Stick&Slip: Altering Fingerpad Friction via Liquid CoatingsAlex Mazursky

Jacob Serfaty

Pedro Lopes

University of Chicago
alexmazursky@uchicago.edu

University of Chicago
jserfaty@uchicago.edu

University of Chicago
pedrolopes@uchicago.edu

Figure 1: We propose Stick&Slip, an approach to friction modulation that works by coating the user’s fingerpads in sticky orslippery liquids, making it possible to alter the friction of nearly any non-absorbent surface, regardless of its geometry. (a) A
user plays a racing game in mixed reality, driving the virtual car over real-world surfaces, such as a table and a small ramp.
(b) To alter the friction of the user’s finger movements on real surfaces, our device deposits liquid droplets onto the user’s
fingerpad, forming an interfacial layer between the two, which allows us to modulate surface friction corresponding to thevirtual events, such as (c) the ice feels slippery to the user’s touch, and (d) the green-goo left by the opponent feels sticky whiledriving the car on the surface.

ABSTRACTCCS CONCEPTS

We present Stick&Slip, a novel approach that alters friction between
the fingerpad & surfaces by depositing liquid droplets that coat thefingerpad. The liquid coating modifies the finger’s coefficient offriction, allowing users to feel surfaces up to ±60% more slipperyor sticky. We selected our fluids to rapidly evaporate so that thesurface returns to its original friction. Unlike traditional frictionfeedback, such as electroadhesion or vibration, our approach: (1)alters friction on a wide range of surfaces and geometries, making it
possible to modulate nearly any non-absorbent surface; (2) scales tomany objects without requiring instrumenting the target surfaces(e.g., with conductive electrode coatings or vibromotors); and (3)both in/decreases friction via a single device. We identified nine
liquids and characterized their practicality by measuring evaporation rates, etc. To illustrate the applicability of our approach,
we demonstrate how it enables friction in virtual/mixed-reality or,even, while using everyday objects/tools.

• Human-centered computing → Human computer interaction(HCI); Interaction devices; Haptic devices; • Hardware → Emerging technologies.

This work is licensed under a Creative Commons Attribution International 4.0 License.
CHI ’24, May 11–16, 2024, Honolulu, HI, USA© 2024 Copyright held by the owner/author(s).
ACM ISBN 979-8-4007-0330-0/24/05
https://doi.org/10.1145/3613904.3642299

KEYWORDS
haptics, friction, adhesion, grasp, mixed reality
ACM Reference Format:
Alex Mazursky, Jacob Serfaty, and Pedro Lopes. 2024. Stick&Slip: Altering
Fingerpad Friction via Liquid Coatings. In Proceedings of the CHI Conferenceon Human Factors in Computing Systems (CHI ’24), May 11–16, 2024, Honolulu,
HI, USA. ACM, New York, NY, USA, 14 pages. https://doi.org/10.1145/
3613904.3642299

1

INTRODUCTION

Touch plays a vital role in our interactions, providing valuable
information about the objects we contact [3, 16, 30, 35]. In fact, a
crucial tactile property that guides touch and grasping, is perceived
friction between a surface and fingerpad [75]. When we feel that an
object has lower friction, we intuitively know it requires a stronger
grasp to prevent slipping [20]. Given the importance of friction to
our tactile perception, many researchers have engineered devices
that use surface friction as a feedback modality [7, 46, 50].
Unfortunately, while many other haptic cues (e.g., pressure, vibration, or kinesthetics) have seen a plethora of methods developed
over the last four decades, this is not the case for friction. For example, to implement pressure rendering, haptic designers can choose
from a variety of approaches (e.g., motors [33, 70], pneumatics [79],CHI ’24, May 11–16, 2024, Honolulu, HI, USA

Alex Mazursky et al.

Figure 2: Our approach modulates surface friction via a (a) wearable droplet generator that (b) deposits slippery and/or sticky
liquids, which (c) coat the fingerpad to form an interfacial layer that the finger glides on, altering the coefficient of friction.
Moreover, (d) these liquids were engineered purposefully to, over time, evaporate and let the surface return to its original
friction.
magnets [52, 73], smart materials [11, 56], electrotactile [62, 78],etc.), each offering different pro/cons regarding wearability, power
consumption, scalability to real-world objects, etc. However, thehaptic domain of friction rendering has not experienced this abundance of research approaches—in fact, only two key approaches
to modulating surface friction have been deeply explored: ultrasonic vibration and electrostatic adhesion. Ultrasonic vibrations
reduce friction as a finger slides over a surface by vibrating thesurface such that it only contacts the finger intermittently [76].
Electrostatic adhesion increases the friction by means of attractive(electrostatic) forces between surface and finger [65]. These twoworking principles may seem limited in that neither can both increase and decrease friction, but this can be overcome by using thesetechniques in conjunction [29]. Moreover, of greater conceptual
significance, implementing these approaches requires researchers
to instrument surfaces around the user—e.g., either adding vibration actuators or coating the surface so that it can be electricallyattracted to the user’s finger. This need for surface instrumentation prevents achieving the goal that Bau et al. set for the field ofaugmented reality haptics in their seminal work “[to] only require
the augmentation of the user, not the entire environment” [9]. In
fact, besides REVEL [9] (which requires objects to be electricallygrounded to the user and metallic) and a vibrotactile device by
Asano et. al [5], the majority of friction devices are implemented
on top of touchscreens or tabletops [1, 8, 14, 65, 76], rather thaneveryday surfaces.
To explore a novel, alternative, technique that circumvents theselimitations, we propose Stick&Slip, a wearable device that can modulate the friction between the finger and many, everyday objects.
We achieve this by depositing liquid droplets onto the finger toform an interfacial layer between the fingerpad and surface. Bytuning the lubricating and adhesive properties of the liquid coating,
we can either increase or decrease the coefficient of friction by up
to ±60%. Importantly, we identify liquids that influence friction, yetleave behind only minimal residue (such as alcohol which rapidly
evaporates). While we acknowledge that depositing liquids onto
the skin is an unconventional approach to haptics, it provides analternative method that circumvents many of the limitations associated with prior work, enabling friction modulation of a wide
range of objects and surfaces. To illustrate the applicability of
our novel approach, we demonstrate how it adds friction-feedbackin virtual/mixed-reality or, even, while using everyday tools. As
the first work exploring the use of liquids for interactive friction
modulation, we identify not only the benefits of this approach, butalso the limitations (e.g., onset times, residues, etc.) via technical evaluations and a user study. Finally, we distill our findings
into recommendations based on the benefits and limitations of thisapproach and discuss areas for future exploration.
Ultimately, we see Stick&Slip as an exploration of alternative
approaches for friction haptics—an area that unlike pressure orforce-based haptics, has not seen a plethora of technical alternatives.

