of measuring small micropatterned areas or otherwise small objects. A base diameter of microliter droplets is typically larger than 1 mm, whereas with picoliter droplets the base diameter can be as low as 100 micrometers. Then at the same time, when dynamic contact angles or roll-off angles are measured by the tilting method, the drop size of 10 ul is typical. Large drop volumes are needed especially with surfaces that have high contact angle hysteresis, as small drops might not move even if tilting up to 90 degrees is done.

Influence of drop size in microliter range

The influence of droplet size in the microliter range has been widely studied. Drelich [1] has reviewed these studies and concluded that droplet volume has no significant influence on the contact angle with close-to-ideal surfaces, such as clean quartz plates. The ideal surface is considered to be smooth, rigid, chemically homogeneous, insoluble, and non-reactive. However, the larger a contact angle hysteresis is the greater impact droplet volume has on contact angle. Substrate’s disparity from the ideal surfaces, such as chemical heterogeneity and surface roughness, causes the contact angle hysteresis. The contact angle hysteresis can be quantified by dynamic contact angle measurement, in which advancing (the largest) and receding (the smallest) angles are defined. It has been demonstrated that the advancing contact angle is less dependent on the droplet volume than the receding angle, which base diameter is less than 5 mm [2].

The discussion about the effects of droplet size on contact angle has broadened to compare microliter droplets to picoliter droplets. The influence of gravity on the droplet and the rate of drop size reduction due to evaporation are the two major differences between pico- and microliter droplets [3].

Berson et al. [4] showed that contact angle value has a significant effect on the evaporation behavior of picoliter-size water droplets. Droplet mass was shown to decrease linearly when the initial contact angle is small, whereas the decrease was not linear with larger contact angles. Some studies have already shown a comparison between pico- and microliter droplets: Taylor et al. [3] demonstrated that picoliter volume water droplets were comparable with those obtained from microliter volume water droplets on a group of commonly used smooth polymer surfaces. They studied the contact angle behavior as a function of time using a high-speed camera. With microliter droplets, the contact angles were stable with time, excluding the mobile hydrogel polymer surface with which water chemically reacted. With picoliter droplets, contact angle decrease with time occurred in two stages; fast evaporation and spreading during the first 0.5 s, and then a slower stage until it reached the receding value. Thus, contact angle versus time curve indicates also hysteresis of the substrate with picoliter droplets. The initial contact angle value of picoliter droplets correlated well with the microliter droplet contact angle values and was also close to the literature value.

Taylor et al. [3] demonstrated also that with larger droplets and greater influence of gravity, the droplet profile fitting model needs to be chosen with care. With picoliter droplets, both Young-Laplace and circular fitting can be used as the free energy of the system at equilibrium is minimized for a spherical shape [3]. With larger droplets (>1ul) the circular fitting became inaccurate and the Young-Laplace model was shown to give a constant value as a function of droplet volume.

Yang et al. [5] compared pico- and microliter droplet water contact angles on grooved polymethyl methacrylate (PMMA) surfaces coated with plasma polymers as a first study to investigate anisotropic wetting behavior with picoliter droplets. They found significant differences in water contact angles when varying the contact angle from microliters to picoliters, and therefore highlighted the importance of showing drop size alongside contact angle results.

According to the previous studies, it has been demonstrated that droplet volume varying from microliter to picoliter scale has a significant influence on the wetting and drying behavior of water droplets on non-porous substrates. Chemical and topographical heterogeneity highlights the importance of the droplet volume on the contact angle results. 

[1] J. Drelich, “The Effect of Drop (Bubble) Size on Contact Angle at Solid Surfaces”, J. Adhesion, 63, 31 (1997).
[2] A. Marmur, “Soft contact: measurement and interpretation of contact angles”, Soft Matter 2 (2006), 12-17.
[3] M. Taylor, A.J. Urguhart, M. Zelzer, M.C. Davies and M.R. Alexander, “Picoliter Water Droplet Contact Angle Measurement on Polymers”, Langmuir 23,6875 (2007).
[4] A. Berson, E.L. Talbot, P.S. Brown and C.D. Bain, Experimental Investigation of the Impact, Spreading and Drying of Picoliter Droplets onto Substrates with a Broad Range of Wettabilities, NIP27, 27th International Conference on Digital Printing Technologies, Minneapolis, USA, Oct. 2-6, 2011.
[5] J. Yang, F.R.A.J.Rose, N. Gadegaard and M.R. Alexander, “Effect of Sessile Drop Volume on the Wetting Anisotropy Observed on Grooved Surfaces”, Langmuir 25, 2567 (2009).

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