Dynamic wetting on soft solids

• Main Author: Mitra, Surjyasish
• Other Authors: Tuan Tran
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Science::Physics::Descriptive and experimental mechanics; Science::Physics::Optics and light
• Online Access: https://hdl.handle.net/10356/153183
• Institution: Nanyang Technological University

Interaction of liquid drops with soft viscoelastic surfaces bears manifold applications in technology and industry. Applications in tissue engineering, soft robotics, self-cleaning `smart surfaces', cellular biomechanics draw directly from fundamental understanding of how liquid drops wet soft surfaces. Unlike wetting on rigid surfaces, a liquid drop on a soft surface deforms the surface at the three phase contact line. Such deformation is dictated by a balance of liquid capillarity and solid elasticity, a surface property which plays negligible role for wetting on rigid surfaces. Hence, the physics of soft wetting departs signifi cantly from the fundamental principles of rigid wetting. Under static conditions, liquid drop pro les and soft surface response have been extensively investigated and hence their salient features are well known. However, dynamic wetting and how it is influenced by surface elasticity is relatively less known. By dynamic wetting, we refer to situations where either the liquid drop or the surface or both is mobile. Dynamic wetting scenarios arise in manipulating liquid drops in soft PDMS micro fluidic channels, controlling drop impact outcomes in 3D printing, designing reliable slippery surfaces for repelling incoming drops, to name a few. With the motivation of designing durable soft coatings subjected to dynamic wetting conditions, in this thesis, we investigate the response of soft viscoelastic surfaces across a wide range of surface elasticity under two dynamic wetting conditions: spontaneous spreading and impact of low viscosity liquid drops. For both scenarios, we consider systems far from from equilibrium. At the same time, for both the studied phenomenon, the observation time-scale is smaller than the retardation time scale of the soft surfaces used. However, the influence of surface elasticity proves to be less significant for one, while being largely significant for the other phenomenon. We use highspeed optical imaging and highspeed laser interferometry throughout this thesis in investigating the dynamic wetting scenarios. Keeping self-cleaning applications in mind, we fi rst reveal conditions which dictate whether an incoming liquid drop with a fi nite velocity will bounce or wet soft deformable PDMS surfaces. On one hand, we find that within a threshold impact velocity, bouncing prevails, which is caused by an extended air- film lifetime beneath the impacting liquid drops. The bouncing regime observed on soft surfaces is typically more extended than on its rigid counterpart which mostly features early air fi lm rupture and random wetting initiation. On the other hand, impacting drops wet the soft surfaces at higher impact velocity. However, in stark contrast to random wetting initiation commonly observed on rigid solids, here we observe a special case of dimple inversion induced air film rupture at the center and wetting. Further, we reveal how this behavior is caused directly by the retracting air cavity in the bulk of the impacting drop. Subsequently, we experimentally quantify the collapsing mechanism of the air cavity and verify it theoretically. Lastly, we study the jetting behavior post air cavity collapse and propose a theoretical model to predict the velocity of the outgoing liquid jet. For both the impact outcomes, we observe that variation across surface elasticity is nominal. The reason behind this is that the observation time scale of impact is in the range of few milliseconds, much smaller than the relaxation time scale of few seconds. At the same time, the drop never makes contact with the surface and instead skates on a thin film of air, thus no direct forcing is exerted on the soft surfaces. Further, while skating, the dynamical pressure at the bottom of the impacting drop is much smaller compared to the elasticity of the surfaces. Thus, drop-air interface undergoes deformation instead of the soft surface. Compared to impacting liquid drops, a gently deposited liquid drops spreads spontaneously spreads on soft surfaces upon initial contact. The spreading process presents motion of three phase contact line at varying rates. Understanding the motion of three phase contact line is at the forefront of technological applications in coating, printing, lithography, self-cleaning surfaces, etc. In departure from classical rigid wetting, here we reveal characteristics of the distinct stick-slip motion exhibited by moving contact lines on soft PDMS surfaces of varying surface elasticity and thickness. From material design point of view, such stick-slip cycles are unwarranted since they cause an abrupt sharp jump in contact line velocity, generating large shear stresses in the underlying soft surface. We observe that the initial contact line pinning is caused by in-situ surface defects which acts as a barrier to the moving contact line. However, once pinned, the CL is strictly not immobile. Rather, it possesses a fi nite radial displacement, which is markedly different from classical stick-slip characteristics on rigid surfaces. At the same time, the surface starts to deform during contact line pinning. To overcome the growing surface deformation and depin, the contact line increases its dynamic contact angle. We consequently characterize each stick-to-slip (pinning-depinning) transition in terms of the variation of change in dynamic contact angle with the CL velocity during each pinning phase. From our experimental fi ndings, we reveal a scaling law where the change in dynamic contact angle varies inversely with CL pinning velocity. Further, using interference, we observe that though during one pinning phase, short-time dynamic surface deformation below the contact line grows linearly with time. Compared to impact scenarios, here a direct contact line forcing is involved and thus, even though its duration is in milliseconds, it generates sufficient surface deformation enabling stick-slip cycles.