2

OUR APPROACH: STICK&SLIP

Stick&Slip alters the friction of real-world objects by interactively
applying liquid coatings onto the user’s fingerpad. We use liquids because they can alter the surface friction of nearly any nonabsorbent surface, enabling a wider range of augmented surfaces
than possible with prior approaches.

2.1

Working principle of Stick&Slip

Figure 2 shows the working principle of Stick&Slip. When theuser’s fingerpad contacts a surface, they immediately perceive its
surface friction [75], which depends upon surface characteristics
like roughness, surface energy, temperature, etc. [10, 54]. To alter
the surface friction, we deposit a thin liquid coating between thefingerpad and the surface from a nail-worn droplet generator, whichacts as a physical buffer. We specifically mixed liquids that candecrease friction (“slippery”) or increase friction (which we referto as “sticky”). Our slippery liquids (typically acetone and IPA)
decrease the coefficient of friction by forming a lubricating layerthat fills the space and irregularities between the skin and thesurface, allowing the finger to glide smoothly over the surface
with less friction than in a dry condition. In contrast, our sticky
liquids (honey mixed with IPA) increase the coefficient of friction
by forming an adhesive layer between the skin and the surface,causing the finger to feel more resistance when sliding over thesurface.
Notably, for surfaces coated with liquids, the friction is almost
independent of surface material [54]; therefore, the liquid coating
influences the user’s perceived sense of friction regardless of thesurface touched by the user. This is a key advantage of our approach: unlike prior approaches (electroadhesion and ultrasonicvibration), our liquid-based approach enables changing the frictionof practically any non-absorbent object touched by the user. Finally,
to fully maximize our novel method of interactively altering surface friction, we selected & evaluated liquids that either evaporate
entirely (e.g., mixed with alcohol) or which have residues that caneasily be washed away with an antagonist fluid (e.g., sugar dissolved with water). Though our skin naturally leaves behind oil

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CHI ’24, May 11–16, 2024, Honolulu, HI, USA

residues on surfaces we touch, our selection of fluid aims to reduce
any additional residue (see Technical Evaluation).

2.2

Walkthrough: Stick&Slip in a mixed reality
application

We demonstrate our concept in a mixed reality (MR) experience
where the friction of real surfaces is altered. As shown in Figure 3,
a user is wearing our wearable device in which thin tubes (mounted
on the fingernail) deposit liquid droplets onto the fingerpad, which
are pumped from reservoirs worn on the user’s wrist. Our device
is self-contained (i.e., battery-powered, and wireless) and communicates with the HoloLens 2 headset via Bluetooth.

Figure 4: (a) The user drives their car through a rain puddleand (b) our device deposits slippery fluid as their car spins
out. (c) The user then quickly rounds a corner, but (d) slows
down dramatically sliding through virtual honey as the user
feels sticky feedback.
Figure 3: Our user in MR wears our friction modulation
device and interacts with real surfaces and objects.
This user experiences a MR racing game where their index finger
controls a car driving around a track on their table. Because ourwearable does not cover the user’s fingerpad, they can feel the
texture of their table and the different objects. For example, when
the user slides their finger over 3D-printed props corresponding tospeedbumps in the game, they feel their ridged texture. As the user
drives the virtual car with their finger around the track, in-gameevents trigger changes in surface friction. As shown in Figure 4(a) a cloud rains on the track, leaving a virtual puddle. When, (b)driving through the puddle, they feel that the table is slippery andsee their car spin out of control. This change in friction is caused byour device delivering slippery liquid to the user’s fingerpad, leading
to a better match between virtual & physical sensations (see User
Study). Our fluids rapidly evaporate and the surface friction returns
to normal (e.g., on following laps, the table no longer feels slipperyin this location).
Figure 4 (c) depicts as the user rounds a corner, a beehive drips
honey onto the track; thus, (d) as the user drives through the virtualhoney, they feel a resistance to sliding. This increase in surface
friction is caused by our device delivering sticky liquid to the user’s
fingerpad. Like how one feels after getting fruit jelly on one’s
skin, our sticky liquids use dissolved sugars to increase the friction
between the finger and surface via adhesion. However, unlike
jelly, our sticky liquids are designed to quickly evaporate. As our
liquids evaporate, they leave behind a thin film of adhesive residue.
Importantly, this residue is significantly lower in sugar (i.e., ∼230wt% compared to 70wt% in jelly) and its stickiness decreases overtime (dry sugar is not sticky). Moreover, any sugar residue can beeasily dissolved using water, which our device can also deliver (seeExperiments).
The user keeps racing and experiences more stick & slip effects,
as depicted in Figure 5 (a) slowed by an opponent that sprays virtual
green goo onto the track, or (b) losing control of their car due tovirtual ice on the road.

Figure 5: Surface friction effects rendered by our device, e.g.,
slowed down by resistive slime or losing control on slippery
ice.

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

Importantly, our approach enables friction modulation on nearly
any non-absorbent surface, allowing us to alter the friction of everyday objects (without instrumenting them with actuators). Figure 6
depicts this: (a) a virtual boost effect (slippery fluid) makes it feeleasier to glide up the physical ramp prop, while (b) a virtual windeffect (sticky fluid) makes the physical speedbump props feel more
obstructive.

Alex Mazursky et al.

an electrostatic charge on a touch panel to increase attraction between the surface and finger. Others combined electroadhesion
with ultrasonic vibration to achieve both increasing and decreasing
friction [29]. Haptidrag [45] used a novel electrode arrangement
to augment tangibles with electroadhesion friction feedback onreal surfaces, although its underlying principle limits the surface
materials, e.g., no metals or plastics.
Temperature-triggered mechanisms for friction changes
have recently been explored. StickyTouch [28] is a surface with variable stickiness from temperature-sensitive tape that undergoes adramatic increase in adhesion above a transition temperature. Others found that the friction between skin and glass can be controlled
by simply heating certain regions, due to the skin’s temperaturedependent viscoelasticity and moisture content [14].
Considering the goal of ubiquitous haptics, non-wearable approaches face a particular challenge: adding actuators onto every
object is impractical due to cost, size, and power consumption [43].
While these approaches lend themselves well for use in tablets andother standalone devices, they do not easily scale to the demandsof haptics for mixed reality. As such, a more pragmatic solution
to scaling haptics may be to instrument the user (rather than theenvironment) so that the actuator accompanies the user anywherethat haptics is needed.

3.2

Figure 6: Our device alters the friction of everyday objects,
such as on top of this toy ramp or speedbumps.

