Fluid separation and network deformation in wetting of soft and swollen surfaces

Abstract

When a water drop is placed onto a soft polymer network, a wetting ridge develops at the drop periphery. The height of this wetting ridge is typically governed by the drop surface tension balanced by elastic restoring forces of the polymer network. However, the situation is more complex when the network is swollen with fluid, because the fluid may separate from the network at the contact line. Here we study the fluid separation and network deformation at the contact line of a soft polydimethylsiloxane (PDMS) network, swollen with silicone oil. By controlling both the degrees of crosslinking and swelling, we find that more fluid separates from the network with increasing swelling. Above a certain swelling, network deformation decreases while fluid separation increases, demonstrating synergy between network deformation and fluid separation. When the PDMS network is swollen with a fluid having a negative spreading parameter, such as hexadecane, no fluid separation is observed. A simple balance of interfacial, elastic, and mixing energies can describe this fluid separation behavior. Our results reveal that a swelling fluid, commonly found in soft networks, plays a critical role in a wetting ridge.

Modulus and adhesion of Sylgard 184, Solaris, and Ecoflex 00‐30 silicone elastomers with varied mixing ratios

“…Softer materials usually display higher adhesion as a result of increased contact and deformability, 56,57 but this trend is likely exacerbated by an increase in free chains that may also lead to capillary behaviors. 24,33,52,58,59 When comparing the different material systems, EF is generally less adhesive at similar modulus values to SY and SO. For example, if we consider the W db at a modulus of $10 kPa, SY has the highest W db at $3.0 J/m 2 with SO at $2.7 J/m 2 , and EF has the lowest adhesion at $1.0 J/m 2 .…”

“…The difference in the wetting properties of these three different commercial PDMS elastomers may be due to differences in surface tension, fillers, or the amount of free chains in the elastomers. 24,65,66…”

“…In general, increases as decreases for all samples. Softer materials usually display higher adhesion as a result of increased contact and deformability, 56,57 but this trend is likely exacerbated by an increase in free chains that may also lead to capillary behaviors 24,33,52,58,59 . When comparing the different material systems, EF is generally less adhesive at similar modulus values to SY and SO.…”

“…Their tunable properties and easy moldability make them ideal candidates for stretchable and wearable electronics, 3,4 microfluidics, 4,5 biomaterials, 6–8 cosmetics, 1,9 and soft robotics 1,4,10–15 . Additionally, the ability to tune their mechanical and interfacial properties, combined with their commercial availability in simple, two‐part systems, make silicones useful for fundamental studies on soft materials, including but not limited to adhesion, 9,16–20 friction, 21–23 wetting, 24,25 cavitation, 26 wrinkling, 17,27–29 fracture, 22,26,30 and cell‐surface viability 8,14,31–33 . These elastomers mostly comprise polydimethylsiloxane (PDMS) and, in some cases, additional fillers such as nanoparticles 1,34,35 .…”

Hydrophobic surface patterning with soft, wax-infused micro-stamps

“…The geometry of the deposited paraffin patterns matches that of the original PDMS stamp with very high fidelity. This is significant because PDMS swells with absorption of many of the liquids to which it is permeable [26][27][28]; however, Dangla et al [29] reported no significant swelling of PDMS upon absorption of liquid paraffin, which allows the fully infused PDMS stamp in our method to maintain its original geometry. To verify this, we imaged microscale features on the PDMS stamp before saturation with paraffin and compared it to the resulting pattern deposited using the same stamp (Fig.…”