A moving contact line as a rheometer for nanometric interfacial layers

Abstract

How a liquid drop sits or moves depends on the physical and mechanical properties of the underlying substrate. This can be seen in the hysteresis of the contact angle made by a drop on a solid, which is known to originate from surface heterogeneities, and in the slowing of droplet motion on deformable solids. Here, we show how a moving contact line can be used to characterize a molecularly thin polymer layer on a solid. We find that the hysteresis depends on the polymerization index and can be optimized to be vanishingly small (<0.07°). The mechanical properties are quantitatively deduced from the microscopic contact angle, which is proportional to the speed of the contact line and the Rouse relaxation time divided by the layer thickness, in agreement with theory. Our work opens the prospect of measuring the properties of functionalized interfaces in microfluidic and biomedical applications that are otherwise inaccessible.

Contact angle dynamics on pseudo-brushes: Effects of polymer chain length and wetting liquid

“…The principle of measurement shown in Figure 2a is simple: the capillary rise is measured while the sample surface is plunged in or withdrawn from a liquid bath at a constant, controlled velocity [12,[19][20][21][22][23][24]. With careful implementation of this so-called "dip-coater" geometry, a remarkable precision and range in the measured contact line dynamics can be obtained.…”

“…In this section we briefly recall a simple model of viscoelastic dissipation in a pseudo-brush, which was developed and tested for one particular good solvent [12] but which we will use here to compare the dynamics between different N and different liquids.…”

“…These studies were limited to measuring quasi-static contact angles and did not investigate the dynamics of the contact line. Recently, we examined the wetting of decane on pseudo-brushes of polydimethylsiloxane (PDMS) with a small set of intermediate polymerization indices [12]. We showed that in fact, this type of simple system can exhibit an extremely small hysteresis of < 0.07 • .…”

Dynamics of Soft Wetting

“…The droplets deform the soft solid into a "wetting ridge" that moves along with the contact line, and thus also probe the viscoelastic response of the polymer [21,26]. It has been shown that the response of nanometer scale polymers grafted or adsorbed at a surface presents a scaling law consistent with the picture emerging from statistical physics [72]. However, in order for droplets to be fully useful as a rheological tool, one must have a perfect theoretical understanding of the processes at work.…”

“…From an engineering perspective, there is a continued effort in designing surfaces with low contact angle hysteresis for purposes of hydrophobicity, self-cleaning, or anti-fouling coatings [91,92]. Recently, wetting of soft surfaces has generated a large interest: The liquid-like surface properties of reticulated polymer networks and brushes can offer nearly hysteresis-free substrates [72,80]. However, despite the absence of hysteresis, drops on these soft surfaces do exhibit a stick-slip motion when forced to spread beyond a threshold velocity as observed in chapter 2 and [33,35,79].…”