3

RELATED WORK

Our work builds upon devices that provide friction feedback. With agoal of haptic ubiquity, we focus on wearable approaches, especiallythose that can overlay haptics on real-world surfaces (e.g., [4, 72,77]). Additionally, we reflect on recent alternatives to traditional
actuators (e.g., liquid chemicals), which aligns with Stick&Slip’s
ethos.

3.1

Non-wearable Friction Devices

The most traditional approach to achieving friction feedback is byadding actuators to surfaces. There are four methods that havebeen commonly used.
Mechanical devices have long been used to generate resistive
forces like those presented by friction. For example, the Haptic
Tabletop Puck presented a tangible with a retractable rubber stopperthat increased friction when dragged on a surface [42].
Vibration devices are commonly used for tactile feedback andsimulating surface properties, typically integrated into flat surfaces
and tablets. By vibrating the surface at high frequencies, the finger only intermittently contacts the surface, leading to a reduced
coefficient of friction [37, 76].
Electroadhesion devices increase friction by electrostatic adhesion [44, 64, 65]. TeslaTouch [8] popularized this approach, using

Wearable Friction Devices

Wearables offer a solution to providing haptics at scale because
they are always with the user and do not require further instrumentation. The common approaches use similar actuators to their
non-wearable counterparts.
Mechanical wearables include devices like Frictio [25], a ringwith motor-controlled rotational resistance as an eyes-free information display. Similarly, FrictShoes [71] proposed wheeled shoes
with brakes for simulating the sensation of walking on surfaces
with different frictions in VR. Some have simulated the sense offriction via skin stretch on the fingerpad by placing motors aroundthe finger and end effectors on the pad [55, 61, 63].
Vibration wearables include devices like HapCube [32], which
generates tangential pseudo-forces (that can be interpreted as friction to users) by means of asymmetric vibrations [48, 57].
We take inspiration from wearable approaches to rendering friction but identify a key challenge: in most cases, the body is instrumented with actuators that impair interactions with real-world
objects. While these approaches work well in VR, these cannot
overlay friction onto real-world objects, a key goal in MR haptics [6]and also our focus. To solve this, we need devices that leave thefingerpad free to feel both physical and virtual sensations.

3.3

Fingerpad-free Friction Devices

REVEL [9] laid out the vision for friction modulation in MR by
expanding electroadhesion to be applicable to everyday applications outside of using tablets (e.g., mixed reality, tangibles, etc.).
REVEL proposed reverse electrovibration, where rather than charging the surface, charge is applied to the user, enabling feeling friction changes when interacting with metallic, grounded objects.

Stick&Slip: Altering Fingerpad Friction via Liquid Coatings

REVEL scales to objects in one’s surroundings, as long as the objects have been coated in layers of conductive electrode and insulation. We draw inspiration from REVEL’s approach while aspiring
to eliminate all object-side instrumentations.
Alternatively, Asano et. al [5] developed a vibrotactile device
that leaves the fingerpad free and modulates roughness of realsurfaces, which is related to surface friction. Because this approachis based on tactile masking of the real-world, the technique requires
strong vibrations that the authors note to interfere with the “natural
fusion of real and artificial stimuli” [5]. Our work is motivated bythis challenge and aims to build upon wearable and scalable haptic
devices by improving upon the sensory consistency between real
and artificial stimuli.
Both REVEL and Asano et. al made great conceptual strides
towards friction modulation anywhere. However, we speculatethat to go beyond the capabilities of conventional actuators, a lessconventional route may be needed. Thus, we explore friction modulation on a wide range of surfaces via coating the user’s fingerpad
with liquids.

3.4

Chemicals as Alternatives to Traditional
Actuators

In recent years, there has been a push to experiment with different
approaches to influencing perception, such as with chemicals, toovercome challenges of traditional actuators [12, 24, 31]. For example, Chemical Haptics proposed using liquid stimulants as analternative to Peltiers, vibromotors, and electrotactile stimulation[40]. We draw inspiration from the ethos of these works when we
explore using liquids as opposed to traditional actuators for friction
modulation.
Water as a touch medium has been investigated in HCI [23, 53,
59]. However, to the best of our knowledge, liquid lubricants andadhesives have not been investigated within an interactive context.
While unconventional, we demonstrate that carefully chosen liquids can circumvent many of the limitations associated with priorapproaches to variable friction, enabling friction modulation of awide range of objects and surfaces.

4

BENEFITS, CONTRIBUTIONS, AND
LIMITATIONS

Our key contribution is that we propose, explore, and engineer a
new approach to friction modulation based on liquid droplets thatcoat the fingerpad. Our approach provides three key benefits: (1) It
enables altering friction on a wide range of surfaces and geometries,
making it possible to modulate nearly any non-absorbent surface;(2) Since our device is a wearable, friction modulation easily scales
to many objects in the environment; (3) It can both increase anddecrease surface friction using a single hardware device. Ultimately,
we see Stick&Slip as an exploration of alternative approaches forfriction haptics, an area that unlike pressure or vibrotactile haptics,
has not seen many alternatives.
Our approach is not without limitations: (1) Liquids are ineffective in absorbent materials (e.g., fabric); similarly, there are edge
case materials that resist our approach (e.g., “non-stick” surfaces
such as PTFE onto which not even a gecko can adhere [27]) or are
incompatible with solvents (e.g., acetone can strip some surface

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

finishes [58]); (2) Because our sticky fluids are diluted to enable
faster evaporation, they first feel slightly slippery to the user before
increasing in friction as their solvent evaporates; (3) While ourapproach delivers small droplets, their wetness can be noticed insome use cases. That said, we strive to characterize these limitations
so that we can reduce them and improve the overall interaction
experience. Further insights into understanding these limitations,best practices, and areas for ongoing investigation are detailed in
our Recommendations and Future Work section.
Finally, we are not proposing to replace existing friction modulation techniques such as electrostatics and vibration, but rather
we aim to widen the range of objects and surfaces that can be
modulated with a new approach.

5

IMPLEMENTATION

To help readers replicate our design, we now provide the necessary
technical details. To accelerate replication, we provide all the sourcecode of our implementation1 . The key components of our haptic
device are the finger-worn droplet generator, the pumps, and thecontrol electronics, as shown in Figure 7. Our wearable device
uses its pumps to draw liquids from their reservoirs (worn as abracelet around the wrist), through tubing, and then to the holesthat generate droplets on the skin.

