A silicone bubble filled with ferrofluid as a haptic actuator
A haptic actuator has to deform the skin enough to be felt, and do so at a precise point. This thesis studies a way of achieving that without rigid parts: a silicone dome with a 4 mm radius, filled with ferrofluid, that deforms when a magnetic field is brought close. The work built a numerical model that couples the field with the deformation of the membrane, and fabricated sealed prototypes that respond to a magnet. The comparison between the two is qualitative for now: the deformation of the prototypes was not measured with instruments.
Context
A ferrofluid is a suspension of magnetic particles that moves towards regions of higher field. If it is enclosed in an elastic membrane, that displacement becomes pressure on the wall and the membrane bulges. The idea is attractive for tactile interfaces because the actuator is soft, has no moving parts and can be repeated in an array.
The literature reviewed in the thesis deals with ferrofluid flow in channels or with elastomers that have embedded magnetic particles. The intermediate case, a magnetic liquid inside a hyperelastic shell, is the one this work addresses.
Model
Simulated module: Ecoflex 00-50 silicone dome filled with EFH-1 ferrofluid
The model was implemented in COMSOL Multiphysics and solves two problems in sequence. It first computes the field of permanent neodymium magnets. It then applies the Kelvin force to the ferrofluid, proportional to the gradient of the squared field, and solves the deformation of the 1 mm thick membrane with a Yeoh hyperelastic model and large deformations. The ferrofluid is not simulated as a flow: it is treated as a medium that transmits pressure.
Material properties come from data sheets and the literature. They were not characterized in the laboratory.
Computed displacement of the dome next to a magnet
Simulation results
Computed maximum deformation as a function of magnetic flux density, with a third-degree fit
Deformation grows nonlinearly with the field. On the computed curve the dome moves about 0.4 mm at 0.4 T and close to 1.4 mm at 0.9 T. The thesis also reports internal pressures between 2 and 7 kPa.
Two arrangements of three magnets under a row of four domes were compared. With aligned poles the field is more uniform in the central region. With opposed poles the field is stronger and its gradients larger, and the deformation is more concentrated.
Magnetic flux density with three magnets with opposed poles
Deformation of the four domes in that configuration
Fabrication
The molds were 3D printed, in PLA and in resin. The silicone cures in the mold, the ferrofluid is injected through a channel, and a second layer of silicone seals the assembly. The main problem was containing the liquid, and it was solved by adding to the mold a hemisphere that defines the cavity.
Printed mold for a 4 × 4 array of domes
Arrays of 4 × 4 modules at the one-centimeter scale and a miniaturized version were obtained. The fabricated radius was 4.1 ± 0.1 mm, against a design value of 4.0 mm. When a magnet is brought close, the domes deform visibly and remain intact.
Miniaturized array
What is missing
Quantitative validation is missing, and the thesis says so explicitly. There is no experimental curve of deformation against field, and no force or pressure measurements. The figure of 1.0 to 1.1 mm for the prototype is a qualitative estimate.
The numerical values in the document should be read with care, because they are not consistent across sections. The maximum deformation of 1.2 mm is associated with 0.93 T in the abstract and with 0.15 T in the conclusions, while on the computed curve it corresponds to a field close to 0.8 T. The relative permeability of the ferrofluid appears with values between 1.1 and 2.6. Fitting those parameters with the lab's own measurements is part of the pending work.
The fields on the curve, several tenths of a tesla, are obtained with permanent magnets placed very close. A haptic interface needs electromagnets that modulate the field, and that was not addressed. No user tests were carried out either, so it is not known whether the deformation produces a distinguishable sensation. As the immediate next step, the author proposes a test bench with controlled electromagnets and force and displacement sensors.
How it fits in Robiolab
Haptics and soft actuators are two of the lab's central interests, and this thesis joins them with an actuation principle different from the pneumatic one that dominates its other projects. It relates to the characterization of the impedance between a soft actuator and a finger, which provides the kind of measurement that is missing here, and to the piezoresistive sensor array, which tackles the complementary problem of sensing rather than stimulating. It leaves a coupled model ready to be checked and a fabrication process that produces sealed modules.