From Wobble-and-Slide to Rock-and-Roll... and Jump!

Transporting small portions of water is ubiquitous in nature, industrial applications, and microfluidic studies. A common approach is to control water drop motion on chemically coated substrates, creating a hydrophobic (water-repelling) surface that allows the drop to slide without adhering. Alternatively, drops wrapped with hydrophobic powder form liquid marbles, which prevent direct contact with the substrate. Interestingly, these two methods result in completely different actuation modes, even under the same external force.

In this talk, we will demonstrate this difference by rolling water-based ferrofluid drops and marbles with an external torque. Furthermore, we will show how these magnetic liquid marbles self-regulate their modes of motion in response to an applied magnetic torque. A ferrofluid drop on a hydrophobic substrate wobbles back and forth when driven by a rotating magnetic field, generating an asymmetric inertial flow that propels the drop in one direction. In contrast, when a ferrofluid marble rolls, it behaves like an ellipsoid. In the absence of liquid-solid adhesion, the ferrofluid marble can even transition from rolling to jumping under the right conditions. Moreover, we show that evaporating liquid in the ferrofluid marble leads to a much stiffer shell, allowing us to successfully model its rolling behavior using the equation of motion for a force-driven ellipse. This model captures the critical angular velocity for the transition from rolling to jumping. This system offers a new approach for synthesizing soft and stiff non-spherical actuators and provides insight into how softness enhances actuation in complex and compliant environments. For example, we will show how soft liquid marbles behave quite differently from rigid ones: they can climb against gravity and work together to bridge gaps.