5.1

Finger-worn Droplet Generator

We wrap y-split channels around the sides of the finger, there thechannels narrow from 1mm to a 0.5mm nozzle that pinches thefluid into a droplet (droplet size: ∼15uL). These droplets run down
the side of the finger and coat the fingerpad. The nozzle diameteris small enough such that shaking the device or touching a surface
does not cause leakage. Importantly, our droplet generator keeps
the user’s fingerpad free to feel real surfaces—even if fingers areslightly wetted from friction modulation, they still feel some of thesurface’s texture [22]. The droplet generator weighs 0.35 grams andis adhered to the fingernail with double-sided tape. We use silicone
tubing for sticky liquids and PTFE tubes for slippery liquids (toprevent evaporating through silicone).

5.2

Pumps, Sensors, and Microcontroller

At the core of our device is an ESP32C3 microcontroller (Seeeduino
Xiao), which communicates with external applications (e.g., MR
experiences) via Bluetooth LE and is responsible for controllingthe micropumps. Our entire wearable device weighs 60 grams
including filled liquids. We also piloted a three-channel version byadding another pump, tubing, and y-split in the droplet generator
to include a channel for depositing water to wash away sugar (asshown in Figure 7a). For the sake of miniaturization, our final
device uses just two channels (sticky/slippery).
Pumps. We use peristaltic micropumps (Takasago RP-Q) to
pump our liquids from their reservoirs to the droplet generator.
These specific pumps were chosen for their siphon prevention (i.e.,liquid cannot be shaken or sucked out preventing accidental fluid
discharge, unlike piezoelectric diaphragm pumps). We run ourpumps at 3.7V via DRV8837 H-Bridges and control fluid volume
based on timing (adding thermistors or flow meters to close the loop
1 http://lab.plopes.org/#stickslip

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Alex Mazursky et al.

Figure 7: (a) Our nail-worn droplet generator. (b) Our bracelet device and its components. (c) Electronics schematic of our PCB.
is possible, but we found in pilots that these pumps are consistenteven in open loop). We found that 1 second of pumping is sufficient
to coat the fingerpad, which is what we use in typical interactions.
Given our 1mL fluid reservoirs and our pump flow rate of 1mL/min,our device enables 120 interactions (60 droplets x 2 channels). This
number is proportional to the reservoir size (e.g., a 2mL reservoir
will double interactions).
Battery life. Our 350mAh battery fits directly under our custom PCB. Since our pumps are low-power (3.7V x 80mA = 0.3W),
the battery life is >4 hours of continuous pumping. However, as
outlined above, we only need to pump fluid for ∼1 second to coat
the fingerpad. Therefore, our battery tends to last a full day of
on-demand use.
Latency. The total latency of our device is ∼200ms as measured by a highspeed camera, i.e., from a keyboard trigger in Unity
to a droplet beginning to dispense. This overall latency includes
Unity/OSC/BLE communication (∼40ms), microcontroller (<10ms),
and pump (∼150ms).

6

DEVELOPING OURFRICTION-MODULATING LIQUIDSThere are two primary factors that influence a fluid’s frictional characteristics: viscosity and surface energy. Increased viscosity leads
to greater resistance to flow, while increased surface energy leads togreater adhesion and cohesion [18, 67]. As such, we experimented
and selected a set of liquids with various viscosities and surface
energies aiming to find fluids that induce sticking and slipping. As
a subgoal, we uniquely want fluids that can either quickly evaporateaway or whose effects can be negated by another fluid, so that we
may interactively switch between frictions. As this is a first workin using liquids for interactive friction modulation, we detail the rationale for selecting candidate fluids: (1) We chose only from fluids
deemed skin-safe (i.e., found in commercially-available healthcare
products), which ruled out UV-curable resins and chemically corrosive fluids. (2) We selected only fluids that are relatively clean
to deposit into environments, which eliminates fluids that cause
miscoloring such as liquid metals—all our fluids are clear and werecolored only for visibility in photos. (3) We chose only from fluidsthat can be easily pumped, which rules out very viscous fluids likepure honey or silicone damping fluids. (4) We did not choose fluidsthat require hardware beyond pumps (e.g., skin-safe super glue

requires a closing mechanism to protect it from curing in air). (5)We did not choose liquids that are not liquid at room temperature(e.g., hot glue). This yielded the following set of initial fluids that
we evaluate later:
Isopropyl Alcohol. While not commonly regarded or studied
as a lubricant, isopropyl alcohol (IPA: 99.9% v/v) acts as a slippery
liquid while also offering a rapid rate of evaporation. This is highly
desirable for our use case—IPA can lubricate a surface to make it
feel slippery and then evaporate away without a trace. If the user
touches the surface after the liquid has evaporated, it will feel as
though it had never been lubricated. IPA is skin-safe under normaluse [26, 34], but can lead to dry skin when the skin is exposed tohigh volumes of fluid for a long time (e.g., 20mL for 1 hour [34]);
this exposure level required for extreme dryness is considerably
higher (>1000x the volume) and longer than that of our droplets,
which evaporate in minutes as shown in Experiment 2. Moreover,widespread clinical studies have found IPA to be less irritating to
the skin than hand washing with detergents [38, 68]. Thus, it
is commonly found in hand sanitizers, cosmetics, and householdcleaning products. Similarly, isopropyl alcohol is compatible witha wide range of surfaces spanning metals, glass, electronics, andmost plastics. It should be noted that some plastics and surface
finishes are degraded by alcohol and are thus incompatible with
IPA [39].
Acetone. Acetone is a solvent that can act as a lubricant, butwith a faster rate of evaporation than IPA. This makes it a greatcandidate for interactive applications in which the friction must
only be briefly reduced. Acetone, like IPA, is skin-safe under normal
use. While acetone is commonly thought to lead to dry skin [41, 80],studies in dermatology have found that it does not disrupt the skinbarrier, even when exposure is much greater and longer than usedin our application (e.g., a cotton ball soaked in acetone applied for>12 mins continuously) [2, 60]. It is commonly found in nail polishremover, often accompanied by additives such as glycerin to ensureskin moisturization [13, 49]. While acetone has the advantage of
rapid evaporation, it should be noted that it is a stronger solvent
than IPA [58]. Thus, if a material is incompatible with acetone, IPA
may be used as an alternative.
Honey-IPA Solution. Honey is well-known for its viscosity
and stickiness owing to sugar’s hydrogen bonds with water [74].
Unfortunately, honey’s viscosity makes it difficult to pump. Therefore, to create a sticky liquid, we dilute honey with IPA to reduce

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CHI ’24, May 11–16, 2024, Honolulu, HI, USA

its viscosity. Importantly, honey’s stickiness remains despite being
diluted (as we show in our Technical Evaluation, just 2wt% honeycan cause a 50% increase in static friction). Because we dilute honeywith a solvent, the honey can effectively coat the fingerpad. As
the IPA evaporates, it leaves behind a thin, adhesive film on thefingerpad. Importantly, we chose honey because it is water-soluble;
thus, it can simply be washed away with water after use, returning
the fingerpad and surface to their original friction. Note, we chose
IPA (IPA: 70% v/v, water: 30% v/v) as the solvent rather than acetone
because sugar water is insoluble in acetone. This is beneficial: slippery liquids that do not rehydrate sugar residue prevent accidental
stickiness.
Thickened Water. Water can be made into a viscous gel with avery small amount of thickening agent (e.g., 1 wt% of Xanthan gum,a food-grade polysaccharide). Despite the small concentration of
thickening additive, it feels adhesive to the touch while very little
residue behind.
Additional safety measures. We exclusively applied our liquids
in small droplets (∼15 uL) to the fingerpad, with brief exposure
times enabled by rapid evaporation. This approach mitigates potential side effects, notably dry skin. Moreover, the small volumes
employed, coupled with the direct delivery of droplets to the sideof the fingerpad, not only ensures immediate coating but also minimizes the risk of inhalation or application to other skin areas.

mass-pulley tribometer, as illustrated in Figure 8. By increasing theload of the hanging mass, a sliding mass approximating the finger’s
frictional properties slides across the surface. The coefficient offriction can be calculated by measuring the block’s acceleration
along with the hanging mass. For each fluid, 10uL of liquid was
pipetted between the testbed surface and the sliding block. We
evaluated both the static and kinetic friction coefficients for nine
potential liquids: (1) sugar water (66.6wt% sugar), (2) 30wt% honeyin IPA, (3) 2wt% honey in IPA, (4) thickened water (1wt% xanthangum), (5) water, (6) water-based lubricant (Shibari), (7) acetone, (8)IPA, and (9) oil (3-IN-ONE Multi-purpose).

7

Our sliding mass approximated the finger’s frictional properties
during a light, exploratory touch. The sliding mass weighed 25g.
To approximate skin, the bottom of the sliding mass was coated inpolyurethane (commonly used in mechanical approximations of theskin [15], Smooth-On KX Flex 40). The surface area of the block was
20mm2 to approximate the contact area of a fingerpad under 25g ofcompression [17]. The testbed was made from smooth acrylic (notethat for well-lubricated surfaces, the friction coefficient is nearly
independent of the material [54]). Prior work in contact mechanics
indicates that the friction coefficient for an index fingerpad on
acrylic under 25g and sliding at low velocity is ∼1.60 [3, 36]. In our
testing, we found our sliding mass to be a reasonable approximation
for a finger because its kinetic coefficient of friction on acrylic
measured 1.6±0.125 over 25 trials.
Static friction procedure. First, we tested the effect of liquids on the coefficient of static friction, determined by finding theminimum force required to initiate sliding. To do so, the hanging
mass was slowly increased by pouring sand into a suspended cup(∼1.5grams/sec). Once the mass began to slide, a hardware limitswitch was released, and no more sand was added. The hanging
mass was then weighed. The static coefficient of friction is calculated by taking the ratio of the hanging mass to the sliding mass.
For each liquid, five trials were performed, and the apparatus wascleaned between trials; we confirmed the cleaning effectiveness bymeasuring and comparing it to the no-liquid baseline value.
Static friction results. Figure 9 (a) shows change in static friction coefficient that fluids achieved (relative to a no liquid baseline,
the surfaces’ original friction). We found greatest reductions instatic friction from oil (-62.9±3.3%), IPA (-59.7±2.9%) and acetone (59.2±4.3%), while the greatest increases in static friction were foundfrom sugar water (83.0±15.7%), 30wt% honey in IPA (78.3±16.1%)
and 2wt% honey in IPA (56.1±9.5%). Note that because there is

EXPERIMENTS 1-4: FLUID FRICTION &PRACTICALITY

To understand the effects of our candidate liquids on friction as
well as their practicality (i.e., how long do their frictional effects
last?), we performed four technical experiments. We then apply
these findings to our user study where participants experienced
our approach in an interactive context. To our knowledge, theseexperiments have not been performed in related work. Thus, we
designed these to focus on our specific application of temporary
friction modulation between a finger and surface.
To aid the reader in understanding the four technical experiments we conducted, we present an overview of their results: In
experiment 1, we measured the fluid effects on friction coefficients: we found the greatest friction reduction from oil, IPA,
and acetone, and the greatest friction increase from sugar water,
30wt%, and 2wt% honey in IPA; In experiment 2, we measured
the fluid evaporation rates: we found that acetone, IPA, 2wt%,
and 30wt% honey in IPA evaporated the fastest, and we eliminatedoil, water-based lube, water, and sugar water for their poor evaporation; In experiment 3, we measured friction over repeatedtrials: we found that acetone and IPA initially decreased friction,then returned to the baseline. We found that 2wt% and 30wt%
honey in IPA caused stickiness faster than thickened water; finally,in experiment 4, we measured washing away residue: wefound that water washes away sugar residues, where effectivenessincreases with water volume.

7.1

Experiment 1: fluid effects on friction
coefficients

To determine the extent to which each of our candidate fluids
impacts the coefficient of friction between surfaces, we built a

Figure 8: We built a mass-pulley tribometer to mechanicallyevaluate the effect of our liquids on coefficient of friction.

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

Alex Mazursky et al.

Figure 9: Relative changes in: (a) static friction, and (b) kinetic friction for each fluid.
an onset time for sticky liquids, we depicted maximal values (seeRepeated Trials for a characterization of this onset).
Kinetic friction procedure. As we are interested in sliding
interactions, we measured the kinetic friction coefficient, which
can be determined by finding the acceleration of the sliding block.
To do so, we apply a hanging mass equal to the average mass that
initiated sliding for each fluid in our earlier static experiment. Whenthe mass begins sliding, the start limit switch is released and theblock slides into the end limit switch. The distance (6cm) and timedifference between triggering the switches is used to calculate theblock’s acceleration (𝑎 = 𝑑/𝑡 2 ). The kinetic friction coefficient isthen calculated using: 𝜇𝑘 = (𝑚 2𝑔 − (𝑚 1 + 𝑚 2 )𝑎)/𝑚 1𝑔.
Kinetic friction results. Figure 8 (b) shows the relative change
in kinetic friction coefficient for each liquid. As shown, the overall
trend held between the static and kinetic friction tests. Consideringthat friction coefficients span a small range (i.e., typically from 0.0
to 4.0), even a ±30% change is dramatic.

7.2

Experiment 2: evaluating fluid evaporation

With a goal of using liquids to alter friction interactively, we aim tochoose liquids that not only alter sensation, but are also practical,
i.e., evaporate quickly and leave minimal residue so that we may
design applications that can switch easily between sensations. Thus,
we measure the weight of a fluid droplet over time, to characterize
its evaporation.
Procedure. To measure the rate of evaporation, we pipetted a
10uL droplet onto a plastic weigh tray and placed it inside a labgrade scale (Mettler Toledo, 0.0001g precision). The droplet weight
was recorded every 30 seconds.
Evaporation results. Figure 10 shows the relative remaining
droplet mass after five and 60 minutes (only two points depicted
for clarity, measurements taken at 30s intervals).
We found that solvents like acetone and IPA evaporated thefastest and left behind no residue. The second fastest evaporatinggroup of fluids were solvents containing honey, where a lower concentration of honey resulted in faster evaporation. Notably, thesolvents evaporated out of these mixtures leaving behind residueof approximately the mass of the honey contained within the mixture along with retained water (after 60 mins, 2wt% honey had
3.4% remaining mass and 30wt% honey had 32.4% remaining mass).
Water-based liquids evaporated slower than solvent-based liquids,
where thickened water evaporated faster than water, which itself

Figure 10: Fluid evaporation at two points (5 and 60 minutes).
evaporated faster than water-based lubricant. Finally, oil showedno signs of evaporation in 60 minutes.
Eliminating unsuitable fluids. At this point, we found that oil,water-based lube, water, and sugar-water do not evaporate rapidly(i.e., the four slowest evaporations in Figure 10). Moreover, giventhat we found that solvents had comparable lubricating properties in our previous experiment and that these solvents evaporate
rapidly, we deem oil, water-based lube, water, and sugar-water as
no longer fluids of interest. That said, these may still have niche
use cases, which we elaborate upon in Recommendations and Future
Work.

7.3

Experiment 3: fluid friction over repeatedtrialsHaving characterized each fluid’s maximal effects on friction alongwith their evaporation rates, we now look to characterize frictionand evaporation in conjunction over multiple sliding motions. Additionally, we aim to shed light on the timing (onset and offset) of
each fluid’s effects.
Procedure. We used the same apparatus and procedure described in Static Friction. However, instead of cleaning the testbed
and sliding block after a single trial, we placed the sliding blockback on the starting position and tested the static friction again.
After a droplet was placed on the block, sliding was repeated 10
times. Each time, the block slid 6cm, for a total distance of 54cm.
Repeated trial results. Figure 6 shows the relative change infriction for each fluid over the sliding distance (each point represents the average of 5 trials). We found that all our fluids cause anStick&Slip: Altering Fingerpad Friction via Liquid Coatings

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

Figure 11: (a) Fluid friction over repeated trials (distance in cm); and (b) offset/onset distance (i.e., the sliding distance required
for a slippery sensation to disappear or a sticky sensation to appear).
initial reduction in the static friction, with solvent-based fluids being the most lubricating. Acetone and IPA return to approximately
the no-liquid baseline as the fluid evaporates. The fluids containing
honey and xanthan gum result in increased friction after sufficientspreading of the fluid during sliding and evaporation.
On/offset results. To understand how the onset/offset timesof friction changes affect perception, we contextualize themwithin psychophysics literature by comparing to established justnoticeable-difference (JND) values. Specifically, the Weber fraction
for surface friction is known to be ∼11% [19]. Therefore, we candetermine the sliding distance required for a change in friction to
be felt, as depicted in Figure 11 (b). All tested liquids initially reduce
friction greater than 11% and thus immediately feel slippery. In thecase of liquids that intentionally are slippery, we aim to select for
those that can quickly return to within 11% of the baseline friction
coefficient (determined with linear interpolation), i.e., the slippery
effect has disappeared, and the surface feels unaltered. In the caseof IPA, we found that after 44.7cm, the change in static friction
returns to within 11% of the original. Acetone exhibited a faster
return to baseline friction, at just 5.7cm. This suggests that acetone
is better for brief lubrication, while IPA can provide a sustained
sensation for the same dispensed volume (while still disappearing
over time). While sticky liquids initially reduce friction by a small
amount, after some sliding and evaporation, they dramatically increase friction. Therefore, we are interested in how long it takes
for sticky liquids to perceivably increase the friction by more than
the 11% JND. We found that 2wt% honey in IPA took the shortest
distance (14.7cm), followed by 30wt% honey in IPA (18.2cm) andthickened water (44.7cm).
These are conservative estimates. We discuss the implicationsof these timings in Recommendations but it is important to notethat mechanical analysis results in very conservative estimates of
performance: (1) we slid the block repeatedly over the same path as
opposed to one continuous long slide (worst-case scenario); (2) ourelastomer skin does not absorb any fluid unlike actual skin [40];and (3) our sliding block is at room temperature, while our skin issignificantly warmer which aids in fluid evaporation [47].
Eliminating unsuitable fluids. Because thickened water has
a long onset time to increasing friction, we eliminate it, but discuss
other potential uses for it in Recommendations.

7.4

Experiment 4: washing away sticky residue

As we found in our second experiment (Figure 10), sticky liquids
leave residue behind due to sugar content. Moreover, our third
experiment (Figure 11) found that, in the short-term, this residue
increases friction as long as it retains water (dry sugar is not sticky).
While waiting for this stickiness to naturally dry might fit some
applications, we argue that understanding whether this process canbe sped up is of value for a wider interactive application of our concept. To tackle this, we propose using water to dilute and dissolveaway sugar residue. To this end, we conducted an experiment.
Procedure. On sticky liquids, we performed static friction tests
until the friction reached within 5% of the peak reported in our firstexperiment (Figure 9). Then, we added 10uL or 20uL of water andcontinued performing static friction tests.
Washing stickiness results. We found that without washing,
the friction decreases over time for both 30wt% and 2wt% honeyin IPA. Adding water causes an initial decrease due to lubrication.
Then, the water dissolves the sugars, and the friction decreases
toward the baseline condition, where more water led to greater reduction in friction. Moreover, we also confirmed that using acetone
and IPA as wash fluids also reduced the stickiness immediately, but
the stickiness eventually rises back to peak values as the solvents
evaporate without having diluted the sugars.

7.5

Summary of findings and narrowing thebest liquids

Our four experiments characterized fluids based on their ability tomodulate friction and their practicality. While all fluids we investigated offer unique properties which we discuss in Recommendations, we optimize for our applications (requiring strong modulation
changes while being easily removed). As such, we selected 30wt%
honey in IPA as our sticky fluid and acetone as our slippery
fluid for all our interactive applications.

8

EXPERIMENT 5: USER-STUDY ON
INTERACTIVE USE

While our first experiments examined our fluids’ effects on friction along with their evaporative properties, our final experimentfocused on observing our approach in an interactive application.

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

Alex Mazursky et al.

was made of acrylic to minimize issues with the Hololens’ handtracking.
We designed the timings of pumping our liquids based on the
results of Experiment 3: (1) Slippery sensations produced byacetone: Because there is no onset time for slippery liquids toreduce friction, we pump for one second immediately when a user’s
finger enters a slippery zone. (2) Sticky sensations produced by
30wt% honey in IPA: Sticky liquids have an onset distance due
to time required for fluid to evaporate and leave behind a sticky
film. Informed by Figure 11, we designed our experience to begin
pumping 30wt% honey in IPA approximately 18cm before the effectappears so that it feels sticky when the user reaches it.

Figure 12: Applying water to sticky surfaces reduces friction.

Specifically, we assess the extent to which our approach influences
the sense of immersion and enjoyment in a mixed-reality experience with physical surfaces and props. Our main hypothesis forthis study was that the MR experience with Stick&Slip would feel
more immersive and enjoyable than a baseline without our device
because of the haptic experience.

8.1

Conditions

Participants experienced two interface conditions in counterbalanced order: (1) Stick&Slip (worn on the right-hand index finger
and wrist) and (2) a no-haptics baseline in which participants did
not wear our device. We chose this baseline rather than depositinga “neutral” fluid because any fluid, even water, still causes changes
in friction, as found in our experiments and literature [51]. Additionally, the baseline depicts the current state of MR with passive
props. Wearable devices offer the benefit of always-available haptics, often at the expense of, to some extent, encumbering the user.
To assess whether adding our liquid-based friction haptics to mixed
reality justified this encumbrance, we compared between our device
and a no-haptics baseline.

8.2

Participants

We recruited twelve participants (seven identified as female, five as
male, average age of 23.4 years old, SD=3.3). Participants received$10 for their time.

8.3

Figure 13: Study 2’s apparatus: a mixed reality experience
with physical props (image taken from 3rd person perspective).

Apparatus

We built a prop-based MR experience to evaluate our device asshown in Figure 13. Specifically, as informed by our prior experiments, we tested this experience using acetone as a slippery liquidand 30wt% honey in IPA as a sticky liquid. In the experience,
participants wore a Microsoft Hololens 2 MR headset. The table

8.4

Procedure

We utilized our MR racing experience, in which participants drivea virtual car on a track by dragging their finger (the same application described in our Walkthrough). In this version of the MR
racing game, participants experienced eight surface haptic events(rainfall, ice, slime, and honey—each was presented twice). Eachtrial took ∼10 minutes and participants were interviewed between
trials. Interviews consisted of Likert ratings (1-7 scale) of the experience (immersion, enjoyment, sensory engagement [66, 69]) alongwith open-ended questions regarding what contributed to theirexperience. Participants’ fingers were cleaned with soap and water
before each trial. Finally, besides these virtual effects, participantsalso experienced physical props, such as the texture of the table,
3D-printed speedbump props and an acrylic ramp prop.

8.5

Results

Figure 14 presents our main findings. After testing the Likert data
for normality, we used a Mann-Whitney test (two-tailed) to test our
hypothesis that Stick&Slip leads to improved experience. We foundsignificant differences between the two interface conditions, where
Stick&Slip received greater ratings than the baseline for immersion(p<0.001), enjoyment (p<0.001), and sensory engagement (p<0.001).
Moreover, all twelve participants indicated that they preferred the
Stick&Slip condition over the no-haptics baseline. Altogether, this

Stick&Slip: Altering Fingerpad Friction via Liquid Coatings

supports our main hypothesis that Stick&Slip improves the experience.

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

commented on any odors originating from the fluids, which suggests that no odors were perceivable (e.g., from possible vapors of
the liquids as they evaporate). Finally, no participants reported any
dry skin or irritation.

9

FURTHER APPLICATIONS FOR STICK&SLIP

To illustrate the versatility of our approach, we implemented two
additional and unique applications in which we use Stick&Slip toenhance user experience via variable friction.

9.1

Figure 14: (a) Participants’ ratings for both conditions. Error
bars show standard deviation. (b) Participants’ preference.

8.6

Qualitative Feedback

We analyzed all responses to our questionnaire. First, all participants felt that friction haptics added to the experience. To illustrate
this, P10 stated “when touching one single surface, you don’t feelmuch but now with different road situations its quite interesting.”
P2 & P6 both stated that the device made them want to explore tactilely and P9 expressed that it “sparked more childish excitement.”
While one may expect some potential novelty effect, all participants preferring Stick&Slip gives some indication that participants
felt the fluids were not unpleasant and added to the experience;for example, the liquids “added a whole other dimension to thegame through sensations” (P6 & P4). This suggests that the gain in
immersion was worth the cost of wearing our device.
Beyond adding haptics, five participants expressed a desire for“the digital and physical to correspond” (P11). P5 explained “seeing
the visuals line up with the feeling led to better immersion”. P7
noted that “the ice felt slippery” and that their “finger went fasterthan expected”. Similarly, P9 stated, “if I were driving through ice,
it’s how I’d expect it to be”. Along this line, P8 recalled that “[I] felt
a sense of traction going through honey effect”.
Our haptics also influenced how participants experienced props
on the track. For example, P1 felt that it was harder to go up theramp after going through slime. Similarly, P3 & P9 felt going down
the ramp was more slippery after it had rained.
Finally, our approach is not without limitations and participants’
responses also indicate opportunities for further research. First,
two participants (P1 & P9) noted feeling that the ice and rain felt
cold, likely either from acetone’s evaporative cooling or from apseudo-haptic effect driven by the visual suggestion (P1 noted theice felt colder than the rain, despite identical actuation parameters).
Second, three participants noted that they felt a delay between the
sticky visuals and the sticky sensation. Third, only two participants
noted feeling any residue, despite us opting not to use any wash
fluid during the MR experiment, suggesting that the once the sugar
has dehydrated, it is no longer perceived as sticky. No participants

VR using static props with dynamic friction

Figure 15 depicts a VR escape room experience in which the userneeds to repair three pipes to open the next door. When a pipe
cracks and sprays water, the user must place their hand on thecrack to stop the water, which will now feel slippery via our device
rendering the virtual water with real water droplets. To patch thepipe, the user turns around, dips their finger into virtual sealantputty, and returns to rub it over the crack. The user then discovers
that another pipe has rusted and feels the rust’s roughness renderedby our sticky fluid. To remove the rust, the user sprays a dissolver
and rubs the pipe until the rust sheds off, feeling the pipe’s surface
return to its smooth texture as our device washes this stickinesswith water.

Figure 15: Stick&Slip adds friction feedback to VR with props.
We use this example of enhancing VR with props to demonstrate
a key benefit of Stick&Slip: no existing technique can both increaseor decrease surface friction on a prop of this size and material (notelectrically conductive).

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

Alex Mazursky et al.

Figure 16: Stick&Slip with tools: (a) A user tightens a nut via a digital torque screwdriver. (b) At the maximum torque, ourdevice pumps slippery liquid to provide an additional haptic cue. (c) A user unsuccessfully tries to open their glue. (d) The user
activates on-demand stickiness, which increases friction and improves grip, (e) which assists the user to unscrew the cap.

9.2

Augmenting friction of everyday tools

RECOMMENDATIONS AND FUTURE
WORKwith rapid evaporation, there is opportunity considering liquids
that intentionally leave a trail of liquid as a feature of the interaction.
Residue. Sticky liquids may leave residue in proportion to theadded sugar. Once water has evaporated from this sugar, it is nolonger sticky, but this is slower than desired for most interactions.
Thus, based on our technical experiments, performance can be
improved by either adding a third channel or opting to use a waterbased liquid as the slippery liquid, which can also serve the dual
purpose of acting as a washing fluid.
Wetness. Our approach to friction modulation might also be
accompanied by a sensation of wetness. Surprisingly, only twoparticipants mentioned this (and could be due to pseudo-haptics
from visual suggestions of ice/water puddle). Still, we recommend
designing with this user expectation in mind. For example, in ouruser study, we used visual effects that would not feel out of place if
the participant experienced wetness.
Mixing liquids. As the first exploration of interactive liquid
friction, we selected liquids that both alter friction and are practicalto use, which yielded simple mixtures. It is likely possible to synthesize more advanced liquids. Because this is an early exploration,
it is likely that future works may identify liquids that alter friction
with fast onset and offset times but are entirely inert when interacting with skin and sensitive materials (e.g., surface lacquers). In themeantime, we can draw inspiration from products that contain our
liquids (nail polish remover, hand sanitizer, etc.), but also include
common additives like glycerin for improved practical long-term
use [13, 49]. Moreover, liquids offer a wider design space thanjust friction, such as affective responses (e.g., pleasantness), which
offers opportunity for further investigation [21, 22].

We now condense our recommendations based on the benefits andlimitations of our approach, as informed by both our technicalevaluation, user study, and pilots.
Timing. Because our sticky fluids are diluted with solvents like
IPA to enable greater evaporation, they first feel slightly slipperyto the user before their dramatic increase in friction as the solvent
evaporates. Therefore, it is important to design with this in mind.
We conservatively characterized this delay in our Experiments andused it to design our MR experience. For example, depositing sticky
fluid on top of the ramp prop in anticipation of an upcoming sticky
event did not interfere with immersion because participants feltthat the initial slipperiness corresponded to sliding down the ramp.
Similarly, we investigated liquids with a range of evaporation rates.
While we implemented further studies and applications using fluids

We proposed, engineered, and validated Stick&Slip, a new approach
to altering everyday surface friction by coating the user’s fingerpads
with liquid. Unlike traditional actuators such as vibromotors orelectroadhesion, which are confined to specific surfaces, our method
uses liquid lubricants and adhesives. These substances can modulate
the friction of nearly any non-absorbent surface without requiring
specific surface instrumentation, resulting in a more universally
applicable friction modulation.
We demonstrated the versatility of our technique in various
applications, including mixed reality, virtual reality with props,and everyday objects. Our approach was validated through four
technical experiments and a user study.

We leverage liquid lubricants and adhesives because they modulate
the friction of nearly any non-absorbent surface without the needfor any surface instrumentation. Therefore, our approach can alter
the friction of everyday objects and tools. Figure 16 (a-b) demonstrates our first example of how Stick&Slip can alter friction whileinteracting with everyday tools. Here, a user is tightening a nut
with a digital torque screwdriver (Figure 16a). When they reachthe maximum allowable torque, our device deposits slippery liquid,causing the user’s fingers to slip off the tool and stop tightening(Figure 16b). This provides an additional haptic cue when maximum
torque is reached.
Figure 16 (c-e) depicts a second example of altering the frictionof everyday tools. Here, our device can improve users’ grip. The
user tries to unscrew a stuck glue cap but is unable (Figure 16c).
They activate Stick&Slip on-demand via their smartwatch, whichcoats both their index and thumb’s fingerpads (Figure 16d). This
improves their grip on the cap, assisting them to ultimately unscrew
the cap with less effort Figure 16e).
Each of these examples demonstrate that our device can augment
everyday tools with variable friction, providing cues and aid to theuser. This highlights another benefit of Stick&Slip: it would be
impractical to augment these tools with any other existing approach(i.e., coating/grounding every tool for electroadhesion).

10

11

CONCLUSION

Stick&Slip: Altering Fingerpad Friction via Liquid Coatings

Stick&Slip points towards a new direction in HCI, in which
engineered materials/chemicals offer alternative methods to achieve
haptic sensations. Having shown that liquids can both modulate
friction and be improved in their practicality, we hope to inspire
future research exploring the use of liquids for haptics beyond
friction modulation. Ultimately, we hope to provoke alternative
strategies in areas of haptics that may otherwise be limited bytraditional approaches.

ACKNOWLEDGMENTS
We would like to express our sincere gratitude to Wen Li for his
generous help with droplet mass measurement. We would also liketo thank our colleague Romain Nith for his guidance on our PCB
design. Finally, we thank Shan-Yuan Teng for his help with video
production. This work was supported by NSF grant 2047189 andthe Sloan Foundation. Additionally, this work was partially supported by the University of Chicago MRSEC, which is funded by the
NSF under award number DMR-2011854. Any opinions, findings,
conclusions, or recommendations expressed in this material arethose of the authors and do not necessarily reflect the views of any
funding agencies.

SPECIAL ACKNOWLEDGEMENTS
We would like to acknowledge that the ACM CHI 2024 conference
has decided to take place in Hawaii, offering extremely limited
options for remote participation. While some authors have chosen
to present their work in person or bring its physical prototype to
the CHI community, they have done so with compassion towards
Native Hawaiians and with a heavy heart. We urge that the ACM’s
process for conference selection should more carefully consider the
impact our conferences have on local communities. We especiallywant to acknowledge Prof. Josiah Hester, a Kānaka Maoli Professor
in Computing, who organized comprehensive resources that educated us about the negative impacts of over-tourism and climate
degradation in Hawaii2 . Finally, the decision of some authors toparticipate in this conference does not represent the views of othermembers of our lab, who have chosen not to engage with this edition of the CHI conference due to its impact on Native Hawaiians.

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